Nitrogen isotopes in bivalve shells from the Colorado River estuary:
Evaluating a potential proxy for changes in riverine nutrient delivery
by
Robert D. Dietz
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THE UNIVERSITY OF ARIZONA
2008
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ABSTRACT
Could stable isotopes of N be used to detect the source of nutrients in modern and past
estuarine ecosystems? We examined δ15N from organic material contained in the shells of livecollected and sub-fossil bivalve mollusks from the Colorado River and its estuary. We compared
δ15N in 3 species: Corbicula fluminea, an introduced freshwater species, Mulinia coloradoensis,
a brackish-water species once abundant in the estuary, and Chione fluctifraga, a fully marine
species. All three species are found alive today, and the latter two also occur in historical shell
deposits. δ15N values obtained from modern C. fluminea are low (δ15N = 9.6 ‰) with respect to
live-collected M. coloradoensis and Ch. fluctifraga (δ15N = 14.77 and 13.87, respectively),
documenting a distinct difference between fluvial N signals and marine N signatures. We
hypothesized that a significant difference in δ15N values would also exist between live-collected
individuals and sub-fossil individuals of the same species. Riverine nutrients are largely absent
from the modern Colorado River estuary because the river no longer reaches the sea, yet prior to
dam construction and large-scale water diversions, such nutrients were likely available to
estuarine biota. However, no significant difference in δ15N was found between modern and predam shells. To test for possible diagenetic distortion of the δ15N signal preserved in fossil shells,
we subjected modern shells to conditions of strong heat (60oC in a drying oven) and intense
sunlight (rooftop exposure in Tucson, Arizona) for 180 days. Whereas no significant directional
shift in δ15N was detected in rooftop shells relative to controls, heated shells became depleted in
15
N. Diagenesis therefore cannot be ruled out as a source of bias in δ15N measurements. The
similarity of δ15N in modern and pre-dam shells nevertheless suggests that either riverine
nutrients were not generally a significant component of the estuarine nutrient pool or that their
isotopic character was lost during biogeochemical transformations prior to incorporation into
bivalve shells.
INTRODUCTION
Under natural conditions, estuaries function as sites of material exchange between
continental and marine environments. Rivers deliver sediments and nutrients generated on the
land, as well as an authochthonous component derived from aquatic primary production, to their
deltas. Nitrogen transported in the fluvial environment enters the estuary in multiple forms,
namely dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN), and particulate
organic nitrogen (PON). These “riverine nutrients” may then be used by estuarine biota. Tidal
flushing and oceanic circulation mix and redistribute the river-derived materials among the
marine nutrient pool.
Freshwater flows and nutrient delivery to an estuary can vary over broad timescales in
response to climatic shifts, geomorphologic processes (e.g. stream capture), and changes in
landscape ecology (Correll et al. 1992; Spencer and Pearthree 2003; Jickells et al. 2000, Ingram
et al. 1996). In recent times, the timing and quantity of river discharge and nutrient delivery has
been altered by anthropogenic activities, including agriculture (water withdrawals, fertilizer
runoff), urbanization (polluted stormflow, wastewater discharge, increased overland flow),
vegetation clearance (reduced terrestrial organics), and damming and water diversions
(diminished flow and suspended load).
3
It is important to document these changes because they can affect estuarine productivity
and ecosystem composition (Mallin et al. 1993). Whereas recent changes can be observed and
measured, historical variation can be discerned only indirectly from aquatic archives.
Reconstructing former estuarine conditions requires the use of chemical and biological indicators
preserved in sediments or the remains of organisms. In this study, we determine whether stable
isotopes of nitrogen contained in the shells of bivalve mollusks record changes in riverine
nutrient delivery to modern and past estuarine ecosystems.
Suspension-feeding bivalves filter and assimilate phytoplankton, detritus,
microzooplankton, and suspended organic sediments – and in some cases they additionally
absorb dissolved organic material directly from water (Péquignat 1973; Manahan et al. 1982).
Ferguson (1982) suggested that the uptake of dissolved free amino acids (DFAA) can support as
much as 20% of bivalve metabolism, and Rice (1999) estimated that DFAA can account for up
to 14% of total organic N uptake in the clams of Narragansett Bay, Rhode Island. Changes in the
relative availability of these various nutrient types and their source regions (e.g. terrestrial,
freshwater, or marine) are relevant to bivalve feeding and may be reflected as isotopic
differences in their tissues and shells.
Nitrogenous materials derived from terrestrial and marine sources possess distinct
nitrogen isotope signatures (Sweeney et al. 1978; Sweeney and Kaplan 1980), with the latter
being more enriched in 15N than the former. Fixation of atmospheric nitrogen (δ15N = 0 ‰) by
land plants produces organic matter with δ15N values also near 0‰, typically between –3 ‰ and
+1 ‰ (Kendall 1998). Partial degradation of these materials as they are transported downstream
may raise their δ15N signatures some, but they typically remain strongly depleted relative to
marine organic materials. Nitrogen-fixing phytoplankton (namely cyanobacteria) also generate
organic matter with low δ15N and can be important in riverine environments. They are not
abundant in the ocean, however, and usually comprise < 1 % of the phytoplankton biomass or
they are absent altogether (Howarth et al. 1988). Marine N pools are strongly influenced by
denitrification, which discriminates strongly against 15NO3- and drives the δ15N of residual N to
high values (Liu and Kaplan 1989).
The δ15N of PON is most relevant in our study, as this is the dominant fraction used by
benthic consumers. Isotopic separation between marine and terrestrial suspended organic matter
is often quite large. Marine end members for the δ15N of PON are geographically variable but
generally are within the range of 6.3 ‰ to 10 ‰ (Liu and Kaplan 1989). Mariotti et al. (1984),
using δ15N to trace the source of suspended PON in the Scheldt Estuary in northern Europe,
applied a marine end member (North Sea) of 8 ‰ and terrestrial end member of 1.5 ‰
(Δ15Nmarine-terrestrial = 6.5 ‰). Wada et al. (1987) calculated a maximum Δ15N of 7 ‰ for marine
and terrestrial primary producers in the Otsuchi River-Otsuchi Bay environment of Japan.
Isotopic separation between only the autochthonous fraction of riverine and marine PON can
also be large. Cloern et al. (2002) calculated mean δ15N values of 5.0 ‰ and 8.0 ‰ for
freshwater and estuarine-marine phytoplankton, respectively, in the San Francisco Bay Estuary.
Although the range of possible δ15N values for terrestrial, freshwater, and marine PON is large,
these trends (terrestrial < freshwater < marine) appear to be widespread and systematic (France
1995). These distinct isotopic signals are also recorded in consumers, thus opening the
possibility of using consumer δ15N values to determine local nutrient sources. Synthesizing the
data available in the literature at the time, France (1994) determined that a mean separation of 3
‰ existed between marine and freshwater zooplankton as well as marine and freshwater
zoobenthos. Reanalyzing data from Peterson et al. (1985), he also determined that estuarine
4
mussels (Geukensia demissa) have δ15N values reflecting their position along a freshwatermarine gradient. Similarly, Kasai and Nakata (2005) showed a clear relationship between the
δ15N of PON and the δ15N of Corbicula japonica foot tissue in different regions of the Kushida
Estuary in Japan. PON in the Kushida River had a mean δ15N of 5.5 ‰ while marine PON was
elevated at 9.0 ‰; estuarine PON δ15N was of an intermediate value (6.3 ‰). Co. japonica δ15N
values increased from 8.6 ‰ in the upper estuary to 11.8 ‰ in the lower estuary, reflecting the
physical mixing of nutrient sources and the increasing dilution of riverine PON down estuary.
The offset between tissue and PON δ15N values, about 3.0 ‰, could be explained by normal
trophic enrichment. Animal tissues are usually 2-5‰ enriched relative to diet (mean = 3.4‰,
Minagawa and Wada 1984).
Numerous other studies have reported δ15N values for bivalve soft tissues (see Table 1).
