TRACING NITROGEN THROUGH THE FOOD CHAIN IN AN URBANIZED TIDAL
CREEK
Kimberley Duernberger
A Thesis Submitted to the
University of North Carolina Wilmington in Partial Fulfillment
of the Requirements for the Degree of
Master of Science
Center for Marine Science
University of North Carolina Wilmington
2009
Approved by
Advisory committee
Craig Tobias
Co-Chair
Michael Mallin
Co-Chair
Lawrence Cahoon
Accepted by
DN: cn=Robert D. Roer, o=UNCW,
ou=Dean of the Graduate School &
Research, email=roer@uncw.edu,
c=US
Date: 2009.12.15 11:12:59 -05'00'
Dean, Graduate School
TABLE OF CONTENTS
TABLE OF CONTENTS ................................................................................................................ ii
ABSTRACT ................................................................................................................................... iv
LIST OF TABLES .......................................................................................................................... v
LIST OF FIGURES ....................................................................................................................... vi
INTRODUCTION .......................................................................................................................... 1
METHODS ..................................................................................................................................... 4
Site Description ........................................................................................................................... 4
Experimental Setup and Sampling Design ................................................................................. 6
Field Sampling ............................................................................................................................ 8
Water Column sampling ......................................................................................................... 9
Benthic sampling .................................................................................................................. 10
Laboratory Analysis for N content and 15N enrichment ........................................................... 11
RESULTS ..................................................................................................................................... 15
Salinity Distribution .................................................................................................................. 15
Physical Exchange .................................................................................................................... 15
DIN Dynamics .......................................................................................................................... 17
Gross Distribution of N pools ................................................................................................... 19
Water Column N Inventories – Microautotrophs ................................................................. 20
Water Column N Inventories - Consumers ........................................................................... 21
Benthic N Inventories – Primary Producers ......................................................................... 23
Benthic N Inventories – Epifauna and Infauna ..................................................................... 23
Temporal Changes in N distribution ......................................................................................... 25
ii
Natural abundance isotopic signatures of biota ........................................................................ 27
Isotopic Tracer Enrichment....................................................................................................... 29
15
N Incorporation by Autotrophs .............................................................................................. 31
Transfer of 15N through the Food Web ..................................................................................... 35
Water Column ....................................................................................................................... 35
Benthos ................................................................................................................................. 38
Bulk Sediments ..................................................................................................................... 41
Turnover times for N pools ....................................................................................................... 42
15
N storage ................................................................................................................................ 46
15
N redistribution ...................................................................................................................... 47
15
N retention .............................................................................................................................. 48
Loss of 15N ................................................................................................................................ 51
DISCUSSION ............................................................................................................................... 53
Temporal and Spatial Movement of N and Contributions of Watershed N to Biota ................ 53
Hewlett‟s Creek – Transformer or Attenuator? ........................................................................ 57
APPENDICES .............................................................................................................................. 62
REFERENCES ............................................................................................................................. 66
iii
ABSTRACT
In order to investigate the fate of nitrogen inputs to tidal creek ecosystems we used an in
situ stable isotopic 15N tracer to label the dissolved inorganic nitrogen (N) pool. With this
approach we were able to identify dominant N pathways, quantify N transfer rates, and
determine overall ecosystem N throughput in a southeastern urbanized tidal creek.
Consequently, we were also able to identify the relative importance of attenuators of watershed
N at an ecosystem scale as well as quantify the ecosystem dependence on watershed-derived
nitrate under base flow conditions.
The study site consisted of a 2 km reach of tidally dominated marsh with relatively high
inputs of nitrate (yet uniformly low reach-concentrations of DIN). The tide-mediated addition of
15
N-NO3 significantly labeled the DIN pool and was subsequently taken up, recycled, and
stored/exported by the ecosystem. Microautotrophs (phytoplankton and benthic microalgae)
were instrumental in the rapid utilization and recycling of DIN, exhibiting a turnover rate of 2 3.5 days. Trophic transfer in the phytoplankton-zooplankton pathway was rapid and these two
compartments were tightly coupled. Of the higher trophic levels, oysters showed the most
significant enrichment and appeared to be a keystone component of the tidal creek N processing
at the ecosystem scale.
Bulk sediments were the primary fate of N (76-90 %) however oysters and marsh
macrophytes were also retainers of N (13-17 % and 2-6 %) due to their longer turnover times.
Using gas N2 enrichments we were able to estimate 26-34 g 15N (12-16 %) lost by denitrification
and 19 % by tidal export. Based on a creek 15N mass balance, the creek is equal parts
transformer of dissolved and particulate N forms and an attenuator of N through denitrification.
iv
LIST OF TABLES
Table
Page
1. Subreach characteristics of the south branch of Hewlett‟s Creek. ...................................... 6
2. Sampling frequency of N pools. “d” indicates days after start of isotope addition. ........... 9
3. Nitrate and ammonium surface concentrations per subreach for all sampling periods
throughout the isotope addition experiment. .................................................................... 18
4. Zooplankton densities during the experiment ................................................................... 22
5. Turnover times for the various N pools within the creek. ................................................ 45
6. Snapshot 15N inventories (g 15N excess) and percent of total excess 15N for all organism
pools on the final day of the experiment (d22) ................................................................. 47
v
LIST OF FIGURES
Figure
Page
1. South Branch of Hewlett's Creek, New Hanover County, NC ........................................... 5
2. Conductivity distribution of Hewlett‟s Creek south branch at spring high tide ............... 15
3. A) Rhodamine dye concentrations in the surface waters over 4 consecutive high tides; B)
Total rhodamine mass inventory over time with a single pulse dosing of dye at the
dripper location. ................................................................................................................ 16
4. Rhodamine dye concentrations at various stages of the tidal cycle from a continuous
rhodamine release using pumping rates used for isotope addition ................................... 16
5. Distribution of N biomass (g N m-2) standardized for area averaged over the whole
experiment......................................................................................................................... 20
6. Phytoplankton contribution to total PON within each subreach of the creek. .................. 21
7. Distribution of N (g m-2) contained in active and biomass dominant N pools during the
first week of the addition .................................................................................................. 25
8. N biomass for tidal creek primary producers and zooplankton per week of the
experiment......................................................................................................................... 26
9. Natural abundance isotopic signatures for sampled pools in Hewlett‟s Creek (A) .......... 28
10. Spatial distribution of δ15N- NO‾3 above background levels for sequential high and low
tide creek transects. Data measured on day 14. ................................................................ 29
vi
11. Ambient high tide nitrate concentration in the oligohaline subreach coupled with daily
rainfall and tidal data (National Weather Service). .......................................................... 30
12. Change in δ15N- NO‾3 enrichment for the oligohaline and upper mesohaline (upstream)
subreach measured at high tide ......................................................................................... 30
13. (A) Change in enrichment of BMA and phytoplankton-PON in the oligohaline and upper
mesohaline portions of the creek in response to rainfall................................................... 33
14. Enrichment trajectories for Spartina and Juncus shoots and roots, rinsed to remove mud
and epiphytes. ................................................................................................................... 34
15. Zooplankton biomass and change in enrichment over time (days)................................... 37
16. Enrichment trajectories (change in enrichment over time) for major fish species. .......... 37
17. Enrichment trajectories for oysters composing the three major reefs............................... 38
18. Enrichment trajectories Palaemonetes captured upstream versus downstream ............... 39
19. Enrichment trajectories for Uca at each subreach of the creek. ....................................... 40
20. Enrichment trajectories for Littorina per subreach ........................................................... 40
21. Enrichment trajectories for bulk sediments ...................................................................... 41
22. Representative linear regression plots used to calculate turnover time of specific N pools
........................................................................................................................................... 43
vii
23. Proportional distribution of active biota excess 15N (excluding storage in sediments)
between autotrophs and consumers (µg 15N m-2).............................................................. 49
24. Distribution and collection of 15N within the principle uptake pools (BMA and
phytoplankton) and long term retention pools through the course of the isotope addition
and post sampling events .................................................................................................. 50
25. Enrichment of N2 gas measured from the surface waters of Hewlett's Creek during the
final week of the addition experiment .............................................................................. 52
26. Distribution of 15N (µg m-2) within the consumer pool at the end of the experiment. ..... 55
27. Food web diagram of N cycling within this tidal creek ecosystem .................................. 56
28. Correlation plot of ammonium and phytoplankton enrichment in the upper three
subreaches of Hewlett's Creek .......................................................................................... 57
29. Mass balance of 15N in Hewlett's Creek ........................................................................... 59
viii
INTRODUCTION
Salt marsh – dominated tidal creeks are common features of temperate passive
continental margins. As conduits for diffuse imports of watershed derived nutrients to the
coastal ocean, they also function as highly active ecosystems that efficiently convert those
nutrients into primary and secondary production. High levels of biological productivity, faunal
diversity, and unique geomorphology help define these systems as primary nursery areas for
finfish and shellfish species and feeding grounds for birds and larger fish (Sanger et al, 2004;
Mallin & Lewitus 2004).
High rates of ecosystem production are maintained by inputs of dissolved inorganic
nitrogen (DIN) from adjacent watersheds and through internal nitrogen (N) - cycling (Bronke et
al. 1994, Capone et al. 2008). Although high productivity is driven by N availability, excessive
N loading can alter ecosystem structure, patterns of trophic transfer, and subsequent function in
the landscape (Nixon & Buckley 2002). Human development along these tidal creeks can lead to
a variety of pollution inputs, including the nutrients N and phosphorus, which can stimulate algal
blooms and lead to hypoxia (Rabalais 2002, Mallin et al. 2004). Quantifying the resilience of
tidal creeks to excess N loading is important for understanding system responses to future
loadings.
Tidal creeks can be viewed simultaneously as interceptors of watershed N, transformers
of N into biomass, and modifiers of N export to the coastal ocean. As N transits tidal creeks (or
estuaries), it can be cycled and recycled biologically before it is deposited in the sediment or
exported (Boyer et al. 1994). Understanding the efficiency with which tidal creeks perform these
functions requires assessment of how they process new N (pathways) and at what rate they
recycle N internally among specific trophic levels and as a whole ecosystem. From an
experimental perspective, these creeks show many estuarine characteristics (e.g., salinity
gradient, species gradient, stratification/destratification patterns) on a compressed spatial scale.
Therefore they are analogues for estuarine processes but operate on a scale that can be
experimentally manipulated.
Whole-ecosystem isotopic tracer 15N experiments have proven useful in a variety of
settings to investigate N processing. In general the technique involves in situ dosing of an
ecosystem with 15N-DIN tracer. The 15N is followed into multiple ecosystem compartments and
used to calculate the rate of N movement between pools under natural conditions. This approach
eliminates many of the ambiguities associated with natural abundance isotope studies that result
from overlapping source endmembers and complex patterns of isotopic fractionation. Although
the isotope tracer approach remains somewhat limited in the spatial scale and duration of its
application, it has proven well suited for experiments in streams, groundwater, marshes, and
small estuaries. In these applications the method has provided simultaneous estimates of DIN
cycling, removal via denitrification, and trophic transfer under in situ conditions (Gribsholt 2007,
Bohlke et al. 2004, Tobias et al. 2001, Holmes et al. 2000, Hughes et al. 2000, Mullholland et al.
2000). Of these experiments, the closest analogue to marsh-dominated creeks of the
southeastern United States are those conducted in small New England estuaries (Tobias et al.
2003, Holmes et al. 2000, Hughes et al. 2000,).
Most recently, ecosystem-scale enriched 15N fertilization of marsh creeks in New
England has helped to quantify nitrogen uptake in Spartina spp. (Drake & Deegan 2007). While
the approach is similar, the differences in geomorphology (of New England tidal creeks),
application of the tracer (as ammonia versus nitrate), and the impact of the nutrient on a “steadystate” system (i.e., a fertilization experiment versus labeling) lead us to modify the approach for
southeastern tidal creeks. Here we present an application of the 15N in situ tracer approach
2
conducted in a marsh-dominated tidal creek in southeastern North Carolina. These tidal creeks
differ substantially from other tidal systems subject to this approach in tidal energy,
stratification/destratification, and biological composition. Application of this common tracer
approach to tidal creeks in the Southeastern US will permit comparison with other systems and
specifically address the role of other ecosystem processes (e.g. the role of filter feeders, in situ
denitrification) not captured in previous tracer studies. We will 1) identify the dominant N
processing pathways; 2) quantify the N transfer rates of these pathways; and 3) determine the
overall N retention time within a tidal creek. To our knowledge, this is the first time this method
has been applied to the tidal creek ecosystems in the Southeastern USA.
3
METHODS
Site Description
Hewlett‟s Creek is a tidal brackish tributary of the Intracoastal Waterway in Southeastern
North Carolina. Located at 34 deg 11‟ N latitude, 77 deg 50‟ W longitude, this creek drains a
watershed of 2393 ha, with a population of approximately 16,000 (Mallin et al. 2008). Land
cover is approximately 22% impervious surface coverage, mostly due to commercial and
residential properties along its watershed. It also drains two golf courses (Johnson 2005). The
experiment focused on a 2 km reach of the south branch of Hewlett‟s Creek (Figure 1).
Phytoplankton production is among the highest measured for North Carolina estuaries
(with maximum peaks during the summer) due to the degree of development along Hewlett‟s
Creek (and subsequent anthropogenic loading) (Johnson 2005). The upper areas produce algal
blooms in which the chlorophyll a biomass exceeds NC water quality standards, and hypoxia
occurs at times as well (Johnson 2005, Mallin et. al. 2004). Phytoplankton productivity is
primarily N limited, although on occasion phosphorous or light limitation from phytoplankton
self shading occurs (Mallin et. al. 2004).
4
LE
to ICW
1 km 
UE
LM
*
UM
O
Figure 1: South Branch of Hewlett's Creek, New Hanover County, NC. Divisions indicate the O, UM, LM,
UE, and LE subreaches; the star indicates the 15N addition site.
High tide volume of water (tidal prism) in the experimental area is ~ 22,000 m3 with a
tidal excursion ~1.5 km. During this study, the creek received fresh water from the watershed at
an average rate during the study of 0.8 m3 sec-1. At low tide, the experimental reach is
characterized by an average low tide volume of ~ 6,000 m3 with a creek bed area of 32,500 m2.
The experimental reach was divided into subreaches based on salinity for the purpose of this
experiment (Table 1).
5
Table 1: Subreach characteristics of the south branch of Hewlett’s Creek.
Sub reach ID
oligohaline (O)
upper mesohaline
(UM)
lower mesohaline
(LM)
upper euryhaline
(UE)
lower euryhaline
(LE)
total
Station
Salinity
number
1-2
0-5.5
Length
(m)
345
Average
width (m)
7-10.5
Area (m2)
3046
high tide
Volume
(m3)
1518
2-3
5-18
300
7.5-12
2850
2831
3-4
5-18
535
11-15
6891
5906
4-5
18-35
505
15-32
9913
4598
5-7
18-35
315
18-44.5
9844
7298
32544
22151
2000
Upstream (low salinity) reaches are characterized by sandy sediments and are bordered
by Juncus roemerianas and Spartina spp. Further downstream, the marsh vegetation yields to
monospecific stands of Spartina alterniflora and the silty subtidal sediments are intermittently
populated with oyster reefs. An average salinity of 12.5 ± 5.5 and nitrate (N) concentration of
4.96 ± 2.88 µM (Johnson 2005) have been reported previously at the creek midpoint.
Atmospheric deposition of N to the watershed is an indirect source of N to estuarine systems
(Richardson et. al. 2003, Paerl et. al.1995). While this is difficult to quantify, pulse rainfall
events provide a method of transport for this and land-derived N to the creek ecosystem.
Experimental Setup and Sampling Design
Nitrate is the dominate species of DIN delivered from the watershed. It is delivered at
the head of the creek and is also entrained upstream into the study reach during flood tide from
an adjacent creek branch at the bottom of the study reach. The overall experimental approach
was to execute a 3-week steady state supply of isotopically-labeled 15NO3‾ to the creek. Multiple
ecosystem components were sampled over time to track the incorporation of 15N into the creek
ecosystem.
6
A series of single pulse rhodamine dye releases was first conducted in 2007 (prior to the
tracer addition) to estimate mean tidal exchange of water in the study reach (~90% d-1) and
establish the optimum location along the creek for retention of
15
N tracer addition (Figure 3).
For each dye release approximately 2 gallons of Rhodamine-WT (various concentrations) were
released as a pulse during high tide. Transect samplings along the creek were conducted for
each subsequent high tide for two days. Rhodamine along the creek axis was measured
fluorometrically to calculate total rhodamine inventory. The decline in inventory permitted
calculation of daily water exchange in the study reach. A continuous rhodamine dye release
prior to the tracer addition using the pumping system described below showed the likely
distribution of tracer in the reach.
15
N tracer was added for 23 days continuously, beginning on 17 July 2007. The isotope
enrichment solution consisted of 0.2M, 20 atom % enriched K15NO3‾ that was dripped into the
reach at a rate of 16-18 L d-1 (23 d; 47.07 g N-NO‾3 d-1; 1083 g N total) at the midpoint of the
creek using a series of metering pumps (Fluid Metering Inc.). Due to the volume change of the
tidal prism during a tidal cycle (22,000 m2 and 6,000 m2), three metering pumps with varying
pump rates (2, 6.5 and 10 ml min-1) were used. Each was activated by a float switch mounted at
different tidal heights located within the channel that turned on or off with the rise and fall of the
tide. The overall addition rate therefore increased during the rising tide in order to maintain a
more constant concentration of label throughout the tidal prism (Figure 4). This setup provided a
much more even distribution of tracer to the creek compared to a single-pump, single-rate
dripping system used in previous studies. Examination of each ecosystem compartment
consisted of field sampling, followed by laboratory analysis, which yielded the total N content of
7
each pool and its isotopic enrichment (δ15N). These two measures were subsequently combined
to yield estimates of 15N mass movement through each pool.
Field Sampling
Multiple N pools (i.e., ecosystem compartments) were sampled prior to, during, and
following the addition experiment at 6 equidistant stations along the experimental reach, and one
station within an adjacent branch. Data from each subreach were combined to calculate total N
inventory and the amount of 15N incorporated within 5 sub reaches of the creek (Tables 1 and 2).
The frequency of sampling corresponded approximately with the expected turnover time
per pool and ranged from 12 h to 10 day (d) intervals. For example, short-turnover pools like
DIN and microautotrophs (phytoplankton and benthic microalgae) were sampled daily from the
start of the experiment and less frequently as the experiment progressed. Primary consumers
(zooplankton, nekton, and benthic invertebrates) were sampled less frequently (Table 2). Water
column N pools quantified during this study included dissolved inorganic nitrogen (DIN; NO3‾
and NH4+), suspended particulate organic nitrogen (PON) that included phytoplankton and
detrital nitrogen, zooplankton (ZOOP) and various fish species. Benthic N pools included bulk
sediment organic nitrogen (SED), benthic microalgae (BMA), representative benthic infauna and
epifauna and the fringing marsh grasses Spartina alterniflora (SPAR) and Juncus roemerianus
(JUNC). Estimates of N content and δ15N enrichment were performed for each compartment.
8
Table 2: Sampling frequency of N pools. “d” indicates days after start of isotope addition.
N pool
Week 1
Addition phase
Post-tracer sampling
DIN
d0.5, d1,d3, d6
d11, d14, d22
d24, d25, d29
POM
d0.5, d1, d3, d6
d11, d14, d22
d24, d25, d29
BMA
d3, d7
d14, d22
d24, d31
ZOOP
d4
d8, d16
d25, d35
SEDS
d3, d7
d14, d22
d24, d31, d38
SPART/JUNC
d7
d14, d22
d31, d38, d47
Infauna/Epifauna
d3, d7
d13, d22
d24, d31, d38
Bivalves
d7
d16, d22
d31, d38, d51
Fish
d9
d14, d21
d30, d44
Water Column sampling
The water column was sampled for dissolved inorganic nitrogen (DIN), particulate
organic matter (POM), phytoplankton, zooplankton, and nekton. DIN and POM samples were
collected in 2L HDPE containers at each station and stored on ice during transport before
filtration in the lab (47mm ashed Whatman glass fiber filters (GFF and GFD)). Filters were
saved for particulate organic nitrogen (PON), % N and δ15N analysis. Filtrate was frozen for
subsequent NH+4 and NO3‾concentration and isotopic analysis for δ 15N- NH4+ and δ 15N- NO3‾.
Zooplankton were sampled in duplicate during timed tows using a nylon tow net
(diameter = 23 cm; mesh size = 156 µm; 30 L water) at four locations in the creek (O, UM, UE,
LE). Samples were fixed and stained with a mixture of Rose Bengal and Formalin then stored
for identification and quantification. Zooplankton populations were identified and counted (to
genus level, when possible) microscopically in the laboratory.
9
Nekton were captured using block nets (width = 7 m; mesh opening = ¼ inch) and seines
(10 m; mesh opening = ¼ inch). Block nets were deployed in the upstream reaches of the creek
during high ebb tide, positioned in three randomly selected side tributaries to the main channel.
Seining up and downstream collected the fish species that dominated the channel. A sub sample
of total catch was collected for IRMS analysis, in replicate when available. Fish were euthanized
using CO2 and stored in ethanol until preparation for isotopic analysis.
Benthic sampling
The benthos was sampled for bulk sediment organic nitrogen, benthic microalgae
(BMA), benthic infauna, bivalves, epifauna (snails and crabs) and marsh macrophytes. Bulk
sediment was collected using a series of 2 cm diameter cores (depth = 3 cm) in mud flats during
low tide. Cores were sliced at 0-3 and 3-10 mm slabs. Samples were dried, homogenized, and N
content and isotopic enrichment were determined during IRMS analysis.
Benthic microalgae were collected from a set of four replicate sediment cores (diameter =
1.3 cm; depth = 10 cm) in the mudflat, channel, and marsh surface. BMA were separated from
subtidal sediment based on a density gradient using Ludox BMA isolation (Varela 1986) and
collected on an ashed GFA filter for isotopic analysis. Nitex screens (150 cm2; mesh size = 300
microns) were also placed on the mudflats in an attempt to collect surface BMA (Tobias et al.
2003). Screens were washed and filtered onto GFA filters and analyzed as a secondary estimate
of BMA-N content and enrichment during IRMS analysis.
Benthic infauna were harvested from sediment core duplicates (diameter = 10 cm; depth
= 15 cm) along the creek bed at low tide. Once collected, cores were rinsed through a coarse
sieve, then a fine sieve (500 microns) to remove plant matter and sediment particles.
10
Epifauna were sampled at each station by randomly selecting between 2 and 5 individuals
of each taxa and freezing them until isotopic analysis. Quantification of Littorina littorae
biomass was determined by a series of individual counts per square meter quadrat at the marsh
edge. Similarly, the Uca pugnax population was determined by burrow counts per meter quadrat
assuming mud crab densities are about 74% of burrow densities; Mouton 1996, Genoni 1991.
Subsets of Palaemonetes pugio caught in seine nets were collected and preserved in ethanol until
analysis for N content and enrichment. Two to 4 Crassostrea virginica were harvested from
each of the four oyster reefs in the creek. Density, area, and oyster count was measured with a
series of square meter quadrats.
Marsh macrophytes (Spartina alterniflora and Juncus roemerianus) were collected by
carefully removing both roots and shoots of 2-3 individual plants at each station within one
square meter of creek edge. At high tide, water flooded the marsh to approximately 1 lateral
meter of marsh fringe so that all fringing marsh sampling occurred within this area. Density of
plants was determined by counting individual plants within a series of square meter quadrats.
Laboratory Analysis for N content and 15N enrichment
All N pools were analyzed for N content and enrichment. Depending on the pool, N
content was measured with chemical techniques (DIN, BMA, and phytoplankton) or during the
isotopic analysis (bulk sediment, zooplankton, fauna). All isotopic analyses were performed
using a Thermo Delta-V Plus isotope ratio mass spectrometer (IRMS) located at the Center for
Marine Science, University of North Carolina Wilmington. Enrichment was reported as ∆δ15N
values in ‰ where:
𝛿 15 N =
R 𝑠𝑎𝑚𝑝𝑙𝑒
R 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑
and
11
− 1 × 1000
𝛥𝛿 15 N = δ15 Nt − δ15 Nbackground
where Rsample and Rstandard were the 15N/14N ratios of the sample and standard air respectively,
normalized to known glutamic acid reference materials. Enrichment was reported as change in
δ15N values above background, where δ15Nt is the enrichment at any sampling point in time. All
samples were introduced to the IRMS through a Costech 4050 Elemental Analyzer (EA)
interface. In addition to providing isotopic enrichment this also provided N content of each
sample.
Water column filtrate was analyzed for DIN concentration and 15N. NH+4 and NO3‾
concentrations were determined by colorimetric reactions using a Bran Luebbe segmented flow
nutrient auto analyzer (EPA Report 600/R-97/072). Isotopic determinations of δ15 NO3‾ and
δ15NH4+ were conducted using diffusion techniques; NH4+ was isolated by NH4+ diffusion onto
an acidified filter disk sandwiched between two Teflon membranes after the addition of MgO to
raise the sample pH above 10 (Holmes et al. 1997). Determination of δ15N- NO3‾ involved the
same treatment of MgO to elevate the pH, after which the sample was boiled to drive off NH3.
The addition of Devarda‟s alloy (0.3 g) reduced the dissolved nitrate to ammonium which was
collected on the acidified filter and processed like the ammonia diffusions (Sigman et al. 1997).
Filter discs were dried in a dessicator with concentrated H2SO4 and δ15N determined via EAIRMS.
Filter subsamples were split and sampled for chlorophyll a and enrichment of POM. A
fluorometric analysis of chlorophyll a (USEPA standards, Turner 10-AU flourometer,
Welschmeyer 1994) determined concentration of cholorophyll a which was then extrapolated to
phytoplankton-derived PON. Chlorophyll was converted to phytoplankton-N equivalents by
12
assuming a chlorophyll:carbon weight ratio of 1:40 and a C:N molar ratio of 1:7 (Redfield 1958).