Measurements of tissue δ15N, often complimented by measurements of δ13C, have been used to
discern the relative importance of different dietary sources, to address spatial variability in
available nutrient sources, and to track anthropogenic N pollution (see references within Table
1). Some investigators have also attempted to extract seasonal information from bivalve tissues
harvested at different times of the year. Soft tissues cannot be used to study long-term
environmental changes, however, because they are not preserved following the death of the
organism. Bivalve shells, on the other hand, can serve as archives of paleoenvironmental
information. Nitrogen contained within the organic material of bivalve shells may, like the N
contained within soft tissues, record changes in bivalve diet and, thus, in sources of available
nutrients. Shell organic matrices, which form prior to mineralization, are thought to provide a
framework for crystalline growth (Bevelander and Nakahara 1969; Weiner and Hood 1975).
Calcium carbonate and the organic constituents of the shell pass through mantle epithelia to the
extrapallial fluid, where shell deposition occurs (Wilbur 1964; Neff 1972). While much of the
organic material in the shell is contained within the “matrix” component, some of it becomes
encased in the mineral crystals; this latter fraction is perhaps less likely to be affected by
diagenesis and more likely to be preserved in fossil shells (Crenshaw 1980; O’Donnell et al.
2003).
To our knowledge, only two studies have reported δ15N values for organic material
within bivalve shells. O’Donnell et al. (2003) measured 15N (and δ13C, δ34S) in the soft tissues
and shell material (ventral margin) of modern and sub-fossil Mercenaria spp. collected from the
Atlantic coastal plain of the United States. Mae et al. (2007) measured 15N (and 13C) in both
modern and fossil deep-sea bivalve shells for the purpose of evaluating whether historical
populations utilized chemosynthetic production. In this study, we compare the nitrogen stable
isotope composition of modern and sub-fossil bivalve shells collected from the Colorado River
estuary.
The Colorado River estuary serves as an ideal investigatory setting because the
completion of massive dams on the Colorado River during the early-to-middle 20th Century all
but shut down the inflow of river water. Biota in the modern “no-flow” estuary have access to
only marine nutrients, but the same biota in the pre-dam estuary likely received a mixture of
marine and riverine nutrients. The Colorado River would have transported terrestrial organic N
and its degradation products, as well as an autochthonous component from freshwater primary
producers, and possibly heterotrophic bacteria. We hypothesized that pre-dam bivalves would
have “seen” these riverine nutrients and that their shells would therefore record a distinct
(depleted) δ15N signature relative to shells collected from live individuals in the modern
environment. In addition, we tested the hypothesis that the shells of freshwater bivalves
5
collected from several locations within the lower Colorado River drainage basin would be
depleted in 15N relative to the shells of estuarine bivalves (both modern and pre-dam). Our
analyses assume a constant isotopic offset between soft tissue and shell, such that any rise in the
δ15N of ambient nutrients translates into higher shell δ15N and any decline in the δ15N of nutrient
sources also reduces shell δ15N. Knowing the absolute value of that offset is not important for
this study, but it may be critical in future investigations that aim to reconstruct historical trophic
relationships.
Hydrologic transformation of the Colorado River estuary following the cessation of flows
off the river has affected many local fauna adversely (Aragón-Noriega and Calderon-Aguilera
2000; Galindo-Bect et al. 2000; Rowell et al. 2005), including bivalve mollusks and their
predators (Rodriguez et al. 2001a; Cintra-Buenrostro et al. 2005). Kowalewski et al. (2000)
estimated that the standing crop of bivalve mollusks, namely Mulinia coloradoensis and Chione
fluctifraga, has declined by as much as 94% from an original size of about 6 billion individuals.
It is unclear whether the disappearance of riverine nutrients has contributed to population
declines. Despite the sharp reduction in consumer productivity, nutrient levels and primary
productivity in the estuary remain high (Hernández-Ayón et al 1993; Millán-Núñez et al. 1999).
We undertook this investigation not only with the aim to evaluate whether shell δ15N reflects the
availability of nutrients derived from the Colorado River, but also with the objective to shed
some light on whether riverine nutrients were important for benthic production in the pre-dam
environment. Were riverine nutrients a significant component of the bivalve diet?
METHODS
Study site
The Colorado River estuary is located between the Mexican states of Sonora and Baja
California, at the terminus of the 2800-km-long Colorado River. Mean monthly air temperatures
range from 16oC in winter and 34oC in summer, while mean monthly water temperature varies
between 15oC and 30oC (Thompson 1968; Thompson 1999). The area receives less than 60 mm
of annual rainfall (Hastings 1964), making local runoff into the estuary negligible (Dettman et al.
2004). The estuary lies within a shallow basin (1 to 40 m) and is characterized by a very large
tidal range (up to 10 m) (Carbajal et al. 1997; Lavín and Sánchez 1999). Sediment resuspension
by the strong tides generates turbid conditions (Hernández-Ayón et al. 1993; Santamaría-delÁngel et al. 1996), and the circulation of materials in the estuary generally follows a
counterclockwise pattern (Carriquiry and Sánchez 1999).
The completion of massive dams on the Colorado River during the early-to-middle 20th
Century and subsequent diversion of enormous quantities of water profoundly altered the
hydrological and ecological characteristics of the estuary. Prior to the dams and diversions, an
unrestrained Colorado River annually discharged 8 x 109 to 30.8 x 109 m3 yr-1 of freshwater into
the upper Gulf of California (Harding et al. 1995), creating a mixing zone that stretched at least
65 km southward from its mouth (Rodriguez et al. 2001b). For approximately the last 50 years,
the river has rarely reached the Gulf, discharging only during unusually wet periods. Absent its
flow, the modern Colorado River fails to deliver sediments and nutrients from the land to the sea.
Delta wetlands have declined to a small fraction of their original area, oligohaline conditions in
the upper estuary have vanished, and sediments are now eroded – not deposited – in intertidal
areas (Lavín and Sánchez 1999; Thompson 1968; Carriquiry and Sánchez 1999). Cessation of
6
freshwater flows has introduced marine salinity conditions (35-42 psu) and inverted the normal
salinity gradient of the estuary such that higher values occur toward the head (Alvarez-Borrego,
2001; Dettman et al. 2004).
Sample collection
Live specimens of Mulinia coloradoensis and Chione fluctifraga were collected in
December 2005 from Estero del Chayo (31o40.12’ N, 114o41.53’ W), a tidal channel cutting into
the deltaic sediments of Isla Montague (Fig. 1). Both species are infaunal suspension feeders
native to the Colorado River estuary. Whereas Mulinia prefer brackish conditions (which are
today absent), Chione can be considered fully marine. In the absence of inflow from the
Colorado River, shells from these individuals are presumed to represent marine conditions. To
obtain a representation of the modern fluvial environment, Corbicula fluminea were collected
from multiple locations in the Lower Colorado River drainage. C. fluminea is an introduced
freshwater species that colonized the lower Colorado River only after the completion of the
major upstream dams. Live individuals were collected from Mitry Lake and Imperial Lake
(December 2004), and from a section of river flowing through the city of Yuma, AZ (April
2007). The shells of recently expired individuals were obtained from Lake Mead (March 2008)
and Walter’s Camp and Island Lake (November 2007). In addition, two unionid mussels were
obtained – one from Mitry Lake (live collected, December 2004) and one from Smithsonian
archives (shell only, collected in the 1880s from the Colorado River at the Mexican border). All
live-collected specimens were kept in cold storage until preparation for isotopic analysis.
Mitry Lake and Island Lake are small, shallow backwaters connected to the main river
channel. Walter’s Camp, located on the river, is a privately owned concession in the Cibola
National Wildlife Refuge. Each of these 3 sites is found within a short distance (<50 km) to the
north of Yuma, as is Imperial Lake, a major impoundment formed by the Imperial Dam. Lake
Mead, formed by waters impounded by Hoover Dam, is located much further north, near Las
Vegas.