Isotopic enrichment of PON was performed by analyzing a portion of the filter using the EAIRMS after drying it at 50°C. N content was determined from calibration against references and
the known filtered volume. The EA-IRMS calculated the δ15N of bulk POM. Since the filters
contained a combination of live phytoplankton plus detrital N, using the concentration of
chlorophyll a allowed us to calculate the 15N of the phytoplankton fraction according to the
following mixing equation:
  phtyo 
  POM [ N POM ]   DN [ N DN ]
[ N phyto ]
where   POM and   DN were isotopic enrichments of respective pools (POM and detrital
nitrogen:   = 3 per mil) and [N] were the N concentrations of those pools. Similarly, BMA
was quantified by analysis of chlorophyll a using double extraction and spectrophotometry,
where polar differences between two phases of solvent separate degraded pigments from intact
chlorophyll a (Whitney and Darley 1979). To separate BMA from sediment detrital N, a
chlorophyll:C weight ratio of 1:35 and a C:N molar ratio of 1:7 were used in order to convert
BMA chlorophyll a into N mass (N BMA). For isotope analysis BMA were separated from their
substrate based on density and isolated in a Ludox silica mixture and trapped on an ashed-GFA
filter (Varela 1986, Blanchard 1988). This method was inadequate in that it did not concentrate
BMA cells from background detritus as intended. Nitex screens also failed to isolate a clean
BMA sample. The BMA enrichment was determined in a way similar to that of δ15N
phytoplankton using the following mixing equation:
  BMA 
  SON [ N SON ]   SDN [ N SDN ]
[ N BMA ]
13
where   BMA was the isotopic enrichment of BMA,   SON was the enrichment of BMA
plus sediment detrital N,   SDN was the enrichment of sediment detrital N (background and
presumed unlabeled, δ15N - 3.5 ‰), and [N] was the respective N concentration of those pools.
The derivation of δ15N phytoplankton and δ15NBMA assumes that the autotrophs held all the
enrichment against an unlabeled background of detrital N.
Subsets of stained zooplankton were sorted from detritus under a microscope at 40x and
100x in order to achieve a pure sample of zooplankton then rinsed onto an ashed 25mm GFF
Whatman filter and dried for enrichment analysis (via EA-IRMS). Zooplankton counts and
species identification were also performed on subsets of each individual sample to determine
biomass and normalized to tow volume. N content was calculated as 10 % of total dry body
mass where 1 individual = 20 μg (Holmes et al. 2000, Jorgensen 1979).
Samples of nekton and bivalves were dried at 50ºF for 4 days. Stomachs were removed
before individuals were ground and homogenized, and stomach and body were processed
individually for isotopic enrichment. Benthic epifauna were similarly dried and homogenized
for analysis (Littorina was removed from its shell). Subsets of infauna were sorted under a
dissecting microscope (40x and 100x) and prepared the same way. EA-IRMS provided δ15N
and % N of all nekton, bivalves, infauna, and epifauna.
Samples of all ecosystem compartments were collected prior to the isotope addition
served to establish baseline δ15N. Enrichment of biota (Δδ15N) was calculated by subtracting
observed δ15N content from baseline δ15N.
14
RESULTS
Salinity Distribution
Vertical salinity profiles indicated a variety of mixing patterns throughout the reach. High
tide profiles indicated vertical stratification upstream where tracer was predominantly found
(Figure 2). Stratification decreased downstream with the water column well mixed below station
3 (645 m from headwaters). This upstream partially-stratified water column was a consistent
conductivity (mS cm-1)
feature through the study, regardless of tidal stage.
-30
(cm)
(cm)
-30
-90
-90
-150
-150
00
200
200
400
400
800
600
800 1000
1000
distance downstream (m)
600
1200
1200
1400
1400
1600
1600
50
45
40
35
30
25
20
15
10
5
distance downstream (m)
Figure 2: Conductivity distribution of Hewlett’s Creek south branch at spring high tide. Profile from 2007
when watershed discharge was low at creek headwaters. Conductivity (mS cm-1) = ~1.6mS / ppt salinity.
Physical Exchange
The single pulse and steady state rhodamine dye additions provided estimates of physical
tidal exchange and expected tracer distributions, respectively. Peak rhodamine concentration
from the pulse remained within the upper portions of the creek through subsequent tides with
little effect from the watershed discharge (Figure 3). Total rhodamine inventory declined ~ 90 %
d-1 indicating 90% water exchange per day. Spring/neap tidal cycles may cause variation in this
exchange rate however the duration of the experiment was long enough to account for this.
Measurements at various stages of the tidal cycle during the continuous dye addition showed the
peak tracer (varying by a factor of 2) moving up and downstream over ~ 1.75 km (Figure 4).
15
The multi-pump system provided a more even distribution of tracer than previously achieved
with a single pump rate (Figure 4; Tobias et al. 2003, Holmes et al. 2000).
16
12 hours
14
24 hours
12
36 hours
10
48 hours
3000
total rhodamine inventory (g)
surface rhodamine ppb
18
8
6
4
2
0
A
0
500
1000 1500 2000
distance downstream (m)
2500
2500
2000
1500
2
Rr 2 = 0.977
1000
500
B
0
0
10
20
30
time (h)
40
50
ppb Rhodamine
Figure 3: A) Rhodamine dye concentrations in the surface waters over 4 consecutive high tides; B) Total
rhodamine mass inventory over time with a single pulse dosing of dye at the dripper location.
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
high ebb
low
high
0
500
1000
1500
distance downstream (m)
2000
2500
Figure 4: Rhodamine dye concentrations at various stages of the tidal cycle from a continuous rhodamine
release using pumping rates used for isotope addition.
16
DIN Dynamics
Previous studies indicate that the surrounding watershed is the primary source of new
nitrate into the system (Mallin et al. 2004). In non-rain situations there were generally low
concentrations of NO3‾ throughout the experimental reach during our study. Nitrate
concentrations ranged from 0.2 – 9.41 μM, and averaged 1.39 μM, when low tide measurements
directly following a day 12 rain event (3.17 – 19.36 μM) were excluded (Table 3). Ammonium
concentrations ranged from 0.2 – 31.51 μM and averaged 4.43 μM (Table 3). Headwater NO3‾
concentration was ~ 4 µM.
17
Table 3 : Nitrate and ammonium surface concentrations per subreach for all sampling periods throughout
the isotope addition experiment. All concentrations are expressed in micromolar (μM). Lines within cells
denote concentrations for high, ebb, and low tide when available. As with all subsequent data, d1-d23 is the
tracer addition period and the shaded areas indicate pre- and post- addition sampling events. A large rain
event on d12 was noteworthy in that it pulsed in a new load of NO3‾ (and NH+4, perhaps from an increase in
marsh drainage) during the tracer addition which subsequently affected the source δ 15NO3‾ enrichment.
NO3‾
d0
d1
d3
d6
d11
d14
d22
d24
d25
O
1.8
------
0.9
------
0.6
------
0.8
0.8
2.41
0.8
0.9
<0.25
0.8
0.8
<0.25
0.9
1.0
1.4
1.0
1.0
2.2
0.7
------
UM
1.1
------
0.9
------
0.7
------
LM
0.9
------
0.4
------
0.6
------
UE
0.6
------
0.3
------
0.5
------
LE
0.8
------
0.3
------
0.6
------
NH4+
d0
d1
O
13.6
------
UM
6.0
2.8
3.9
4.9
3.6
12.0
4.0
6.9
13.1
2.3
9.2
8.5
2.1
7.0
9.8
1.9
------
2.0
------
2.3
------
0.8
------
2.0
------
1.1
------
0.5
------
2.0
------
0.8
------
0.3
------
3.5
------
0.5
------
0.2
------
1.1
------
0.4
------
d3
d6
d11
d14
d22
d24
d25
5.8
------
1.0
------
7.7
------
5.6
------
1.3
------
LM
9.5
------
4.0
------
0.4
------
UE
8.6
------
3.0
------
0.8
------
LE
4.4
------
1.9
------
1.2
------
1.3
1.6
8.8
1.5
0.6
----1.0
0.6
3.2
0.6
1.3
2.6
0.5
1.0
2.4
3.3
------
10.8
7.1
8.8
10.1
7.7
13.0
9.9
11.7
12.3
8.8
14.2
8.6
7.9
10.8
7.5
3.0
------
9.6
------
7.2
------
2.6
------
16.0
------
3.4
------
2.0
------
13.0
------
2.4
------
2.0
------
13.0
------
1.6
------
1.8
------
11.0
------
1.6
------
0.7
-----0.5
-----0.4
-----0.4
------
3.8
-----4.2
-----5.5
-----5.9
------
18
Gross Distribution of N pools
The majority of N within the creek was located in the creek benthic environment,
including living and dead organic material and sediments (Figure 5). Bulk sediments (0-3 mm)
contained the most N and comprised an average of 4.7 g N m-2 – 20.2 g N m-2 between reaches.
Marsh plants (1 m fringe) accounted for 1.5 g N m-2 – 3.9 g N m-2 in the whole creek and made
up between 30-40 % of the marsh edge N inventory. The whole creek water column inventory
ranged from 0.2 – 6.8 g N m-2.
N located in the upstream reaches was approximately evenly split between marsh edge
and creek bed compartments with negligible amounts of N in the water column. Water column
N contributed more to the total N in the downstream reaches (15-20 %). The water-column N
contribution peaked mid reach (LM) but then decreased downstream where the marsh edge
(Spartina alterniflora) became more important. Higher levels of water column primary
productivity and thus greater water column N (as organic N in phytoplankton) are supported by
the clearer, shallower water downstream that attenuates less light.
19
g N m-2
water column
40
35
30
25
20
15
10
5
0
marsh edge
creek bottom
oligo
upper
meso
lower
meso
upper
eury
lower
eury
Figure 5: Distribution of N biomass (g N m-2) standardized for area averaged over the whole experiment.
Marsh edge includes Spartina and Juncus spp (1 m of regularly flooded fringe), Littorina and Uca. Creek
bottom includes benthic microalgae and bulk sediments (0-1 cm). Water column includes PON, zooplankton,
fish species and bivalves.
Water Column N Inventories – Microautotrophs
The phytoplankton fraction of PON (derived from chlorophyll a concentration) was
highest in the oligohaline subreach. Chlorophyll a in the whole creek ranged from 0.7-108.8 μg
L-1 (Appendix A). Upstream (above addition site) and downstream chlorophyll a concentrations
averaged 21.71 μg L-1 (124 μg phyto-N L-1) and 8.82 μg L-1 (50 μg phyto-N L-1), respectively.
Phytoplankton-derived N was therefore higher upstream (O-LM) with an average phytoplanktonN: Bulk-PON ratio of 0.35 ± 0.04 (SE) compared to a downstream (UE-LE) average of 0.23 ±
0.03 (SE). These values indicate upstream PON was ~ 35 % phytoplankton compared to ~ 23 %
downstream (Figure 6).
20
percentage of PON as phytoplankton
day 3
30%
day 6
25%
day 11
20%
day 24
15%
10%
5%
0%
o
um
lm
ue
le
Figure 6: Phytoplankton contribution to total PON within each subreach of the creek.
Water Column N Inventories - Consumers
The majority of zooplankton fauna consisted of calanoid copepods (~ 80%), at least half
of which were juveniles, followed by cirripede nauplii (barnacle larvae). Acartia tonsa was the
most abundant copepod. Generally, higher densities of zooplankton were found downstream
(UE, LE; Table 4). Zooplankton biomass fluctuated during the experiment where a decline in
the second week was followed with highest zooplankton densities on d25 (66 – 596 individuals
L-1).
21
Table 4: Zooplankton densities during the experiment
tow location day collected
upstream
mesohaline
euryhaline
downstream
0
4
8
16
25
35
0
4
8
16
25
35
0
4
8
16
25
35
0
4
8
16
25
35
zooplankton
-1
-1 μg N L
individuals L
12
2.42
7
1.41
41
8.24
29
5.83
66
13.27
15
3.04
30
6.04
22
4.41
68
13.55
64
12.80
200
40.00
59
11.86
99
19.75
21
4.29
309
61.72
94
18.72
421
84.21
52
10.49
24
4.70
729
145.85
17
3.30
148
29.51
596
119.27
153
30.56
Fish population size and distribution were dependent on location in the reach. Population
distributions were similar to previous studies in tidal creeks of this region (North Carolina
Department of Marine Fisheries database 1987), predominately pinfish (Lagodon rhomboides),
spotfin mojarra (Eucinostomus argenteus), spot (Leiostomus xanthurus), Atlantic menhaden
(Brevoortia tyrannus), and mosquitofish (Gambusia affinis). Mummichog (Fundulus
heteroclitus), Atlantic herring (Clupea harengus), and Atlantic silverside (Menidia menidia)
were also present, but inconsistently. Total biomass was small (0.002 - 0.005 g N m-2, all
22
subreaches averaged). Spot was the most common fish found (~ 85 %) and made up the majority
of catch biomass, followed by pinfish (~ 6 %) and upstream by mosquitofish (~ 2 %).
Benthic N Inventories – Primary Producers
Benthic chlorophyll a concentrations (0-3 mm depth) in mud flat and marsh edge
sediment were similar to each other and similar throughout the study reach (average
47.2 and 36.4 mg chl a m-2, respectively, to a depth of 1 cm) but accounted for < 20 % of the
total sediment N (dominated by detrital N). Mud flat sediment BMA stocks reached a maximum
during the second week of the experiment, ranging from ~13 - 180 mg chl a m-2 sediment to a
depth of 1 cm (25 – 51 μg N cm-3; d13), however there was no discernable difference between
upstream and downstream BMA biomass (Appendix B).
Macrophyte primary producers (Spartina and Juncus spp; 1 m fringe) accounted for 1.5 –
3.9 g N m-2. Juncus was present in the upstream reaches (O, UM, LM) and absent in the lower
subreaches. Conversely, Spartina was sparse upstream (~ 1.5 g N m-2) yet was the only
component of the fringing marsh downstream (~ 6.6 – 7.5 g N m-2). N in roots made up roughly
20 % of total N found in plant biomass for both Juncus and Spartina.
Benthic N Inventories – Epifauna and Infauna
Epifauna were distributed both intertidally and subtidally. Mud crabs (Uca spp) were
abundant throughout the reach with individuals feeding on the exposed mud flats during low
tide. Burrow counts ranged from 25 - 45 burrows per square meter (higher densities in the
upstream subreaches) and N content ranged from 1.5 – 3 g N m-2. Bivalves (Atlantic ribbed
mussel; Geukensia demissa and Eastern oyster; Crassostrea virginica) were scattered throughout
the meso-and euryhaline subreaches. All bivalves except oysters had negligible contribution to
23
the creek N inventory due to low individual counts. The majority of oyster biomass was found in
4 reefs located in the euryhaline reaches of the creek (~2.0 - 6.5 g N m-2; Figure 7). Reef
biomass (as N) ranged from 214 g N (total) to ~ 5,700 g N. Littorina was commonly found
among the marsh grass downstream (average ~ 100 individuals m-2) however these snails were
scarce in the oligohaline subreach (< 1 individual m-2) and biomass was small (0 – 1.6 g N m-2).
The dominant crustacean was grass shrimp (Palaemonetes spp), found throughout the entire
study reach (highest numbers of individuals upstream). Shrimp accounted for 4 – 10 % of total
catch by numbers during seining. Biomass of this pool was insignificant relative to all other
epifauna (< 30 mg N m-2). Infauna included mainly oligochaetes, polychaetes, and insect larvae.
Collected infaunal biomass was less than 0.1 g N m-2.
Sediment composition ranged from compact sediment interspersed with sand bars at the
headwaters to fine muds downstream. The majority of N in the system was found in the bulk
sediments averaging ~ 12.5 g N m-2 (Figure 5). This N content peaked midstream (LM, average
20 g N m-2) with a smaller N inventory upstream (O), which was < 25 % of the average (4.7 g N
m-2). Sediment cores taken from the intertidal mud banks revealed high amounts of plant debris
and minimal biomass of subtidal macrofauna.
Highly active ecosystem N pools (turnover <1 month, e.g. microautotrophs, zooplankton)
comprised 10-30 % of the total N inventory and the greater fraction of total N further
downstream (Figure 7). Oyster beds in the 3 subreaches downstream (LM, UE, LE) accounted
for the majority of biotic N inventory (excluding sediments). Since total N inventory peaks midestuary and biotic N increases with distance, the faster cycling biotic pools become a
proportionally more important part of N inventory downstream relative to the plant and sediment
detrital pools found upstream.
24
g N m-2
9
Oysters
8
Mud Crabs
7
Snails
6
Zoop
5
BMA
4
phytoplankton
POM
3
2
1
0
oligo
upper meso lower meso upper eury lower eury
Figure 7: Distribution of N (g m-2) contained in active and biomass dominant N pools during the first week of
the addition. Fish and infauna each comprise less than 0.05 g N m-2. Sediment N was not included but
averaged ~ 11.7 g N m-2.
Temporal Changes in N distribution
Temporal changes in N distribution of the highest trophic level (crabs, fish) or sessile
biota (oysters) were negligible, but water column organisms (zooplankton, phytoplankton) and
BMA biomass did change throughout the experiment (Figure 8). These organisms with rapid
turnover times showed a response in enrichment variability more readily than those with longer
turnover times. A 3-4 fold increase in phytoplankton and zooplankton abundance in particular
was observed after the influx of watershed-derived nitrate into the system during runoff resulting
from a rain event (Figure 8). In the mesohaline subreach decreases in BMA biomass
accompanied increases in phytoplankton and zooplankton in the overlying water.
25
Week 1
0.6
0.5
Zoop
BMA
phytoplankton
POM
0.4
0.3
0.2
0.1
0
Week 2
0.6
0.5
g N m -2
0.4
0.3
0.2
0.1
0
Week 3
0.6
0.5
0.4
0.3
0.2
0.1
0
oligo
upper
meso
lower
meso
upper
eury
lower
eury
Figure 8: N biomass for tidal creek primary producers and zooplankton per week of the experiment.
26
Natural abundance isotopic signatures of biota
Ecosystem pools were analyzed for δ15N and δ13C prior to the isotope addition in order to
establish accurate isotopic baseline data for each subreach (Figure 9A). Natural abundance
isotope values for 15N were uniform throughout the subreaches for all N pools except for grass
shrimp, which were slightly heavier upstream by ~ 2 – 3 ‰. PON- δ15N ranged from 2 – 7 ‰.
The commonly accepted ~ 3 ‰ δ15N increase between trophic level was observed between food
sources (POM, BMA) and consumers. A clear separation in δ13C between C3 (Juncus) and C4
(Spartina) plants was also apparent. Higher δ13C values in fishes were seen downstream (LE)
although δ15N changed little through the different subreaches (Figure 9B).
27
POM
12
BMA
11
Spartina
10
Juncus
9
bulk seds
δ15 N
8
zooplankton
7
oysters
6
mud crab
5
littorina
4
grass shrimp
3
fish
2
-30
-25
-20
δ13 C
-15
11.75
-10
A
lookdown
mosquitofish
11.5
spot
pinfish
11.25
δ15 N
mummichug
11
spot
pinfish
10.75
10.5
10.25
10
-25
-20
δ13 C
-15
-10
B
Figure 9: Natural abundance isotopic signatures for sampled pools in Hewlett’s Creek (A). Error bars denote
one standard deviation from the mean. Isotope values within fish indicate a heavier δ13C value for
individuals captured downstream (B). The dotted line indicates the separation between isotope values of
upstream (UM, to the left of the line) versus downstream (LE, to the right of the line) catches.
28
Isotopic Tracer Enrichment
The Δδ15 NO3‾ranged between 500 - 2000 ‰ with a time-weighted mean Δδ15 NO3‾
enrichment of ~ 900 ‰. This δ15N enriched source of NO3‾moved back and forth through the
reach with the tide. Ebb and flood tide enrichments were in the same range differing only in the
location of the peak δ15 NO3‾ (Figure 10). Due to stratification in the upstream reaches, bottom
water enrichment was less than the surface, averaging ~ 200 ‰ (measured on d1, d3, d11, and
d22). The isotope addition technique succeeded in supplying tidally- and reach- averaged δ15
NO3‾ enrichment that stayed within a factor of two for the whole study reach.
700
Δδ 15 N-NO3
600
high
500
low
400
300
200
100
0
0
500
1000
distance downstream (m)
1500
2000
Figure 10: Spatial distribution of δ15N- NO‾3 above background levels for sequential high and low tide creek
transects. Data measured on day 14. The dotted line is the location of the isotope addition site (dripper).
The δ15 NO3‾ enrichment depended on several factors in addition to pumping rate
including ambient [NO3‾], watershed inputs of NO3‾, and tidal supply of NO3‾ (concentration
and water volume). A major rainfall event on d12 caused an influx of watershed-derived
unlabeled nitrate (Figure 11) and subsequently diluted the isotopic enrichment of the pool
(Figure 12).
29
8
6
7
1.4
5
4
4
3
3
2
rainfall (cm)
[NO3 ] µM
6
Tidal height (m)
5
2
1
1
0.0
0
0
-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
duration of experiment (d)
Figure 11: Ambient high tide nitrate concentration in the oligohaline subreach coupled with daily rainfall and
tidal data (National Weather Service). Low tide [NO3‾] reached 20 μM (station 3, d13). The dotted lines and
shaded area indicate uncertainty in the trajectory since no sampling events occurred during this time. Bars
indicate rainfall. The black line indicates the end of the isotope addition period (d23). The beginning of the
experiment coincides with a spring tide, adding a dilution in the measured nitrate values.
1400
1200
6
5
800
4
600
3
400
2
200
1
0
0
0
5
10
time (d)
15
20
25
rainfall (cm)
Δδ15N-NO3
1000
30
Figure 12: Change in δ15N- NO3‾ enrichment for the oligohaline and upper mesohaline (upstream) subreach
measured at high tide. The gray shading indicates areas of uncertainty based on sampling and analytical
methods. The black line indicates the end of the isotope addition period (d23). This enrichment moved
through the tidal prism with the ebb and flood of the tidal cycle. The black bar indicates the termination of
the isotope addition period.
30
The amount of new NO‾3 imported via rainfall on day 12 initially diluted the δ15N by a
factor of 2 however the enrichment of this pool rebounded after a few days (Table 1, Figure 12).
This changing baseline enrichment of δ15N was then propagated through faster turnover time
organisms like phytoplankton and BMA (Figure 13), but the temporal variability in enrichment
was smoothed in pools further up the food chain.
15
N Incorporation by Autotrophs
15
N labeled NO3‾ was incorporated within the phytoplankton pool within 1 day of
addition, reflecting the rapid turnover time of the phytoplankton pool. Maximum enrichment for
phytoplankton was found consistently in the oligohaline reaches of the creek, and peaked at d11
(1344 ‰ – O, 310 ‰ - LE) for all creek areas before sharply declining on d14 in response to the
rainfall lowering of δ15 NO3‾ (Figures 12 and 13). BMA also incorporated tracer following a
slight lag time (~3 d), however it reached a lower maximum enrichment relative to
phytoplankton of 949 ‰ (Figure 13C). Higher enrichment further upstream was due to closer
proximity to tracer. Enrichment of BMA also declined in response to the rainfall/dilution event
(Figure 13A)
Enrichment of creek-edge marsh macrophytes was measured in both shoots and roots and
showed a difference in enrichment pattern based on tissue type (Figure 14). Both grasses
incorporated the highest amount of tracer directly above and below the dripper (UM and LM).
For Spartina, both locations upstream from the addition site showed immediate incorporation of
tracer. Enrichment declined downstream with a maximum Δδ15N of 5.3 - 8.5 ‰ for either marsh
plant. Spartina roots showed higher peak enrichment at every station compared to the shoots
(max roots - 41 ‰; shoots – 25 ‰). Juncus showed comparable peak enrichment in shoots and
31
roots (26 ‰ vs. 18 ‰, respectively), but 15N was clearly incorporated into Juncus roots prior to
transfer to shoot tissue.
32
Δδ 15 N- producers
1400
A
6
BMA - oligohaline
5
1200
BMA - UM
1000
phyto PON - oligohaline
4
phyto PON - UM
800
3
600
2
rainfall (cm)
1600
400
1
200
0
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
duration of experiment (d)
B
C
oligo
1400
1000
1200
lower meso
1000
upper eury
Δδ15 N - BMA
∆δ15N - phytoplankton
upper meso
lower eury
800
600
oligo
900
upper meso
800
lower meso
700
upper eury
600
lower eury
500
400
300
400
200
200
100
0
0
0
2
4
6
time (d)
8
10
12
0
2
4
time (d)
6
8
Figure 13: (A) Change in enrichment of BMA and phytoplankton-PON in the oligohaline and upper mesohaline portions of the creek in response to
rainfall. Downstream productivity was less affected by the influx of nitrate from the watershed. A lack of sampling for BMA for between d7 and d13
causes a skew in the enrichment trajectory; the decline in enrichment most likely does not occur until the first rain event on d12. The black bar
indicates the end of the isotope addition period (d23). Rising enrichment trajectories for phytoplankton-PON are plotted for all sections of the creek
prior to rain-induced dilution of tracer (B). Log transformed trajectories were used to estimate production turnover times. Rising enrichment
trajectories are also plotted for BMA prior to dilution of tracer from rain (C).
45
upper meso
lower meso
30
Δδ 15 N
45
40
35
30
25
20
15
10
5
0
oligo
40
35
25
20
15
10
5
0
0
10
30
40
0
50
time (d)
Spartina shoots
Δδ 15 N
20
Spartina roots
45
45
40
35
40
35
30
25
30
25
20
15
20
15
10
5
10
5
0
0
0
10
20
30
10
40
50
0
10
30
40
50
40
50
time (d)
20
30
time (d)
time (d)
Juncus shoots
20
Juncus roots
Figure 14: Enrichment trajectories for Spartina and Juncus shoots and roots, rinsed to remove mud and
epiphytes.
Transfer of 15N through the Food Web
Water Column
Tracer moved from autotrophs into higher trophic levels in the water column and
benthos. Because the time scale of the experiment was shorter than the time required for the
seasonal conversion of macrophytic leaf litter to useable detritus, enrichment in higher trophic
levels was assumed to originate from the microautotrophs (phytoplankton and BMA) and not
from marsh plants.
Enrichment of the zooplankton pool followed the enrichment trajectory of PON during
the first week of addition; however, it sharply declined on day 8. The decline is coincident with
an increase in zooplankton numbers, potentially diluting the enrichment signal (Figure 15). An
increased use of detrital N in order to support the larger zooplankton biomass may be another
cause for the decline in zooplankton enrichment.