Subfossil shells of Mulinia and Chione were collected from cheniers, or long shelly
beaches, on the western shores of the estuary. Amino acid racemization and radiocarbon dates
(Kowalewski et al. 1994, 1998) indicate that the shells in these locations were produced by
bivalves living prior to the construction of upstream dams. Oxygen isotope data (Rodriguez et
al. 2001b; Dettman et al. 2004) confirm that the bivalves were influenced by freshwater inflows
off the Colorado River. Pre-dam Mulinia specimens used in this study were obtained from a
chenier at Las Isletas (December 2001); pre-dam Chione were collected at a chenier on Isla
Sacatosa (November 2005) (Fig. 1). All shells were obtained from > 25 cm depth to minimize
any diagenetic effects from surface exposure.
Sample preparation
All shells were thoroughly cleaned and rinsed prior to taking δ15N measurements. Soft
tissues were removed with a scalpel from live-collected bivalves. Both live-collected and predam shells were scrubbed with a stiff nylon brush to remove attached dirt and debris. The
periostracum, an organic coating on the exterior shell surface that can differ isotopically from
shell material (unpublished data), was removed using a dental drill. Most pre-dam shells
appeared to have lost their periostracum, but the exterior shell surface was still cleaned with the
7
drill to be certain that all of the coating had been removed. Shell surfaces were rinsed with
deionized water after brushing and drilling to remove loosened material.
One valve of each shell was approximately bisected using a low-speed ISOMET lapidary
saw with diamond-tipped wafer blade. One of these halves was then ground into a powder and
homogenized using a mortar and pestle (Fig. 2). δ15N measurements on these “whole-shell”
powders reflect the average isotopic composition of shell material over the lifetime of the
bivalve. Some Chione shells were too thick to grind with a mortar and pestle; in such instances a
cross-sectional slice (about 2-3 mm wide) was cut along the axis of maximum growth between
the umbo and ventral margin. The slice was then easily ground with mortar and pestle. Powders
obtained from ground cross-sections should be equivalent to “whole-shell” powders in their
average isotopic composition.
For some bivalve samples, shell material was additionally obtained from the ventral
margin of the other valve half. Drilling at the margin produced a fine powder that did not require
further grinding with a mortar and pestle. δ15N measurements on “margin” powders reflect the
isotopic composition of the most recently deposited shell material only.
The remaining valve was, for some individuals, spot sampled along a vertical transect
between the umbo and ventral margin. Shallow circular drill holes were made at semiregular
intervals in the exterior shell surface to extract small amounts of powder for carbonate δ18O
measurements. The δ18O data were then used to identify portions of the shell deposited during
early summer, which, in the pre-dam environment, is when the strongest freshwater flows off the
Colorado River would have occurred (Harding et al. 1995). These summer growth bands were
subsequently sampled for δ15N measurements.
Isotope measurement
For δ15N measurements, homogenized shell powder, usually in a quantity between 40 mg
and 90 mg, was loaded into tin capsules. Sample capsules were combusted using a Costech
elemental analyzer coupled to a Finnigan Delta Plus XL continuous-flow gas-ratio mass
spectrometer. δ15N values were reported with precisions between + 0.1 ‰ and + 0.5 ‰ (1σ), as
the amount of sample powder available for analysis was not always sufficient to obtain
maximum precision. For simplicity, a standard error of + 0.5 ‰ should be assumed for all δ15N
values reported in this study. Mass spectrometer standardization is based on IAEA-N-1 and
IAEA-N-2 for δ15N. Shell carbonate δ18O was measured using an automated carbonate
preparation device (Kiel-III) coupled to a Finnigan MAT 252 gas-ratio mass spectrometer.
Powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70oC. The
isotope ratio measurement is calibrated based on repeated measurements of NBS-19 and NBS-18
and precision is + 0.1 ‰ for δ18O (1σ).
Diagenesis experiment
To test for possible diagenetic distortion of the δ15N signal preserved in pre-dam shells,
we subjected a subset of modern shells to conditions of strong heat and intense sunlight – the
most likely causes of any diagenetic effects – for ½ year, between April 4, 2007 and September
29, 2007. For each shell (n=10; 4 Corbicula, 3 Mulinia, and 3 Chione), one valve was used for
experimentation and one valve was kept aside as a control. The experimental valve was bisected
using the low-speed lapidary saw; one half was then placed in a drying oven set to a constant
8
temperature of 60oC and the other was rested upon a rooftop in Tucson, Arizona to receive
maximum sunlight exposure. A thin wire meshing was fixed over the rooftop shells to prevent
translocation by wind or animals, and a HOBO® datalogger placed 10 cm away recorded local
temperature and light intensity beginning on April 19. Maximum daily temperatures frequently
exceeded 60oC after April (Table 2) and averaged < 40oC. Maximum daily light intensity
between 1.6 x 105 and 2.3 x 105 lux were common.
RESULTS
Fluvial vs. marine δ15N signals
To be able to interpret temporal δ15N shifts (pre-dam vs. modern Mulinia and Chione) in
terms of changes in riverine nutrient delivery, we first needed to establish whether fluvial and
marine δ15N signals are indeed distinct in the area of investigation. For this, we compared the
shells of Corbicula collected from portions of the lower Colorado River drainage to the shells of
modern estuarine clams, which have been subject to only marine conditions since the starvation
of river flow. Mean whole-shell δ15N values for Corbicula at each sampling site (Table 3) range
from 4.88 ‰ (Mitry Lake) to 12.51 ‰ (Lake Mead). Reasons for this extremely large range are
postulated in the discussion section. Each site mean is lower than the mean whole-shell δ15N
values obtained for modern Mulinia and Chione (13.87 ‰ and 14.77 ‰, respectively; Table 4).
Likewise, the overall location-based mean for Corbicula (9.55 ‰) is far lower than the marine
signal obtained from Mulinia and Chione. If each sampling location is not given an equal weight
and a mean is instead constructed from all Corbicula shells, regardless of site, the value falls
slightly to 9.04 ‰.
In addition, 2 unionid mussels, 1 from Mitry Lake and the other from a stretch of river at
the international border, record depleted δ15N signals (whole-shell values < 6 ‰), which further
illustrate that bivalves in the fluvial environments of the Colorado River are isotopically distinct
(lighter) from bivalves in the marine conditions of the modern Colorado River estuary.
Modern vs. Pre-dam δ15N signals
Whole-shell δ15N values for modern Mulinia exhibit a fair amount of variability (std .
dev. = 1.11) and range from 11.49 ‰ to 14.71 ‰. The overall whole-shell mean equals 13.87
‰. This differs slightly from the whole-shell mean of pre-dam Mulinia (12.14 ‰). The predam mean is pushed downward by shells D11 and D12, which possess anomalously low wholeshell values (7.85 ‰ and 7.96 ‰, respectively) compared to all other whole-shell data for
Mulinia and Chione. If these two values are removed, the pre-dam whole-shell mean becomes
13.22 ‰ and the remaining values show a standard deviation of 0.74. However, even using the
full dataset, pre-dam and modern populations of Mulinia do not show statistically significant
differences in whole-shell δ15N (1-way ANOVA, p = 0.406).
Whole-shell δ15N values for modern Chione range from 14.26 ‰ to 15.43 ‰ and are
fairly consistent (std. dev = 0.42). The overall whole-shell mean equals 14.77 ‰. Pre-dam
values are more variable (std. dev = 1.46) and range from 12.41 ‰ to 16.10 ‰. The whole-shell
mean is nearly identical to that of modern Chione, however, and there is no statistically
significant difference between the two populations (p = 0.760). Comparisons between species
(not across time) reveal statistically significant differences. Whole-shell δ15N values for Mulinia
9
were consistently lighter than for Chione (p= 0.006 for modern shells; p = 0.031 for pre-dam
shells)
Intra-shell δ15N variation
Table 5 displays multiple series of δ18O values obtained from drill samples of surface
carbonates in the shells of 3 pre-dam Mulinia (D10-D12), 3 pre-dam Chione (G1-G3), 1 modern
Mulinia (C4), and 1 modern Chione (B4). Pre-dam δ18O series include much more negative
values than modern δ18O series, reflecting the presence of Colorado River water (Dettman et al.