Overall, the Δδ15N of zooplankton decreased downstream, with a maximum value of 140
‰ (O, d4; maximum value LE - 21 ‰). The simultaneous decrease in zooplankton δ15N with an
increase in zooplankton numbers could be attributed to 1) a mixing of south branch zooplankton
with zooplankton from other branches and/or further downstream (unlabeled), 2) feeding on a
less enriched phytoplankton pool downstream, or 3) a shift away from highly-labeled
phytoplankton to a more detrital (unlabeled) food source as zooplankton production increases.
For fishes there was a separation in enrichment levels among fishes with similar feeding
strategies (Figure 16). By d9, all fish species incorporated tracer from 5 to 20 ‰ with water
column feeders (menhaden) incorporating the tracer more rapidly. Menhaden and mojarra
showed the highest uptake of tracer (d21) of 48 ‰ and 45 ‰, respectively. Highest values for
35
pinfish were 30 ‰ whereas mosquitofish and spot (detrital feeders) reached a maximum
enrichment of only 12 ‰ (d 44) and 18 ‰ (d 21), respectively.
Fish species caught upstream and downstream also showed spatial distribution patterns in
enrichment. Mojarra reached maximum enrichment downstream (37 ‰) however pinfish
incorporated more of the tracer upstream (24 ‰). The result suggests either movement of some
species within the creek with utilization of upstream resources and/or a progressive movement of
15
N tracer from upstream to downstream.
36
oligohaline
160
mesohaline
349.5
140
140
oligohaline
120
mesohaline
120
100
80
80
60
60
40
40
20
20
0
0
0
4
8
time (d)
16
25
gN
Δδ 15 N
100
35
Figure 15: Zooplankton biomass and change in enrichment over time (days). Euryhaline subreaches did not
show significant enrichment. Lines represent enrichment (Δδ15N) and bars represent biomass (g N).
40
60
upstream mojarra
downstream mojarra
downstream pinfish
35
upstream pinfish
50
upstream mosquitofish
30
upstream menhaden
upstream spot
40
Δδ15 N
Δδ15 N
25
20
15
30
20
10
10
5
0
0
0
10
20
30
40
0
50
10
20
30
time (d)
time (d)
Figure 16: Enrichment trajectories (change in enrichment over time) for major fish species.
37
40
50
Benthos
Epifauna (oysters, mud crabs, grass shrimp) dominated 15N uptake in the benthic food
web. During the first week of the experiment, δ15N values increased in oysters as they fed on
water column PON and resuspended BMA (Figure 17). Oysters reached a maximum ∆δ15N
value of 43.1 ‰ on d16 (UE). There was similar enrichment for LM, UE, and lower for LE as
these oysters were further from the tracer source. These oyster reefs encountered the lowest
source of enrichment based on their location both within the water column as well as within the
experimental reach. At high tide, the submerged reefs saw minimal tracer since the enriched
nitrate/PON pool was predominantly upstream. As this labeled food source moved downstream
with the receding tide, the oyster reefs only encountered some of the label because the reef
eventually became exposed as the water level dropped below the elevation of the live oysters;
therefore, the downstream oyster reef saw the least amount of enrichment of all four reefs. A
moderate enrichment in this pool can also be explained by the fact that the oysters feed on a mix
of labeled and unlabeled suspended organic matter (phytoplankton, zooplankton and unlabeled
Δδ 15 N
detritus), as well as the extremely large size of the internal N pool.
lower meso
50
45
40
35
30
25
20
15
10
5
0
upper eury
lower eury
0
10
20
30
time (d)
40
50
60
Figure 17: Enrichment trajectories for oysters composing the three major reefs. The black bar shows the end
of the tracer addition period (d23).
38
Other epifauna incorporated lower levels of tracer relative to oysters but they did show
enrichment patterns with lag times as well as localization of tracer incorporation. Grass shrimp
exhibited a maximum enrichment of 73 ‰ (d21) and displayed a lag time in tracer uptake
downstream compared to upstream, indicative of the spatial movement of the tracer downstream
throughout the experiment (Figure 18).
grass shrimp upstream
80
grass shrimp downstream
Δδ 15 N
70
60
50
40
30
20
10
0
0
10
20
time (d)
30
40
50
Figure 18: Enrichment trajectories Palaemonetes captured upstream versus downstream. The bar indicates
the end of the isotope addition period.
Uca and Littorina followed enrichment trajectories similar to each other, exhibiting a
delayed enrichment of up to 7 d (Figures 19 and 20). These trajectories differ from the
microautotrophs and zooplankton previously mentioned in the duration of uptake lag time,
indicating a greater reliance on recycled N. Enrichments of Uca (mud crabs) and Littorina
climbed during tracer addition and continued to rise after the tracer was shut off, giving further
evidence of their dependence on recycled N (maximum values: 42 ‰, UM, mud crabs; 115.8 ‰,
O, snails).
Benthic infauna showed a maximum enrichment of 28 ‰ after the tracer was shut off
(d38, O). N biomass was variable among samples, from 0 - 0.5 g N m-2. Due to this low
biomass, we were unable to determine turnover time from the samples in this experiment. Even
39
with a rapid turnover time, the benthic infaunal N pool was too small to be a dominant pool for
processing N for higher trophic levels at the time of the study. The minimal role of infaunal N
transfer at an ecosystem scale is in close agreement with results from other N and C isotope
tracer experiments (Middleburg et al. 2000).
45
oligo
Δδ 15 N
40
upper meso
35
lower meso
30
upper eury
25
20
15
10
5
0
0
10
20
time (d)
30
40
Figure 19: Enrichment trajectories for Uca at each subreach of the creek.
Δδ 15 N
115
110
105
oligo
lower meso
~~
45
40
35
30
25
20
15
10
5
0
upper eury
lower eury
0
10
20
time (d)
30
40
Figure 20: Enrichment trajectories for Littorina per subreach. The trajectory for the oligohaline subreach
continues to a maximum value of 116 ‰ on d24.
40
Bulk Sediments
Subtidal creek bulk sediments contained the majority of N biomass and consisted of a
mixture of detritus, BMA, and settled phytoplankton. Bulk sediment exhibited a maximum
change in enrichment of 23 ‰ in both subreaches upstream from the addition site (Figure 21).
Below the isotope dripper, enrichment averaged 3.8 ‰ with a maximum value of 11 ‰ (LM,
d24). Peak enrichment was achieved after the dripper stopped indicating slow turnover and
contribution from recycle-detrital N. By the end of the sampling period (d38), creek sediments
upstream (O, UM, LM) were still enriched between 6 and 10 ‰.
O
UM
LM
UE
LE
Δδ 15 N - sediments
25
20
15
10
5
0
0
10
20
30
40
time (d)
Figure 21: Enrichment trajectories for bulk sediments. Retention of 15N extending beyond the addition
period indicates storage of N within the benthic N pools. The black bar indicates the end of the tracer
addition.
41
Turnover times for N pools
The rising enrichment trajectories were used to calculate turnover time for N pools (Table
5). Rates were calculated by plotting the natural log transformed δ15N against time (days).
Turnover time was equal to the inverse slope of the plot (Tobias et al. 2003). Two examples for
each trophic level are shown in Figure 22.
Microautotrophs turned over fastest. Phytoplankton and BMA pools turned over N
within 2-3 days. Turnover time for these pools increased downstream. The faster rate upstream
may be related to the availability of watershed-derived nutrients and the longer residence time of
the water in the oligohaline subreach of the creek. The calculated rate of turnover for the BMA
pool is consistent with previous studies (~3.0; Tobias et al. 2003).
The variability in the enrichment trajectories of the zooplankton due to changing
population size made calculation of turnover times difficult. The times reported are rough
estimates and likely to be high (specifically, LE). Previous studies have reported zooplankton
population turnover times of 5 days (Holmes et al. 2000).
Mud Crabs and Littorina showed similar turnover times averaging 30 days. Similar to
BMA and phytoplankton, shorter turnover times were calculated at the upstream reaches.
Shrimp turned over more rapidly, ~ 9 days, with minimal difference between upstream and
downstream populations (Table 5).
Oyster nitrogen turn-over was ~ 17 days and also showed minimal differences in
enrichment, turnover time, and biomass between subreaches (Table 5). Fish turnover times were
highly variable and ranged from 4 d to 118 d, depending on species and location. High variance
may reflect some immigration and emigration effects between the creek and the ICW.
42
Uca
Oyster
4
4
6
3
3
4
2
= 0.5508
r2R= 0.5508
2
upstream
2
ln δ15 N
8
ln δ15 N
ln δ 15 N
Phytoplankton PON
2
R
0.9024
r2 == 0.9024
2
Rr2 ==0.9673
0.9673
2
1
1
0
0
downstream
0
1
2
time (d)
3
4
0
ln δ15 N
ln δ 15 N
4
2
0.7679
Rr2 =
= 0.7679
0
2
4
time (d)
0
5
10
6
20
25
L. rhomboides
5
4
3
2
2
Rr == 0.8926
0.8926
2
2
Rr2 == 0.9154
0.9154
3
2
1
0
0
8
15
time (d)
1
upstream
downstream
0
40
4
R = 0.8677
2
30
Littorina
r2 =2 0.8677
6
20
time (d)
BMA
8
10
ln δ15 N
0
0
10
20
time (d)
30
0
5
10
15
20
time (d)
Figure 22: Representative linear regression plots used to calculate turnover time of specific N pools. ∆δ15N values were log transformed and plotted
against time in order to determine the slope of the regression line. PON and BMA were divided into upstream and downstream based on similar
turnover times for specific subreaches. Other organisms shown include mud crabs (LM), Littorina (LM), oysters (LE), and upstream pinfish. A
complete list of turnover times is found in Table 5.
25
Although previous studies assumed bulk sediments (0-3 mm) to be a long-term storage
pool for N, the observed turnover time in Hewlett‟s Creek was relatively fast (specifically
upstream; 4 – 21 days). In order for this sediment N pool to exhibit a fast turnover, fresh
microautotroph detritus must comprise a high proportion of the total sediment N (versus old
detritus, slow turnover). A sediment pool consisting of a higher proportion of unlabeled „old‟
detritus would exhibit less 15N enrichment and subsequently a slower turnover rate. Another
possibility for high turnover would be a high bacterial uptake and remineralization of N.
Table 5: Turnover times for the various N pools within the creek.
N pool
phytoplankton
BMA
ZOOP
C. virginica
Uca
Littorina
Palaemonetes
Palaemonetes
Bulk Seds
E. argenteus
(mojarra)
E. argenteus
(mojarra)
L. rhomboides
(pinfish)
L. rhomboides
(pinfish)
L. xanthurus
(spot)
Location
o
um
lm
ue
le
o
um
lm
ue
le
upstream
downstream
le
lm
ue
le
o
um
lm
ue
le
o
lm
ue
um
ue
o
um
lm
ue
le
Turnover Time
2.72 d
2.77 d
2.85 d
3.41 d
3.61 d
1.30 d
1.33 d
1.70 d
1.87 d
2.36 d
4.76 d
6.93 d
17.27 d
19.21 d
16.72 d
18.94 d
12.33 d
26.67 d
34.97 d
32.26 d
28.82 d
9.08 d
27.17 d
25.97 d
9.03 d
9.90 d
4.27 d
4.22 d
12.79 d
13.05 d
21.51 d
r2
0.592
0.5225
0.5766
0.607
0.7352
0.9112
0.8274
0.8244
0.8016
0.6894
0.3219
0.2217
0.184
0.8119
0.75
0.9673
0.9489
0.4708
0.9024
0.8505
0.6872
0.9991
0.8926
0.7498
0.9154
0.9505
0.88
0.86
0.8038
0.89
0.7193
n=6
n=6
n=6
n=6
n=6
n=3
n=3
n=3
n=3
n=3
n=6
n=6
n=3
n=5
n=5
n=4
n=4
n=4
n=4
n=4
n=4
n=3
n=5
n=3
n=3
n=3
n=3
n=3
n=3
n=3
n=3
um
90.1 d
0.9795
n=3
ue
16.21 d
0.9996
n=3
um
44.05 d
0.814
n=4
ue
39.37 d
0.7866
n=3
um
117.65 d
0.9725
n=4
45
15
N storage
Given the turnover times and N transfer of organism pools listed above, Table 5 shows
where 15N resides after 3 weeks of isotope addition (d22). Deposition into the active sediment
pool (versus storage) was the fate of much of the excess 15N found in the estuary (76 – 90 %),
similar to previous in situ tracer studies (Tobias et al. 2003, Holmes et al. 2000). Of the biota
measured (not including the sediment pool), the majority of excess 15N resided in the oyster beds
downstream (46%), followed by marsh macrophytes (21%).
Faster turnover time pools such as phytoplankton and BMA retained similar amounts of
15
N, each roughly between 1 - 5 % of the total. Phytoplankton 15N retention decreased
downstream related to their decrease in total biomass (Figure 8) and coinciding with a longer
turnover time. BMA, however, retained similar amounts throughout the experimental reach,
peaking upstream (O) and downstream (LE). The distribution of BMA populations during
sampling was highly variable on the mud flats (ie. high variability among replicate samples) yet
uniform across creek (Figure 8). Zooplankton retained very small amounts of 15N even though
their biomass was greatest during the last week of the experiment.