2004). The lowest values in each of the pre-dam bivalve profiles, highlighted in the Table 5 and
boxed in Fig. 3, are interpreted to represent the early summer growth period (approx. May
through July), when strong pulses of snowmelt produced most of the Colorado River’s annual
flow (Harding et al. 1995). Because the δ18O profiles of modern bivalves are not affected by
river flow, they record a simple temperature cycle. The lowest values mark shell growth during
the warmest part of the year (i.e. summer).
Once identified, the summer shell bands in both pre-dam and modern bivalves were
drilled for 15N measurements. In a few instances, marked differences in δ15N between summer
shell bands and whole-shell material are apparent, but these differences are not consistent in sign.
Although the summer-shell δ15N means reported in Table 4 for each bivalve population are less
than corresponding whole-shell means, they are based on fewer data points and may be biased
toward shells that, for whatever reason, have lighter isotopic values than the “average” bivalve
within the population. It is therefore more useful to turn to Table 6, which compares summershell δ15N values and whole-shell δ15N for individual bivalve specimens. These data show no
obvious relationship between the nitrogen isotopic signatures of whole-shell and summer-shell
material. In addition, there is no statistically significant difference between the pre-dam wholeshell values and pre-dam summer shell values for either species (Mulinia, p = 0.406; Chione, p =
0.760)
Interestingly, δ15N values obtained from shell material at the ventral margin are, with a
single exception, lighter than whole-shell values, regardless of bivalve species. In many cases,
there is a wide separation (> 3 ‰) between margin and whole-shell values, particularly in
modern Mulinia. There is a statistically significant difference between margin δ15N and wholeshell δ15N for modern Mulinia (p = 0.007) but not for modern Chione (p = 0.257).
Diagenesis experiment
δ15N values obtained from shells placed on a Tucson rooftop for approximately ½ year
were similar to δ15N values obtained from controls. Differences in isotopic signature were minor
and bi-directional in sign (Table 7, Fig. 4), suggesting that preferential degradation of specific N
species within the shell had not occurred. Data obtained from shells placed in the drying oven
appear to indicate a diagenetic shift toward lighter values, as the difference in δ15N between
controls and oven samples was, with only a single exception, always negative in sign. This
suggests that heating may be more important than sunlight exposure in terms of its diagenetic
effect on stable isotopes of N. The δ15N of pre-dam estuarine bivalve shells could have been
shifted toward lighter values due to heat-related diagenesis.
10
DISCUSSION
Fluvial vs. marine δ15N signals
Whole-shell δ15N values for Corbicula varied strongly between sampling locations; this
may reflect differences in denitrification rates, anthropogenic N inputs, and/or other
environmental factors. Yuma and Walter’s Camp are the only sites where Corbicula were
collected from the Colorado River proper (not a deep artificial lake or off-channel backwater
lake), and δ15N means from these locations (9.60 ‰ and 9.52 ‰, respectively) are nearly
identical. Island Lake, located between these two sites, produced shells with a similar signature
(9.88 ‰). Island Lake and Walter’s Camp are located in sparsely populated areas north of the
city and are not likely influenced by wastewater discharges or other anthropogenic N inputs. The
absence of an enriched δ15N signal in the Yuma shells suggests that the collection site is located
upstream from the city’s major effluent releases. δ15N values from these 3 sites may best
represent the pre-dam fluvial N signature.
High δ15N values from Lake Mead shells were not surprising, since the shells were
collected in the lake’s western basin, which receives wastewater from Las Vegas. Human wastes
(and other animal wastes) are enriched in 15N and generally have an isotopic composition
between +10 to +20 ‰ (Kendall 1998; McClelland et al. 1997). Denitrification in deep
sediments may have also contributed to an enriched δ15N signal. No major wastewater inputs to
Imperial Lake are known; intermediate δ15N values obtained from shells collected at this site
likely reflect isotopic enrichment from denitrification. The reasons for much lower δ15N values
in Mitry Lake shells are unclear.
It is notable that whole-shell δ15N means obtained from bivalves at each sampling
location in the lower Colorado River drainage – even locations with enriched δ15N signals due to
wastewater inputs and higher levels of denitrification – are less than whole-shell δ15N means
obtained from bivalves living in the modern Colorado River estuary. This supports the
assumption that riverine nutrients are isotopically distinct (lighter) from marine nutrients.
Interestingly, the mean whole-shell δ15N values for modern Mulinia and Chione differ by almost
1 ‰. Local habitat variation does not explain this difference because individuals of both species
were collected from the same sample site. This instead suggests species-specific differences in
feeding strategy. For example, one species may filter a specific size class of particles more
efficiently than the other.
Modern vs. Pre-dam δ15N signals
Riverine nutrients may be acquired directly or indirectly by estuarine bivalves. PON
generated in the river (freshwater phytoplankton, small zooplankton, and bacteria) may pass into
the estuary and be filtered out of suspension. Likewise, PON produced on land but transported
by the river (terrestrial detritus) may reach the estuary and be removed from the water column.
Dissolved inorganic nutrients (NO3-, NH4+) from the river cannot be acquired directly, but they
may be utilized by estuarine phytoplankton, which are then filtered by the bivalve. DON may be
obtained either directly (absorbed) or indirectly (first consumed by estuarine bacterioplankton or
phytoplankton). Given these several modes of riverine nutrient utilization and the strong
separation of fluvial and marine isotopic signatures discussed above, it is surprising to find an
absence of significant difference between whole-shell δ15N values in the modern and pre-dam
11
estuarine bivalve populations. The similarity of δ15N in modern and pre-dam shells presents
several possibilities:
1) Riverine nutrients were not a significant component of the total nutrient pool utilized
by estuarine bivalves;
2) Riverine nutrients represented a sizeable fraction of the total nutrient pool but lost
their distinctive (depleted) isotopic signature during biogeochemical transformation;
or
3) Depleted signals from riverine nutrients were recorded in the shell but
counterbalanced by processes leading to isotopic enrichment
1. “Riverine nutrients were insignificant”
Page & Lastra (2003) found no evidence from 15N and13C measurements on bivalve
stomach contents and muscle tissue that river-derived PON was a significant portion of bivalve
diets in the Ría de Arosa estuary of northwest Spain. δ15N values did not track distance from
riverine resources. This estuarine system, however, is marine-dominated and freshwater inputs
are small compared to tidal exchange. The pre-dam Colorado River estuary experienced much
stronger freshwater flows.
Unless strongly affected by anthropogenic N loading, most rivers do not contain high
concentrations of DIN, and a significant portion of what does reach the mouth can be intercepted
by coastal wetlands (Schlesinger 1997). The intercepted N may be assimilated into plant
biomass, recycled within the wetland, removed via sedimentation or denitrification, or exported
in dissolved and particulate fractions. N losses due to denitrification are equivalent to 40-50 %
of the DIN entering from rivers (Seitzinger 1988). Organic forms generally dominate the total
nitrogen load transported by rivers (Ittekot and Zhang 1989; Seitzinger & Sanders 1997; Scott et
al. 2007). Some particulate organics may settle in the wetlands as the velocity of surface waters
slows, but tidal action can be expected to mobilize some material and send it into the open waters
of the estuary. Overall, tidal flushing results in a net movement of nutrients away from coastal
marshes and toward the open estuary (Valiela and Teal 1979; Correll 1981; Nixon 1980). We
therefore assume that a significant quantity of nutrients were flushed into the pre-dam Colorado
River estuary. Moreover, because the wetlands in the delta were far more expansive than today,
it is likely that more organic N was produced within these environments and delivered to the
open estuary.
The presence of nutrients does not necessarily guarantee their biologic availability,
however. If too refractory, nutrients cannot be metabolized. Suspension-feeding bivalves
separate particles with low nutritional value, such as refractory detritus, and deposit them as
pseudofeces (Kiørboe and Mohlenberg 1981). According to Fry (1999), most riverine seston
entering the North Bay of San Francisco is refractory, and it comprises only a minor portion of
the diet of suspension feeders. Canuel et al. (1995) generalize that the pool of organic matter in
estuaries consists primarily of refractory components. In the Miya Estuary of Japan, for
example, 90% of PON originates from terrestrial materials and yet this fraction represents only
10% of the diet of resident bivalves (Ruditapes philippinarum and Mactra veneriformis) (Kasai
et al. 2004). Much of the DON that reaches estuarine environments is also refractory, and its
resistance to microbial attack favors its passage through coastal marshes and into the open waters
of the estuary (Valiela and Teal 1979). Unlike the labile, low molecular weight compounds (e.g.
12
amino acids and urea), which generally represent less than 20% of the riverine DON (Thurmann
1985), these components may not be readily utilized by the local biota.
2. “Riverine nutrients lost their depleted δ15N signatures”
Biogeochemical transformations of nitrogenous materials in estuaries and their adjacent
wetlands can exert strong influence over δ15N signals, sometimes to a greater extent than the
physical mixing of riverine and marine nutrient sources (Cifuentes et al. 1988; Sato et al. 2006)
δ15N may vary spatially and temporally within these environments due to changes in the
rate/amount of phytoplankton N uptake and microbially-mediated N conversions (i.e.
nitrification, denitrification) (Montoya et al. 1990). Isotopic fractionation during assimilation by
phytoplankton depends on N availability, the chemical form that is utilized, and the species of
phytoplankton (Waser et al. 1998), and the range of potential fractionation values is quite large
(Needoba and Harrison 2004; Waser et al. 1999). In the Sumida River Estuary of Japan,
seasonal variations in the δ15N of suspended PN were attributed to changes in the taxonomic
composition of phytoplankton (Sato et al. 2006). Denitrification in coastal marshes and estuarine
sediments is important not only because it removes some riverine nutrients (as discussed above)
but also because it raises the δ15N of residual N moving into the water column of the estuary
(Valiela and Teal 1979). Producers utilizing this 15N-enriched nutrient source then become
isotopically enriched themselves. Nitrification has the opposite effect – it creates a pool of 15Ndepleted nitrate – but nitrification and denitrification are often coupled processes, such that
nitrates generated via the former support the latter (Seitzinger 1988; Kemp et al. 1990). In
addition, nitrification has the effect of increasing the δ15N of residual ammonium. Because
phytoplankton preferentially utilize ammonium over other DIN fractions (Glilbert et al. 1982;
Dortch 1990; L’Helguen et al. 1996), this can mean that the enriched δ15N signal is transferred to
phytoplankton and upward to consumers. Mariotti et al. (1984) found that nitrification in the
Scheldt Estuary produced higher δ15NNH4 values downstream and argue that the isotopic
signature of suspended organic matter, which also becomes more enriched downstream, responds
to nitrification. If these various biogeochemical transformations cause a net increase in the δ15N
of DIN, and if primary producers utilizing this enriched nutrient source constitute a significant
fraction of consumer diets, local bivalves will record higher δ15N values than they would if no
biogeochemical alteration took place and only physical mixing of riverine and marine nutrient
sources mattered.
3. “Depleted signals were counterbalanced by other factors”
Any process or influence which raises the δ15N values of pre-dam shells or lowers the
δ N values of modern shells would reduce the expected temporal contrast in δ15N between the
two bivalve populations.
15
 Ontogeny
We assume that ontogenetic changes do not influence the nitrogen isotopic composition
of adult bivalves. Minagawa and Wada (1984) concluded that the δ15N of soft tissues in two
species of marine mussels, Septifer virgatus and Mytilus edulis, is independent of both age and
internal nitrogen mass balance – except during “early stages” of growth. Other studies have
13
noted δ15N offsets between juveniles and adults of the same species (Kang et al. 1999;
O’Donnell et al. 2003), perhaps due to the utilization of different food resources. However, all
isotope measurements in this study were performed on adult individuals.
 Anthropogenic nitrogen loading
Agricultural return flows and sewage effluent supply a very minor quantity of water and
nutrients to the modern Colorado River estuary. Animal wastes are also minimal. We do not
expect these inputs to influence the marine nutrient signal preserved in modern bivalve shells.
Any anthropogenic influences on nutrient types or levels in the pre-1930 estuary should likewise
have been small.
 Diagenesis
The organic matrix of bivalve shells is composed of a complex assemblage of proteins,
glycoproteins, polysaccharides, and lipids (Addadi 2006). Selective degradation of one of these
components, or a subset of one of these components (e.g. specific amino acids), could potentially
produce a shift in δ15N. Vallentyne (1969) measured differential decomposition rates for amino
acids in Plesitocene Mercenaria shells during pyrolization at 174-252oC. He observed the
following order of amino acid stability (from least to greatest): tyrosine < phenylalanine <
isoleucine < leucine < proline < valine < glycine < alanine < glutamic acid. Risk et al. (1997), in
a nuclear magnetic resonance spectral analysis of modern and fossil bivalve shells (Mya
truncata, Hiatella arctica, Macoma baltica, and Mya arenia), found that Chitin, a
polysaccharide, undergoes breakdown early in diagenesis relative to other components, and
analyses on Mya truncata suggested selective degradation of some amino acids over others.
Spatial heterogeneity in the distribution of nitrogenous compounds may also lead to diagenetic
δ15N shifts if degradation processes favor one region of the shell over another. Goodfriend et al.
(1997) found that the concentration of amino acids in each of the 3 structural layers of Ch.
fluctifraga differed, with the middle layer containing the lowest concentration and the inner layer
containing the highest. In addition, diagenesis may occur more rapidly in thin shells than in
thick shells (Risk et al. 1997). Mulinia have much thinner shells than Chione, but the data
obtained in our diagenesis experiment do not appear to indicate a more pronounced isotopic shift
in one species relative to the other.
Our data do suggest that diagenetic alteration of δ15N signals in the shells of pre-dam
Mulinia and Chione is possible and that such alteration is more likely attributable to warm
temperatures rather than sunlight exposure. Shells heated in a drying oven at 60oC for a half year
are characterized by lower δ15N values than controls, while shells resting upon a sunlit rooftop
for the same period showed no obvious directional shift in δ15N values relative to controls.
However, δ15N values obtained from shell M4 (Corbicula) may indicate diagenetic effects from
sunlight exposure. The shell was collected from a paleoshoreline of Lake Mead. One valve,
fully buried in sediment, retained all of its periostracum while the other valve, partly exposed,
had lost most of its periostracum, thus suggesting differential degradation of the two valves.
After cleaning in the laboratory and complete removal of the periostracum, each valve was
separately ground into a powder and homogenized. The δ15N value obtained from the exposed
valve was over 1 ‰ lighter than the value obtained from the buried valve. Pre-dam Mulinia and
Chione shells were collected from below 25 cm depth and were completely buried by other
14
shells. Differential sunlight exposure would not have occurred and total exposure prior to burial
was probably minimal.
We observed that more shell material was needed for pre-dam shells than for modern
shells to obtain precise δ15N measurements on the mass spectrometer. This implies nitrogen loss
from the shell with age. Similarly, O’Donnell et al. (2003) found a 50-70% decrease in the
concentrations of both carbon and nitrogen in Pleistocene Mercenaria shells relative to modern
shells. They warn that this reduction could amplify the influence of any contaminants.
Thus our results confirm the possibility that this N loss bears an isotopic effect and
suggest that prolonged heating can lead to lighter δ15N values, but it is not possible to ascertain
based on available data whether the δ15N signatures of the pre-dam bivalves sampled in this
study were, in fact, altered by diagenesis. If diagenetic effects were significant, they would have
likely lowered δ15N values and increased the isotopic contrast between pre-dam and modern
shells. Because we observe little to no temporal contrast in δ15N values in the first place,
diagenesis cannot be used to explain the “missing” riverine signal.