The dominant epifaunal pool were the oysters downstream, which also comprised a major
portion of total storage for the ecosystem, retaining 13 – 17 % of total 15N excess (Table 6). The
large amount of 15N found in this pool is indicative of N redistribution from upstream throughout
the creek. Similarly, other slow-turnover pools such as marsh macrophytes (2 - 6 %) and
sediments show 15N redistribution evenly over the subreaches by the end of the experiment
independent of their location in relation to the addition site. This is potential evidence of
recycling in other pools prior to incorporation into sediments or plants.
46
Table 6: Snapshot 15N inventories (g 15N excess) and percent of total excess 15N for all organism pools on the
final day of the experiment (d22). POM was not included in the calculation; it was represented by 15N mass
of phytoplankton. Percentages for the lower euryhaline may be skewed due to the lack of data for the
Spartina contribution on this day.
O
UM
LM
UE
LE
POM
0.07
0.08
0.07
0.01
0.02
phytoplankton
0.06 / 4.4%
0.06 / 2.3%
0.06 / 1.3%
0.01 / 0.3%
0.02 / 1.1%
BMA
0.07 / 4.9%
0.01 / 0.4%
0.04 / 1.0%
0.08 / 2.8%
0.07 / 4.8%
Spartina
0.07 / 4.9%
0.12 / 4.6%
0.29 / 6.5%
0.07 / 2.4%
n/a
Juncus
0.02 / 1.7%
0.03 / 1.1%
0.04 / 0.9%
n/a
n/a
Zoop
0.0007 / 0.05%
0.0047 / 0.2%
0.0024/ 0.05%
0.0031 / 0.1%
0.0026 / 0.2%
Snails
n/a
n/a
0.00 / 0.01%
0.01 / 0.2%
0.00 / 0.3%
Mud Crabs
0.10 / 7.5%
0.03 / 1.3%
0.04 / 0.9%
0.04 / 1.2%
0.02 / 1.0%
Oysters
n/a
n/a
0.7315 / 17%
0.3749 / 13%
0.2608 / 18%
Bulk Seds
1.03 / 77%
2.44 / 90%
3.17 / 73%
2.30 / 80%
1.08 / 74%
1.35
2.71
4.37
2.88
1.45
15
Total g N
These slow-turnover pools also retained the tracer well beyond the duration of the
experiment. On the final day of sampling (15 days after the end of the addition period),
sediments in the lower mesohaline subreach still retained ~ 4 g 15N (d38), a larger 15N inventory
than at any time during the addition. On this same day, oysters contained up to 0.5 g 15N (LM)
and still ~ 0.4 g 15N (LM) 4 weeks after the end of the experiment (d51). These longer term
repositories and/or exports are potentially important for extensive recycling and eventual burial
of N within this ecosystem.
15
N redistribution
The incorporation of the tracer was initially located upstream due to uptake by
phytoplankton and BMA. By the end of the first week, 66% of the tracer (including storage in
sediments) was located upstream (O, UM) compared to 11 % downstream (UE, LE). By the
third week, however, 15N was dispersed more uniformly, with 41 % found upstream and 25 %
downstream.
47
The 15N content of the primary producer pool (phytoplankton, BMA, Spartina, and Juncus) was
predominantly found in phytoplankton and BMA upstream (O, 71 % of total 15N in primary
producers) decreasing in content downstream (LM, 30 %). As the tracer was incorporated into
higher trophic levels, 15N was cycled from rapid-turnover pools to longer-term storage. By the
third week of the experiment, 67 % of the added tracer in the active biota had been incorporated
into consumers (Figure 23). Eventual redistribution of 15N downstream through recycling and
trophic transfer caused a more uniform spatial distribution throughout the creek in these
consumers by the end of the study.
15
N retention
While the phytoplankton pool was active in the initial uptake of 15N, other pools
were more instrumental in 15N retention. Phytoplankton and zooplankton, with their relatively
short turnover time, acted as transient processors of N, while pools with longer turnover times
acted as N retainers. As observed in previous studies, incorporation into bulk sediments was the
fate of the majority of 15N (located in the estuary) and they retained it well beyond the duration
of the experiment. As the phytoplankton and zooplankton pools flushed 15N after the addition
experiment was terminated, the 15N content of the sediment pool remained fairly steady and at
some subreaches even increased.
Similarly, the BMA pool still retained 20-50% of their 15N inventory one week after the
tracer was shut off. The ability of this pool to retain 15N longer than the phytoplankton pool can
be related to the deposition of phytodetritus and tight recycling of 15N between BMA uptake and
mineralization back into the ecosystem, similar to previous studies (Tobias et al. 2003). Oysters
and marsh plants, because of their long turnover time, were also retainers of N. Marsh plants
retained up to 30 % of their assimilated 15N on day 47, while the oysters were the predominant
epifuanal retainer, with 48% of their 15N inventory on day 51.
48
Distribution of excess
between consumers
and producers
15 N
Dispersal of excess
in producers by
subreach
15 N
oligo
70%
21%
64%
upper meso
lower meso
upper eury
5%
lower eury
30%
8%
Week 1
2%
45%
58% 42%
46%
1%
7%
2%
Week 2
46%
34%
67%
3%
8% 11%
32%
Week 3
Figure 23: Proportional distribution of active biota excess 15N (excluding storage in sediments) between
autotrophs and consumers (µg 15N m-2). Primary producers are further divided by location within the creek.
The dark green shading in the circles on the right represents the primary producers; the light green shading
represents the consumers.
49
0.5
Benthic
Microalgae
g 15 N
0.4
day 3
day 7
0.3
day 14
0.2
day 22
0.1
day 24
0
day 31
day 1
1.5
Phytoplankton
day 6
1
g 15 N
day 3
day 11
day 14
0.5
day 22
day 24
0
day 25
1
0.8
Marsh
Macrophytes
g 15 N
0.6
day 7
day 14
day 22
0.4
day 31
0.2
day 38
0
day 47
1
0.8
Epifuana
day 3
g 15 N
0.6
day 7
day 14
0.4
day 22
0.2
day 31
day 38
0
4
Bulk Sediments
3.5
3
day 3
g 15 N
2.5
day 7
2
day 14
1.5
day 22
day 24
1
day 31
day 38
0.5
0
oligo
upper meso
lower meso
upper eury
lower eury
Figure 24: Distribution and collection of 15N within the principle uptake pools (BMA and phytoplankton) and
long term retention pools through the course of the isotope addition and post sampling events. Marsh
macrophytes include Spartina and Juncus and epifauna includes mud crabs, Littorina, and oysters. Note the
different y axis scales between bar graphs.
50
Loss of 15N
15
N loss was also examined in order to provide a better estimate of 15N cycling. Loss by
export to the coastal ocean was calculated based on tidal exchange. With a tidal export (the sum
of all non-motile water column compartments; DIN, PON, zooplankton) of 1.4 – 1.8 g 15N day-1,
32-41 g of added 15N was lost during the experiment (14 – 20 %). Although dissolved organic
nitrogen (DON) was not measured in this study, using a typical creek DON concentration of 20
µM N and applying the average enrichment value for the NH4+ pool (19 ‰ at high tide), ~ 8.7 g
15
N may have been exported tidally as enriched DON.
Another process of N removal is denitrification.
15
N that is denitrified into dissolved 15N2
gas would also be exported on each tide. Isotopic measurements of N2 gas were taken at low tide
in the upstream reaches of the creek on d14 and d21 of the experiment (Tobias et al. 2007).
Δδ15N values ranged from 0.7 – 1.9 ‰ and 0.3 – 1.5 ‰ for morning and evening low tides,
respectively (Figure 25). The N2 concentration was assumed to be at air saturated equilibrium,
or ~ 1mM N as N2. We can use these enrichment values, tidal volume, and tidal exchange to
calculate a conservative estimate of 15N lost by denitrification. Converting the Δδ15N2 value to g
15
N excess and multiplying it by the tidal volume and molar concentration of N2 gives an
estimate of excess g 15N as dissolved N2 gas exported with the tides. With a tidal exchange of 90
% d-1 and an addition period of 23 days, denitrification could account for a loss of ~ 26 g 15N.
51
morning
3.5
afternoon
3
background
δ 15 N2
2.5
2
1.5
1
0.5
0
0
500
1000
meters downstream
1500
Figure 25: Enrichment of N2 gas measured from the surface waters of Hewlett's Creek during the final week
of the addition experiment. The dotted lines are linear regression lines for the background enrichment
values.
52
DISCUSSION
The addition of isotopically labeled nitrate allowed us to quantify pathways and trophic
interactions important in the processing of watershed nutrient inputs. The tracer provided a ~ 1
month snapshot during the high production period (summer). It is important to clarify that this
15
N tracer assesses N dynamics only in the summer. Lower temperatures, lower primary
production, and different species composition (of the ecosystem) observed in the winter may
result in a shift in N transformation pathways and/or rates from what are reported here (Mallin et
al. 2004). Nevertheless, this experiment allowed us to examine some fundamental aspects of
tidal creek N dynamics not readily attainable without the in situ use of 15N, including: 1)
temporal and spatial patterns of watershed N processing and contributions of watershed NO3‾ to
biota; 2) the importance of detrital N and N recycling in fueling creek biota; and 3) the overall
ecosystem processing (g N d-1) of watershed-derived N, as well as the mechanisms of overall
export.
Temporal and Spatial Movement of N and Contributions of Watershed N to Biota
The isotopic enrichment of the phytoplankton within the first day of the experiment
(Figure 13B) indicated that this pool quickly incorporated the nitrate tracer. This immediate
uptake by phytoplankton is consistent with past estuarine 15N studies with high productivity
(Holmes et al. 2000) where pelagic diatoms assimilated > 75% of an 15N- NO3‾ tracer. In
Hewlett‟s Creek, the majority (89 %) of this rapidly-assimilated N was initially retained locally
at or above the addition site (O, UM, LM) where water column productivity was generally
highest. High productivity and chlorophyll a biomass in this creek has been noted previously to
occur in the upper region of this creek (Mallin et. al. 2004, Johnson 2005).
Enrichment in BMA was on the same scale as phytoplankton at all locations within 7
days of the isotope addition (Figure 13C). Because of their location at the sediment surface it
53
has been accepted that BMA is fueled by porewater DIN (Rizzo 1990). The magnitude of
enrichment in BMA in Hewlett‟s Creek (200 – 950 ‰ upstream) suggested a highly enriched
source of N, possibly NO3‾ from the water column (200 – 1500 ‰). However, the lag time in
the initial uptake of tracer (and consequently extended enrichment of the BMA post-addition)
suggested that BMA is obtaining significant inputs of recycled N (typically NH4+). To satisfy
the high enrichment with prolonged tracer incorporation required both a highly enriched source
that is also recycled. These criteria are satisfied if BMA preferentially use recycled freshlydeposited highly-labeled phytoplankton. With respect to N uptake by primary producers,
Hewlett‟s Creek behaves like a hybrid of past studies in that both water column and benthic
microautotrophs are a sink for water column NO3‾.
The 15N-DIN and 15N assimilated into the phytoplankton and zooplankton pools moved
back and forth through the tidal prism due to tidal flux. Therefore, what did not get exported to
the coastal ocean was utilized in all subreaches of the south branch of Hewlett‟s Creek. This
redistribution of N fueled secondary and tertiary production in all subreaches of this branch
regardless of the point of initial assimilation, however we have no evidence that the tracer moved
into the other major branches of the creek. The tracer (DIN) was taken up by local water column
organisms and subsequently transferred to organisms in the creek indicating a spatial
redistribution (net downstream transport) of watershed-derived nitrate by cascading through
biota (Figure 23). By the final week of the experiment, the 15N shifted from autotrophs to
consumers (mostly downstream, Figure 23). Of these consumers, mud crabs and oysters made
up the majority of N biomass (Figure 26) and retained the most 15N. While the majority of
labeled DIN was concentrated upstream, on the final day of the experiment 82 % of the total 15N
in the consumers was incorporated downstream into oyster biomass and 17 % in mud crabs.
54
Consumers
Zooplankton
Snails
Mud Crabs
Oysters
Fish
Figure 26: Distribution of 15N (µg m-2) within the consumer pool at the end of the experiment. The total 15N
inventory is a function of both the enrichment and abundance (biomass).
Similar evidence of spatial redistribution was demonstrated by the enrichment trajectories
of fish. The incorporation of tracer within the both the upstream and downstream fish is
consistent with known foraging behavior (Levin 1997, Luczkovich 1995, Kerschner 1985). All
species studied have a varied diet of detritus, phytoplankton, zooplankton, and benthic infauna.
The enrichment trajectories of all fish except mosquitofish show that these fish are not spatially
constrained to one subreach (Figure 16), and were using the entire south branch to forage.
Does watershed N contribute to fish and shellfish? Isotope studies have been able to
identify proportional contributions of isotopically labeled source material to higher organisms.
Cole et. al. (2002), for example, was able to identify the contributions of allochthonous carbon to
food webs in lakes. For Hewlett‟s Creek, the ratio of each pool‟s enrichment relative to the
mean source NO3‾ enrichment (900 ‰) provides an estimate of the fraction of N processed by
each pool that was watershed derived (Figure 27). Δδ15N of both fish and shellfish reached a
maximum of roughly 50 ‰. This enrichment is one twentieth of the mean NO3‾ enrichment
(900 ‰), indicating that 5 % of the fishes and shellfish N requirement can be traced back to the
tracer, and therefore watershed-derived NO3‾ (assuming the watershed is the sole source of
NO3‾).