Intra-shell δ15N variation
In the absence of strong evidence from whole-shell δ15N measurements for isotopically
distinct pre-dam and modern bivalve populations, we sought to identify whether summer shell
deposits recorded unique δ15N values that were masked by whole-shell averaging. Most of the
Colorado River’s annual flow occurs between May and July due to strong pulses of snowmelt in
the upper basin (Harding et al. 1995). For the San Pedro River, an arid-zone tributary to the
Colorado River, Brooks et al. (2007) showed that large flood pulses following summer monsoon
storms can produce substantial increases in the flux and concentration of both dissolved and
particulate organic matter transported downstream. In a subestuary of Chesapeake Bay, Correll
(1981) found that increased exports of total nitrogen from intertidal marshes into the estuarine
basin were synchronous with periods of high flow from the Rhode River. The period of greatest
discharge of the pre-dam Colorado River may therefore be expected to bring the greatest quantity
of riverine nutrients to the estuary relative to marine sources.
Whole-shell δ15N values for pre-dam bivalves represent the average of all organic
material deposited in the shell over the lifetime of the individuals, during flood periods and lowflow periods alike. Because a greater total quantity of shell material is deposited during lowflow periods, whole-shell averages may obscure the brief utilization of low-δ15N nutrients during
the flood season. Given this possibility, we constrained the portions of bivalve shells deposited
during peak river flow and measured the δ15N organic material within these specific shell zones.
Whole-shell and flood-pulse values in Chione, for both pre-dam and modern shells, are fairly
similar (< 1‰ separation). Results from pre-dam Mulinia show variable isotopic separation
between whole-shell values and summer values and do not necessarily indicate greater riverine
nutrient utilization during flood periods. In the pre-dam Mulinia shell D10, a surprisingly large
isotopic separation (4.2 ‰) was observed between the whole-shell value and summer value.
Because the summer value is lower, it may reflect the incorporation of riverine nutrients.
However, in shells D11 and D12, measured whole-shell δ15N values are more depleted relative to
flood-pulse values. This is difficult to interpret, particularly since the whole-shell values are
anomalously low compared to all other whole-shell values obtained from both modern and predam Mulinia. The low whole-shell values, regardless of how they compare to flood-pulse
values, suggest that riverine nutrients reached the bivalves for the majority of their lives. Did
15
these two shells grow during exceptionally wet years? δ18O profiles for D11 and D12 carbonates
do not seem to support this interpretation, as they do not differ substantially from the δ18O profile
for D10, which has a more “typical”δ15N value. However, more pronounced seasonal minima in
18
O are apparent for D11 and D12. Taken together, the results from these 3 shells are
ambiguous. Measurements on additional pre-dam Mulinia shells may reveal a significant trend
that is not apparent from our limited data. Our sample size is too small to reveal the true
complexity and variability of this dynamic ecosystem.
A single modern Mulinia shell (C4) was also examined and showed negligible isotopic
separation between whole-shell and summer values, which is what we expected for the present
no-flow environment.
The absence of a clear riverine nutrient signal in the summer growth bands of the
sampled shells may be due to slow rates of tissue turnover. Because the shell organic matrix is
secreted by mantle epithelia (Wilbur 1964; Neff 1972), its isotopic composition may be subject
to the same temporal averaging as internal soft tissues. Table 8 summarizes published rates of
tissue turnover in bivalves (Unfortunately, the turnover rates of mantle tissue have not been
investigated.) These long turnover rates imply that shell δ15N does not record transient
conditions but rather long-term average conditions. It may therefore be unreasonable to expect
shell organics deposited during flood periods to bear distinctly lower δ15N signatures. This also
suggests that fine-resolution sampling of changes in diet (and of nutrient source) may not be
possible. Moreover, δ15N measurements (unlike δ13C and δ18O) require large quantities of shell
material, often upwards of 30 mg, which precludes sampling within very narrow growth bands.
In northeastern freshwater lakes, McKinney et al. (2002) found that the δ15N of
Unionidae mussel (Elliptio spp.) tissues correlated with long-term nitrate concentrations but not
with nitrate concentrations measured on a single sampling date. This, they argue, suggests that
mussels act as long-term integrators of the δ15N of primary producers and that they do not record
transient shifts. Gustafson et al. (2007) also assert that bivalve tissues cannot be used as proxies
for pulse events or rapid changes in DIN loads. On the other hand, some studies have shown
significant short-term changes in the δ15N of bivalve soft tissues (Goering et al. 1990; Fry and
Allen 2003; Piola et al. 2006). Our own data from other portions of bivalve shells seem to
suggest sub-annual δ15N changes. δ15N values obtained from the ventral margins of the bivalve
shells seem to be lighter than whole-shell δ15N values in most instances. Because all estuarine
bivalve samples were collected in late November or early December, shell margin material –
which represents the most recent growth – was likely deposited just prior to winter growth
shutdown (Mulinia and Chione generally cease to grow for several months during winter because
of low temperatures (Goodwin et al. 2001). River flow is not substantial at this time of year,
suggesting that the shift to lighter δ15N is controlled by factors unrelated to freshwater flow.
If the deposition of organic material in bivalve shells responds to transient conditions,
sub-annual changes in δ15N may reflect seasonal variability in the size of nutrient pools and the
rates of biogeochemical transformations in the estuary and surrounding wetlands (e.g. Kaplan et
al. 1979; Cifuentes et al. 1988; Sato et al. 2006). Reduced denitrification in winter may help to
explain lower margin δ15N values and, in some cases, higher summer values (where they occur).
Rates of denitrification vary as a function of temperature (Jorgensen 1989; Kaplan et al. 1979).
In the Tama estuary of Japan, for example, Nishio et al. (1983) found that denitrification rates
increased by a factor of four between December (9.3oC) and May (18.0oC). Reduction of
dissolved oxygen availability in summer may also promote increased denitrification during
summer. York et al. (2007) noted higher δ15N of ammonia during summer in the Waquoit Bay
16
estuary of Massachusetts, which they attribute to more rapid biologic processing of the nutrient;
they also found higher δ15N of phytoplankton. In the Scheldt estuary of northen Europe, Mariotti
et al. (1984) measured sharp increases in the δ15N of suspended PON. Whereas in winter, the
estuary showed a clear continental-to-marine gradient in the δ15N (most suspended PON during
this season was from the mixing of terrestrial and marine sources), summer δ15N values were
high in both the upper and lower estuary (due to strong authochthonous production that utilized
enriched ammonium generated by nitrification). Seasonal differences in circulation and
upwelling in the Gulf of California are probably unimportant. The northern Gulf is shallow (<
200m) and partially separated from the main gulf by a sill and midrift islands (Santamaría-delÁngel et al. 1994; Pagau et al. 2002). The extent to which southern currents and upwelling
nutrients seasonally affect the northern Gulf is unclear.
SUMMARY
A distinct difference between the whole-shell δ15N signatures of freshwater bivalves and
their estuarine-marine counterparts suggests that bivalve shells can be used to determine
historical nutrient sources and changes in the availability of a particular class of nutrients over
time. However, we found no clear evidence of a riverine nutrient signal in the shells of bivalves
that lived in the pre-dam Colorado River estuary. It may be that riverine nutrients were not a
significant component of the total nutrient pool in the pre-dam estuary. Alternatively,
biogeochemical modification of riverine nutrients in the estuarine environment and its fringing
marshes could have diminished the isotopically-distinct (depleted) riverine nutrient signal. Other
processes that might “mask” a riverine nutrient signal (by raising δ15N) are probably
insignificant. We determined that heat-related diagenesis can reduce the δ15N of older shells, but
a diagenetic shift toward lighter δ15N values would only lead to overestimation of riverine
nutrient utilization. Because we find no clear evidence of a riverine nutrient signal, the
possibility of diagenesis did not influence our analysis. The results of this study do not support
the use of whole-shell δ15N signatures in estuarine bivalves as an indicator of riverine nutrient
availability. However, observed sub-annual differences in shell δ15N raise the possibility of
detecting short-term changes in estuarine conditions. Better understanding of intrashell δ15N
variability, tissue turnover time, and the relationship between soft tissues and shell organics is
needed.
ACKNOWLEDGEMENTS
We thank Majie Fan and Chris Eastoe for processing our sample materials in the
Environmental Isotope Laboratory and for providing helpful suggestions on sample preparation.
We are also grateful to the fishermen of El Golfo de Santa Clara for transporting us to clam
collection sites in the Colorado River Delta. The Smithsonian Institute provided an 1880s
unionid mussel collected from the lower Colorado River, on which we conducted δ15N
measurements. This research project was supported by the National Science Foundation (Grant
No. 044381, awarded to KWF) and the Colorado River Delta Research Coordination Network.
RDD received an Exxon Scholarship and Geosciences General Scholarship (University of
Arizona) and was provided additional support through the Bert S. Butler Fund and Charles
Evenson Fund.
17
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24
Table 1. δ15N values reported for suspension-feeding bivalve mollusks in various environments
LOCATION
ENVIRONMENT
SPECIES
TISSUE
low
Coringa Wildlife
Sanctuary and
Kakinada Bay, Andra
Pradesh, India
San Francisco Bay,
USA
Mangrove creeks and
estuary
Creek
Creek
Creek
Creek
Bay
Bay
Bay
Bay
Bay
Bay
Bay
Salt marsh
Meretrix meretrix
Tellina spp.
Macoma spp.
Placuna placenta
All bivalves - creek
Meretrix meretrix
Tellina spp.
Macoma spp.
Placuna placenta
Paphia undulata
Pinctada radiata
Siliqua albida
All bivalves - bay
Stomach content
Stomach content
Stomach content
Stomach content
Macoma petalum
Whole body
Stomach content
Stomach content
Stomach content
Stomach content
Stomach content
Stomach content
Stomach content
8.1
6.2
8.5
8.7
6.2
9.5
6.8
8.9
9.3
7.7
11.6
7.6
7.6
δ15N
high mean
8.1
8.0
8.5
9.1
9.1
12.0
9.3
8.9
9.5
12.9
11.6
7.6
12.9
8.1
7.2
8.5
8.9
7.6
9.9
7.7
8.9
9.4
9.7
11.6
7.6
9.0
14-15
Comment
Source
Limited utilization of mangrove and
terrestrial sources. Local algal sources
of OM are dominant portion of diet.
δ15N of tissues from bivalves in creek < Bouillon et al. (2002)
bay; ~3.2 permil gradient between the
environments for bivalves and other
benthic invertebrates
No significant differences in δ15N
between salt marsh habitats
Brusati and Grosholtz
(2007)
Estuary
Upper ARE Rangea cuneata (93%)
Middle ARE and Corbicula fluminea
Lower ARE (7%)
All bivalves - ARE
Upper NRE Rangea cuneata (93%)
Middle NRE and Corbicula fluminea
Lower NRE (7%)
All bivalves - NRE
Neuse River Estuary
(NRE) and Alligator
River Estuary (ARE),
North Carolina, USA
Various
Various
Lampsilis sp., Anodonta
Ontario and Quebec,
Freshwater lakes
Canada
sp., Elliptio spp.
Lake Oneida, USA
Freshwater lakes Dreissena polymorpha
Assiminea lutea,
Gamo Lagoon,
Nuttalia olivacea,
Nanakita River
Estuary
Corbicula japonica,
Estuary, Japan
Laternula limicola
Estuary
Pleasant Bay, Cape
Multiple subestuaries Mercenaria mercenaria
Cod, USA
Estuary
Modiolus demissus
Plum Island Sound,
Middle estuary Crassostrea virginica
Massachusetts, USA
Mya arenia
Lower estuary Mya arenia
Antarctic Peninsula,
near Anvers Island
Laternula elliptica
Nearshore marine
Yoldia eightsi
Foot
Foot
Foot
7.4
6.3
6.1
6.4
10.7
10.9
9.3
10.4
Foot
Foot
Foot
?
1.2
9
?
~8
?
~9.5
Whole body
Muscle
Muscle
Muscle
Muscle
Muscle or "body wall
tissue"
Muscle or "body wall
tissue"
8
High variability. δ15N values correlate
Cabana and
with degree of anthropogenic N loading
Rasmussen (1996)
and human population within watershed
δ15N values correlate with %
wastewater
~6.5 ~10.5
7.1
δ15N values of bivalves from NRE >
ARE; POM follows same trend. Reflects
higher wastewater and animal waste N Bucci et al. (2007)
in Neuse. δ15N decline toward sea due
to high-δ15N N loading upstream
7.8
6.5
8.3
8
Carmichael et al. (2004)
Deegan and Garritt
5.7
Dunton (2001)
6.5
Estuary
North Bay
San Francisco Bay,
USA
South Bay - spring
bloom 1990
South Bay - spring
bloom 1991
Potamorcorbula
amurensis
Potamorcorbula
amurensis
Potamorcorbula
amurensis
Whole (minus gut
contents)
Whole (minus gut
contents)
Whole (minus gut
contents)
10.7
δ15N values elevated in South Bay
11.5-11.9
12
relative to North Bay due to stronger
Fry (1999)
14.3 anthropogenic N loading. Values within
North Bay higher upstream due to
13 anthropogenic inputs
River
Baton Rouge Dreissena polymorpha
Illinois Dreissena polymorpha
Mississippi River,
USA
Whole body
Whole body
11
10
16
16
Auke Bay, Alaska,
USA
Macoma nasuta
Saxidomus giganteus
Yoldia spp.
Whole body
Whole body
Whole body
6.1
8.6
Estuary
Piedmont region,
North Carolina, USA
Freshwater streams
Elliptio complanata
Foot
4.9
9.1
Northeast Atlantic
Ocean (Irish Sea,
English Channel,
North Sea)
Offshore marine
Aequipecten opercularis Adductor muscle
4.18
11
7.6
9.3
8.5
11.6
10.3
11.5
Marennes-Oléron Bay,
Estuary
France
Certastoderma edule
Whole body
Estuary
Kushida Estuary,
Japan
Japan
Upper estuary Corbicula japonica
Lower estuary Corbicula japonica
Coastal brackish lakes
Lake Jusan
Upstream
Downstream
Lake Ogawara
Upstream
Downstream
Lake Shinji
Upstream
Downstream
Miya Estuary, Ise Bay,
Estuary
Japan
Rhode Island, USA
Bay of Brest, France
Freshwater lakes and
ponds
Marine
Corbicula japonica
Corbicula japonica
Corbicula japonica
Corbicula japonica
Corbicula japonica
Corbicula japonica
Corbicula japonica
Corbicula japonica
Corbicula japonica
All bivalves
Foot
Foot
Foot
Foot
Foot
Foot
Foot
Foot
Foot
Foot
Foot
Ruditapes philippinarum Muscle
Muscle
Mactra veneriformis
Elliptio complanata
?
Pecten maximus
Pecten maximus
Adductor muscle
Gonad
11.7
10.2
8.5
11.7
9.9
10.2
10.6
11.2
4.9
13.3
Seasonal changes as large as 5 ‰ (in
Baton Rouge); max values at end of
summer phytoplankton bloom and low
in early spring; spatial differences due
to anthropogenic loading
Fry and Allen (2003)
7.6
Significant seasonal differences in δ15N
Goering et al. (1990)
9.1
of Macoma
7.6
Bivalve δ15N correlate with long-term
7.5
Gustafson et al. (2007)
means of dissolved nitrate δ15N
Jennings and Warr
(2003)
7.45
Relative importance of
8.4 microphytoplankton vs. algal POM
Kang et al. (1999)
inferred from δ15N
Enriched values in lower estuary reflect
Kasai and Nakata
8.6 greater % marine POM; depleted
11.8 values in upper estuary reflect greater (2005)
% terrestrial POM
10.4
9
11
11
11.1
10.9
10.9
10.6
11.4
10.8
Gradient in δ15N between river mouth
and downstream areas in Lake Jusan
(increasing toward ocean); less
apparent in other lakes
Kasai et al. (2006)
High % terrestrial POM but little
Kasai et al. (2004)
10.1
utilization by the bivalves
10.5
Elevated δ15N correlate with a higher %
8.4 residential development & wastewater Lake et al. (2001)
N
9.4
7.8 Significant separation in δ15N of
Lorrain et al. (2002)
Bay of Brest, France
Marine
Pecten maximus
Pecten maximus
Digestive gland
Whole body (est.)