55
Water Column
Watershed NO3
Crassostrea
Zoop
Phyto
<1.0 g N d-1
5%
648 g N d -1
3.5%
166 g N d -1
4.9%
Fish
407g N d-1
78-100%
<1.5 g N d -1
<1%
Palaemonetes
58 g N d -1
Littorina
<1%
313 g N d -1 Uca
2%
<1.0 g N d-1
1%
Sediment
Detritus
NH4
Detritus
Benthic
Infauna
BMA
389 g N d -1
30-54%
47746 g N d -1
<1%
Figure 27: Food web diagram of N cycling within this tidal creek ecosystem. Numbers in the ellipses beside
each compartment are the calculated throughput (g N d-1) of the organism pool and the percent of N from the
watershed (assuming that NO3‾ is watershed) based on the ratio of enrichment to 900-1500 ‰. The range
given for microautotrophs includes an estimate based on a nitrate enrichment of 500-900 ‰. An estimate of
N throughput for the marsh macrophyte pool is 105 g d-1 with a watershed contribution of 3% based on
published values of turnover times (Tobias et al. 2003). The size of the arrows is reflective of the %
contribution of watershed N to each pool’s total N requirement.
At the ecosystem scale, based on the 15N tracer, only about 2 % (~1 kg d-1) of the N being
turned over in the system was watershed derived. This value is consistent with the watershed
delivery rate of NO3‾ (3.8 kg N d-1) and small tidal exports (~90 % d-1) of creek NO3‾ (0.3 kg N
d-1). While the watershed contribution to some pools was high (phytoplankton, BMA,
zooplankton, and grass shrimp), as a whole, under base flow conditions the ecosystem seems to
be supported predominantly (49 kg d-1) by detrital N or old recycled N. Nitrogen recycling is
further supported by a strong correlation between the enrichment trajectories of NH4+ and those
of phytoplankton and BMA (p value = 0.001) (Figure 28; Appendix 3). Presumably, the marsh
was the primary source of this N however the tracer only indicates that it is not NO3‾ derived.
56


delnh4 = 0.40 + 0.03 * delphyto
R-Square = 0.61
40
15 N-NH
δdelnh4
4

30

R2 = 0.61

20



10
0

0





400
800
1200
delphyto
δ15 N phytoplankton
Figure 28: Correlation plot of ammonium and phytoplankton enrichment in the upper three subreaches of
Hewlett's Creek. Correlation is significant at the 0.001 level using a 2-tailed t test, with a Pearson correlation
coefficient of 0.779.
The biological residence time of 15N was calculated to be ~12 days by multiplying the
turnover times of each compartment by its contribution of 15N to the ecosystem total on the final
day of the experiment. This residence time is short compared to other estuarine studies however
the length of storage in bulk sediments and marsh macrophytes is also small in comparison
(Tobias et al. 2003).
Hewlett’s Creek – Transformer or Attenuator?
A total of 216 g 15N as NO3‾ was added to Hewlett‟s Creek. On the final day of the
experiment ~13 g 15N (6 % of the total) was measured in the estuary (Figure 29). Seventy seven
percent of the 13 g 15N was stored in the sediments, which acted as both a recycler and as a
repository. Five percent (0.7g 15N) was located in marsh plants, and 10 % (1.3 g 15N) in oyster
biomass
About ½ of the difference between the 15N released and that recovered in biota can be
accounted for by a combination of tidal export and denitrification. Tidal export of DIN, DON,
57
phytoplankton, and zooplankton is a pathway by which the creek transforms watershed NO‾3 and
exports it in useable forms of N to the coastal ocean (i.e. little attenuation).
Tidal export was measured as ~ 1.8 g 15N d-1, or roughly 42 g over the experiment. If
NO3‾ acted conservatively (ie. not utilized biologically), a tidal exchange of 90 % d-1 would
cause 90 % of the added tracer to be lost immediately (~194 g total). The measured exchange
was far less, indicating that not only was NO3‾ transformed, recycled, etc. as discussed
previously, the 15N export was in the form of labeled phytoplankton-POM, zooplankton, and
NH4+, but not NO3‾. Most of it (the difference between the conservative estimate and measured
tidal export) was assimilated by the ecosystem (152 g 15N).
Denitrification, a method by which tidal creeks attenuate watershed N loads, is difficult
to estimate in situ. Based on observed 15N2 enrichment (2 ‰), a [N2] of ~ 1 µM, a creek tidal
exchange of 90 % d-1, ~ 34 g 15N was denitrified and removed. This estimate is likely
conservative in that it does not account for gas exchange of 15N2. This estimate is equivalent to a
per area denitrification rate of ~ ⅓ of rates typical for these types of systems (Tobias et al. 2003).
58
Atmospheric N2|
(surface water gas exchange)
Tidal export to
Coastal Ocean
“Watershed” 15NO3
216 g 15 N
addition
42 g 15 N
(DIN, PON,
zoop)
13 g 15 N in
active biota
26 - 34 g 15 N
Denitrified
(N2 )
Fig 27
Burial
0.8 – 4.7 g 15 N
Figure 29: Mass balance of 15N in Hewlett's Creek. Amount buried was derived from other studies (Tobias et
al. 2003, Thomas et al. 2001).
While 42 g 15N gets exported to the coastal ocean in a usable form (tidal export) and 26 –
34 g 15N is attenuated by denitrification (Figure 28), and 13 g remains in the estuary, still not all
of the added 15N is accounted for. N burial in southeast tidal marsh ecosystems ranges from 9 to
21 g N m-2 y-1 (Craft 2007, Anderson 1997, Merril & Cornwell 2000). Using the average and
maximum δ15N value of the sediment pool in Hewlett‟s Creek (10.8 ‰ and 27 ‰, respectively)
and this yearly estimate, we can calculate a range of roughly 1 – 5 g 15N buried during the
addition experiment. While most addition experiments indicate burial as a large sink for N
(Tobias et al. 2003, Thomas et al. 2001), the highly active ecosystem of this tidal creek and the
uncharacteristically rapid turnover time for the sediments could be a reason that this pool may
not be a dominant sink for 15N.
Based on the existing mass balance that accounts for ~ ½ of the added 15N, the creek is
equal parts transformer of watershed N via tidal export of useable N, and attenuator of watershed
N through denitrification. Because the mass balance is not complete, it is important to identify
59
which N pools may have been underestimated and/or which areas of N assimilation have been
missed. Loss of 15N may have occurred through 15N2 gas exchange with the atmosphere,
applying typical gas transfer rates could raise the denitrification estimate by a factor of 4. Tracer
incorporation during marsh flooding (during spring tides and rain events) was not well
characterized. This flooding could have distributed the tracer well beyond the 1 meter marsh
fringe sampled in this study and a large area of minimally enriched marsh would account for a
lot of 15N mass. It is an unknown and should be quantified in future 15N studies.
For tidal export, uncertainty in DON enrichment could alter the export estimate.
Similarly, fish migration (Hughes et al. 2002) from the tidal creek to the coastal ocean could not
account for the missing 15N because the fish populations do not retain enough 15N mass within
the ecosystem to account for the missing nitrogen. However, oyster 15N mass was significant.
These organisms acted as a repository for N that can then be repackaged as feces and
psuedofeces that represents a potentially long-term retention pool that was not accounted for in
this study (Newell et al. 2002). Future isotope studies should include extensive sampling in the
marsh as well as longer addition duration in an attempt to quantify retention in longer storage
pools as well as lend the ability to rebound from ecosystem disturbances.
The oyster represents a keystone organism in tidal creek N cycling. It sequesters a
significant amount of the N inputs to the creek, some of which is turned into biomass and some
of which is recycled as feces or ammonium. The ammonium excreted by oysters supplies N to
both phytoplankton and BMA. Harvest of oysters removes N from the creek (permanent
removal of biomass), while shellfish bed closures (due to poor water quality from the impacts of
anthropogenic loading) does not permit this. Therefore, proper pollution management on land
can maintain water quality standards that would facilitate the harvesting of shellfish beds and
provide another pathway for N removal.
60
Despite an open 15N mass balance in this study, Hewlett‟s Creek appears to attenuate
roughly 20 – 40 % of the watershed N it processes. It retains approximately 5 – 10 % on
monthly (+) timescales, and exports 20 – 40 % to the coastal ocean as recycled DIN/DON and
biological particulates (phytoplankton, zooplankton, etc.). From an ecosystem perspective, the
creek is fueled by internally recycled N (about 90%) and relies little on watershed N inputs to
support productivity, although large pulsar events (ie. rainfall) may change this conclusion
considerably.
61
Appendix A
Chlorophyll a concentrations of phytoplankton at various tidal stages and water column levels.
GFF and GFD are the separate filters and their chlorophyll a concentrations.
day
0
1
3
22
0
1
3
11
22
1
3
11
22
1
3
11
22
1
3
11
22
22
6
6
14
6
14
6
14
6
14
6
14
7
14
7
14
7
13
7
13
7
13
7
13
station
st1
st1
st1
st1
st2
st2
st2
st2
st2
st3
st3
st3
st3
st4
st4
st4
st4
st5
st5
st5
st5
st7
st1
st2
st2
st3
st3
st4
st4
st5
st5
st7
st7
st1
st1
st2
st2
st3
st3
st4
st4
st5
st5
st7
st7
tide/
area
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
bottom
ebb
ebb
ebb
ebb
ebb
ebb
ebb
ebb
ebb
ebb
ebb
low
low
low
low
low
low
low
low
low
low
low
low
62
GFD
µg/L
23.0
18.2
16.3
20.9
18.2
12.0
14.4
15.1
12.8
11.4
9.8
14.5
11.6
5.2
5.5
11.8
10.8
5.0
4.6
7.3
9.2
7.4
41.3
23.1
1.3
24.0
27.9
15.8
15.3
8.9
6.5
5.0
3.2
2.7
3.1
31.8
4.8
11.0
7.0
14.4
7.3
4.9
17.3
3.5
41.2
GFF
µg/L
1.5
1.9
1.0
0.8
1.5
2.8
1.7
0.7
0.7
1.7
2.4
0.6
0.8
1.4
1.8
1.3
0.7
1.4
1.3
0.5
0.6
0.6
1.7
1.9
1.1
1.3
0.7
1.3
0.5
1.0
0.4
1.6
0.3
0.3
0.8
0.4
1.0
0.2
0.6
0.7
0.5
0.3
0.5
0.4
0.5
Chlorophyll a concentrations of phytoplankton for high tide surface water.
day
14
0
1
3
6
11
22
24
25
29
14
0
1
3
6
11
14
22
24
25
29
0
1
3
6
11
14
22
24
25
29
station
headwaters
st1
st1
st1
st1
st1
st1
st1
st1
st1
st1
st2
st2
st2
st2
st2
st2
st2
st2
st2
st2
st3
st3
st3
st3
st3
st3
st3
st3
st3
st3
GFD
µg/L
2.1
35.3
61.6
37.3
29.3
13.4
25.8
11.5
11.6
108.0
7.3
23.4
19.2
39.1
11.0
23.3
9.1
56.7
7.2
6.4
25.8
13.9
25.3
21.7
7.3
29.6
4.7
56.9
4.9
3.8
10.8
GFF
µg/L
0.4
1.2
3.0
2.4
1.4
0.9
1.0
0.6
0.8
0.8
0.4
1.7
2.8
2.2
1.8
0.7
0.5
0.9
0.6
0.8
0.9
2.0
2.2
2.2
2.3
0.5
0.6
1.0
0.5
1.1
0.7
day
0
1
3
6
11
14
22
24
25
29
0
1
3
6
11
14
22
24
25
29
0
1
3
6
11
14
22
24
25
29
63
station
st4
st4
st4
st4
st4
st4
st4
st4
st4
st4
st5
st5
st5
st5
st5
st5
st5
st5
st5
st5
st7
st7
st7
st7
st7
st7
st7
st7
st7
st7
GFD
µg/L
9.3
18.8
7.3
3.4
15.3
3.0
12.2
3.1
4.6
13.0
5.6
10.8
4.9
2.4
3.0
6.6
9.0
1.5
3.4
8.4
6.3
6.4
5.6
2.0
13.7
3.9
2.0
3.7
4.9
GFF
µg/L
1.7
1.5
2.0
1.9
0.7
0.5
0.8
0.5
0.7
0.6
1.6
1.1
1.3
1.3
0.8
0.4
0.8
0.5
0.8
0.7
1.7
0.8
1.2
1.2
0.7
0.3
0.8
0.5
0.6
0.5
Appendix B
Chlorophyll a concentrations for benthic microalgae in the creek bed (CB) and marsh surface
(MS)
Sample
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
Day
3
7
14
22
24
0
3
7
14
22
24
0
7
13
22
24
31
0
3
7
13
22
24
31
0
7
13
22
24
31
0
3
0
3
7
13
22
24
31
Station
st1
st1
st1
st1
st1
st2
st2
st2
st2
st2
st2
st3
st3
st3
st3
st3
st3
st4
st4
st4
st4
st4
st4
st4
st5
st5
st5
st5
st5
st5
st6
st6
st7
st7
st7
st7
st7
st7
st7
µg/ cm3
Chl a
Sample
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
MS
2.6
5.1
7.7
1.4
8.3
5.8
5.8
2.6
7.4
5.1
5.1
8.4
3.9
18.0
2.6
4.5
1.4
6.4
6.4
5.8
1.9
3.9
3.2
1.9
1.3
2.6
7.7
2.6
1.9
1.3
1.3
1.3
1.3
1.3
1.3
2.6
9.6
12.2
9.0
64
Day
7
14
22
7
14
22
24
7
13
22
31
7
13
22
24
31
7
13
22
24
31
7
13
22
24
31
Station
st1
st1
st1
st2
st2
st2
st2
st3
st3
st3
st3
st4
st4
st4
st4
st4
st5
st5
st5
st5
st5
st7
st7
st7
st7
st7
µg/ cm3
Chl a
0.0
6.4
5.1
0.6
0.0
1.3
1.3
1.7
2.6
5.1
9.0
3.9
3.9
7.7
9.0
10.3
0.0
5.1
5.1
3.9
0.0
7.7
3.9
7.7
6.4
3.9
Appendix C
Ammonium trajectories plotted with microautotrophs for the upper three subreaches.
Oligo
1600
BMA
phyto
NH4
1400
60
50
1200
40
1000
800
30
600
20
400
10
200
0
0
UM
1600
60
50
1200
40
1000
800
30
600
20
400
10
200
0
0
1600
35
LM
1400
30
1200
25
1000
20
800
15
600
400
10
200
5
0
0
0
10
time (d)
65
20
30
Δδ15 N-NH4
Δδ15 N producers
1400
REFERENCES
Anderson IC, Tobias CR, Neikirk BB, Wetzel RL (1997) Development of a process-based
nitrogen mass balance model for a Virginia (USA) Spartina alterniflora salt marsh:
implications for Net DIN Flux. Marine Ecology Progress Series 159:13-27
Blanchard GP-CMR, Dinet A, Samson FL, Gatje C, Amadi, I (1990) Comparison of techniques
for the measurement of microphytobenthic chloropigments after cell isolation with
Ludox-HS. Marine Microbial Food Webs/ Aquatic Microbial Ecology 4:207-216
Bohlke JK, Judson WH, Voytek MA (2004) Reach-scale isotope tracer experiment to quantify
denitrification and related processes in a nitrate-rich stream, midcontinent United States.
Limnology and Oceanography 49:821
Boyer JN, Stanley DW, Christian RR (1994) Dynamics of NH+4 and NO3ˉ uptake in the water
column of the Neuse River Estuary, North Carolina. Estuaries 17:361-371
Bronk DA, Glibert PM, Ward BB (1994) Nitrogen uptake, dissolved organic nitrogen release,
and new production. Science 265:1843-1846
Capone DG, Bronk, Deborah A., Mulholland, Margaret R., Carpenter, Edward J. (2008)
Nitrogen in the Marine Environment, Second Edition.
Christian RR, Thomas CR (2003) Network Analysis of nitrogen inputs and cycling in the Neuse
River Estuary, North Carolina, USA. Estuaries 26(3):815-828
Cloern JE, Canuel EA, Harris D (2002) Stable carbon and nitrogen isotope composition of
aquatic and terrestrial plants of the San Francisco Bay estuarine system. Limnology and
Oceanography 47:713
Cole JJ, Carpenter SR, Kitchell JF, Pace ML (2002) Pathways of organic carbon utilization in
small lakes: results from a whole-lake 13C addition and coupled model. Limnology and
Oceanography 47:1664
Craft CB, Seneca ED, Broome SW, (1993) Vertical accretion in microtidal regularly and
irregularly flooded estuarine marshes. Estuarine, Coastal and Shelf Sciences 37:371-386
Drake DC, Peterson BJ, Deegan LA, Harris LA, Miller EE, Warren RS (2008) Plant nitrogen
dynamics in fertilized and natural New England salt marshes: a paired 15N tracer study.
Marine Ecology Progress Series 354:35-46
Deegan LA, Hughes JE, Rountree RA (2000) Salt marsh ecosystem support of marine transient
species. In: Weinstein M, Kreeger DA (Eds.), Concepts and Controversies in Tidal
Marsh Ecology. Kluwer Academic Publishing, Dordrecht, pp. 333-365
Galloway JN, Cowling EB (2002) Reactive nitrogen and the world: 200 years of change. Ambio
31:64
66
Genoni GP (1991) Increased burrowing by fiddler crabs Uca rapax (Smith) (Decapoda:
Ocypodidae) in response to low food supply. Journal of Experimental Marine Biology
and Ecology 147:267
Gribsholt BS, Tramper A, De Brabandere L, Brion, N, van Damme S, Meire P, Dehairs F,
Middelburg JJ, Boschker HTS (2007) Nitrogen assimilation and short term retention in a
nutrient-rich tidal freshwater marsh; a whole ecosystem 15N enrichment study.
Biogeosciences 4:11-26
Holland AF, Sanger DM, Gawle CP, Lerberg SB, Santiago MS, Riekerk GHM, Zimmerman LE,
Scott GI (2004) Linkages between tidal creek ecosystems and the landscape and
demographic attributes of their watersheds. Journal of Experimental Marine Biology and
Ecology 298:151
Holmes RM, McClelland JW, Sigman DM, Fry B, Peterson BJ (1998) Measuring 15N-NH+4 in
marine, estuarine and fresh waters: An adaptation of the ammonia diffusion method for
samples with low ammonium concentrations. Marine Chemistry 60:235
Holmes RM, Peterson BJ, Deegan LA, Hughes JE, Fry B (2000) Nitrogen biogeochemistry in
the oligohaline zone of a New England estuary. Ecology 81:416
Hopkinson CS, Schubauer JP (1984) Static and dynamic aspects of nitrogen cycling in the salt
marsh graminoid Spartina alterniflora. Ecology 65:961
Hughes JE, Deegan LA, Wyda JC, Weaver MJ, Wright A (2002) The effects of eelgrass habitat
loss on estuarine fish communities of southern New England, USA. Estuaries 25:235-249
Hughes JE, Deegan LA, Peterson BJ, Holmes RM, Fry B (2000) Nitrogen flow through the food
web in the oligohaline zone of a New England estuary. Ecology 81:433
Johnson, VL (2005) Primary productivity by phytoplankton: temporal, spatial, and tidal
variability in two North Carolina tidal creeks. University of North Carolina Wilmington
Jorgensen SE (1979) Handbook of environmental data and ecological parameters, Pergamon,
Oxford, UK
Kerschner BA, Peterson MS, Gilmore RG, Jr. (1985) Ecotopic and ontogenetic trophic variation
in mojarras (Pisces: Gerreidae). Estuaries 8:311-322
Levin P, Petrik R, Malone J (1997) Interactive effects of habitat selection, food supply and
predation on recruitment of an estuarine fish. Oecologia 112:55-63
Luczkovich JJ (1988) The role of prey detection in the selection of prey by pinfish Lagodon
rhomboides (Linnaeus). Journal of Experimental Marine Biology and Ecology 123:15
Lewitus AJ, Koepfler ET, Morris JT (1998) Seasonal variation in the regulation of
phytoplankton by nitrogen and grazing in a salt marsh estuary. Limnology and
Oceanography 43(4): 636-646
67
Mallin MA, Paerl HW (1994) Planktonic trophic transfer in an estuary: seasonal, diel, and
community structure effects. Ecology 75:2168
Mallin MA, Lewitus AJ (2004) The importance of tidal creek ecosystems. Journal of
Experimental Marine Biology and Ecology 298:145
Mallin MA, Parsons DC, Johnson VL, McIver MR, CoVan HA (2004) Nutrient limitation and
algal blooms in urbanizing tidal creeks. Journal of Experimental Marine Biology and
Ecology 298:211
Mallin MA, McIver MR, Spivey MIH, Tavares ME, Alphin TD, Posey MH. (2008)
Environmental Quality of Wilmington and New Hanover County Watersheds 2006-2007.
Report No. 08-01, Center for Marine Science, University of North Carolina Wilmington,
Wilmington NC
Merrill JZ, and JC Cornwell (2000) The role of oligohaline marshes in estuarine nutrient cycling.
In: Weinstein, M, Kreeger DA (Eds.), Concepts and Controversies in Tidal Marsh
Ecology. Kluwer Academic Publishing, Dordrecht, pp. 425-442
Middleburg JJ, Barranguet C, Boschker HTS, Herman PMJ, Moens T, Heip CHR (2000) The
fate of intertidal microphytobenthos carbon: An in situ 13C-labeling study. Limnology
and Oceanography 45:1224
Mouton EC Jr, and DL Felder (1996) Burrow distributions and population estimates for the
fiddler crabs Uca spinicarpa and Uca longisignalis in a Gulf of Mexico salt marsh.
Estuaries 19:51
Mulholland PJ, Tank JL, Sanzone DM, Wollheim WM, Peterson B J, Webster, JR, Meyer JL
(2000) Food resources of stream macroinvertebrates determined by natural-abundance
stable C and N isotopes and a 15N tracer addition. Journal of the North American
Benthological Society 19:145-157
Newell RIE, Cornwell JC, Owens MS (2002) Influence of simulated bivalve biodeposition and
microphytobenthos on sediment nitrogen dynamics: A laboratory study. Limnology and
Oceanography 47:1367
Nixon SW and BA Buckley (2002) "A Strikingly Rich Zone": Nutrient enrichment and
secondary production in coastal marine ecosystems. Estuaries 25:782
Paerl HW, Mallin MA, Donahue CA, Go M, Peierls, BL (1995) Nitrogen loading sources and
eutrophication of the Neuse River estuary, North Carolina: Direct and indirect roles of
atmospheric deposition. Water Resources Research Institute of the University of North
Carolina 291
Peterson BJ, Wollheim WM, Mulholland PJ, Jackson RW, Meyer JL, Tank JL, Marti En,
Bowden WB, Valett HM, Hershey AE, McDowell WH, Dodds WK, Hamilton SK,
Gregory S, Morrall DD (2001) Control of nitrogen export from watersheds by headwater
streams. Science 292:86
68
Rabalais NN (2002) Nitrogen in aquatic ecosystems. Ambio 31:102
Redfield AC (1958) The biological control of chemical factors in the environment. American
Scientist
Richardson C, Saball D, Johnson A, Wright D, Gould D, Vanarsdale A, Weiss J, Smith BB,
Doering E (2003) Estimating Estuarine Pollutant Loading From Atmospheric Deposition
Using Casco Bay, Maine as a Case Study. A report by the Casco Bay Air Deposition
Study Team. Sonoma Technology, Inc. Petaluma, CA
Rizzo WM and RR Christian (1996) Significance of subtidal sediments to heterotrophicallymediated oxygen and nutrient dynamics in a temperate estuary. Estuaries 19:475-487
Ryther JH and WM Dunstan (1971) Nitrogen, phosphorus, and eutrophication in the coastal
marine environment. Science 171:1008
Sanger DM, Holland AF, Hernandez DL (2004) Evaluation of the impacts of dock structures and
land use on tidal creek ecosystems in South Carolina estuarine environments.
Environmental Management 33:385
Seitzinger S, Harrison JA, Bohlke JK, Bouwman AF, Lowrance R, Peterson B, Tobias C, Drecht
GV (2006) Denitrification across landscapes and waterscapes: a synthesis. Ecological
Applications 16:2064-2090
Sigman DM, Altabet MA, Michener R, McCorkle DC, Fry B, Holmes RM (1997) Natural
abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate: an
adaptation of the ammonia diffusion method. Marine Chemistry 57:227
Silliman BR and JC Zieman (2001) Top-down eontrol of Spartina alterniflora production by
periwinkle grazing in a Virginia salt marsh. Ecology 82:2830
Thomas CR and RR Christian (2001) Comparison of nitrogen cycling in salt marsh zones related
to sea-level rise. Marine Ecology Progress Series 221:1-16
Tobias CR, Cieri M, Peterson BJ, Deegan LA, Vallino J, Hughes J (2003) Processing watershedderived nitrogen in a well-flushed New England estuary. Limnology and Oceanography
48:1766
Tobias CR, Macko SA, Anderson IC, Canuel EA, Judson WH (2001) Tracking the fate of a high
concentration groundwater nitrate plume through a fringing marsh: A combined
groundwater tracer and in situ isotope enrichment study. Limnology and Oceanography
46:1977
Varela M (1986) The efficiency of colloidal silica Ludox-TM for separating benthic microalgae
from their substrate. Boletin del Instituto Espanol de Oceanografia 3:85-88
Watts SH and SP Seitzinger (2000) Denitrification rates in organic and mineral soils from
riparian sites: a comparison of N2 flux and acetylene inhibition methods. Soil Biology
and Biochemistry 32:1383
69
Welschmeyer NA (1994) Fluorometric analysis of Chlorophyll a in the presence of chlorophyll b
and pheopigments. Limnology and Oceanography 39:1985
Welsh BL (1975) The role of grass shrimp, Palaemonetes pugio, in a tidal marsh ecosystem.
Ecology 56:513
Whitney DE and WM Darley (1979) A method for the determination of Chlorophyll a in samples
containing degradation products. Limnology and Oceanography 24:183
70