Estuary
Childs River Estuary
Sage Lot Pond Estuary
Mya arenia
Mya arenia
Whole body
Whole body
Narragansett Bay,
Rhode Island, USA
22 Salt marshes
Geukensia demissa
Whole body
Rhode Island
19 Freshwater lakes
Elliptio spp.
Waquoit Bay, Cape
Cod, Massachusetts,
USA
Rhode Island
Usujiri coast,
Hokkaido, Japan
Freshwater ponds, lakes,
reservoirs
Quidnick Reservoir Elliptio spp.
Elliptio spp.
Elliptio spp.
Elliptio spp.
Tucker Pond Elliptio spp.
Elliptio spp.
Elliptio spp.
Elliptio spp.
Browning Mill Pond Elliptio spp.
Elliptio spp.
Elliptio spp.
Elliptio spp.
Spring Lake Elliptio spp.
Elliptio spp.
Elliptio spp.
Elliptio spp.
Mishnock Lake Elliptio spp.
Elliptio spp.
Elliptio spp.
Elliptio spp.
Tiogue Lake Elliptio spp.
Elliptio spp.
Elliptio spp.
Elliptio spp.
All bodies Elliptio spp.
Elliptio spp.
Elliptio spp.
Septifer virgatus
Marine
Mytilus edulis
Marine
Assateague Channel,
Mercenaria mercenaria
Chincoteague, VA
Mercenaria mercenaria
Mercenaria mercenaria
Mercenaria mercenaria
Mercenaria mercenaria
Mercenaria mercenaria
Mercenaria
Cedar Key, FL
campechiensis
Gomez Pit, Virginia
Mercenaria mercenaria
Beach, VA
Atlantic coastal plain
Mercenaria mercenaria
Mercenaria
campechiensis
Bass site, North Carolina
Mercenaria mercenaria
shelf
Mark Clark Pit,
Mercenaria mercenaria
Charleston, SC
Jones site, Skidaway
Mercenaria mercenaria
Island, Georgia
Ria de Arosa, Spain
Estuary
Northern Argentina
Coastal marine
Great Sippewissett
Marsh, Falmouth,
Massachusetts, USA
Salt marsh
Manning River
Estuary, New South
Wales, Australia
Estuary
All sites Mercenaria mercenaria
Cerastoderma edule
Cerastoderma edule
Tapes decussatus
Tapes decussatus
Mytilus galloprovincialis
Mytilus galloprovincialis
Amiantis purpurata
Solen tehuelchus
12.1
Foot tissue
4.9
12.6
Adductor muscle
Foot tissue
Mantle tissue
Mean of 3 tissues
Adductor muscle
Foot tissue
Mantle tissue
Mean of 3 tissues
Adductor muscle
Foot tissue
Mantle tissue
Mean of 3 tissues
Adductor muscle
Foot tissue
Mantle tissue
Mean of 3 tissues
Adductor muscle
Foot tissue
Mantle tissue
Mean of 3 tissues
Adductor muscle
Foot tissue
Mantle tissue
Mean of 3 tissues
All muscle
All foot
All mantle
Whole body
Whole body
6.2
4.7
4.3
6.5
5.1
4.8
6
5.1
5
6.5
5.2
5.3
6.9
5.9
6
7.2
6.7
6.3
8.8
8.2
8.2
8.9
8.3
9
9.9
8.6
8.3
10.6
9.6
9.3
13.1
11.7
11.2
13.4
11.9
11.6
Modern shell (Inner &
middle layers, ventral
margin)
Modern shell (ventral
margin)
Foot
Mantle
Muscle
All soft tissue
Modern shell (middle
layer)
Fossil shell (Late
Pleistocene; inner &
middle layers, ventral
margin)
Fossil shell (Mid
Pleistocene; inner &
middle layers, ventral
margin)
Fossil shell (Early
Pleistocene; inner &
middle layers, ventral
margin)
Fossil shell (Holocene;
inner & middle layers,
ventral margin)
Fossil shell (Late
Pleistocene; middle
layer)
Fossil shell (Late
Pleistocene; middle
layer)
All shells
Stomach
Muscle
Stomach
Muscle
Stomach
Muscle
Stomach content
Stomach content
Geukensia demissa
Muscle
Saccostrea glomerata
Saccostrea glomerata
Adductor muscle
Ctenidia
Viscera (= whole body ctenidia & adductor
muscle)
Whole body
Saccostrea glomerata
Saccostrea glomerata
Estuary
9
11
13.7
10.1
11.6
7.2
9.9
4.5
13.2
7.2
9.3
6.5
9.3
3.7
9.8
3.8
4.5
7.2
3.7
7.2
13.7
Significant separation in δ N of
6 different tissues
8.5
Bivalves in Childs River Estuary
~7.9 enriched relative to Sage Lot Pond
~5.6 Estuary due to stronger wastewater
influence
Bivalves with diets dominated by
terrestrial and marsh sources show
depleted δ15N; where marine sources
dominate = enriched δ15N. Values
correlate with land use practices
Correlation with % residential
development and often with dissolved
nitrate concentration
6.4
4.9
4.6
5.3
6.3
5.2
5.1
5.5
7.1
6.3
6.2
6.5
8.8
8.2
8.5
8.5
10.3
9.2
8.9
9.5
13.2
11.9
11.4
12.2
8.7
7.6
7.5
9
8.7
~8.5
8.5
6.9
10.4
10.1
4.6
2.9
8.7
10.1
McClelland et al. (1997)
McKinney et al. (2001)
McKinney et al. (2002)
Adductor muscle consistently enriched
relative to other tissues. All tissues
enriched in locations with stronger
anthropogenic N loading
McKinney et al. (1999)
δ15N independent of age and nitrogen
mass balance in bivalve body
Minagawa and Wada
(1984)
Very large intrashell variation. Mean
separation between tissue and shell
material at the ventral margin = 0.7 ‰
O'Donnel et al (2003)
10.6
11.5
11
12.2
11.3
9.5
10.7
10.1
11.3
9.6
10.8
12.3
10.9
~5.5
Lorrain et al. (2002)
Muscle tissue enriched relative to
stomach tissue. Spatial pattern in δ15N
values from multiple stations does not Page and Lastra (2003)
suggest utilization of riverine N
(freshwater inputs small relative to tidal
exchange)
Values suggestive of low trophic
Penschaszadeh (2006)
position
Generally lower δ15N values for
individuals from innermost marsh,
increasing toward the ocean
Peterson et al. (1985)
Bivalves transplanted from Wallis Lake,
New South Wales. Measured after 3
mo. δ15N records position in estuary
Piola et al. (2006)
(enriched seaward) and sewage outfall.
Suggests seasonal information can be
6.8 recorded in tissue.
Passeonkquis Cove
(Upper Bay) Geukensia demissa
Prudence Island (Middle
Bay) Geukensia demissa
Fox Hill Salt Marsh
(Lower Bay) Geukensia demissa
Sphaerium striatinum
Pleurobema sintoxia +
Eagle Creek,
Freshwater creek
Fusconaia flava
Michigan, USA
Pleurobema sintoxia +
Fusconaia flava
Estuary
Oosterschelde Estuary Cerastoderma edule
Oosterschelde Estuary Crassostrea gigas
Scheldt Estuary,
Oosterschelde Estuary Mytilus edulis
Netherlands
Westerschelde Estuary Cerastoderma edule
Westerschelde Estuary Crassostrea gigas
Westerschelde Estuary Mytilus edulis
Estuary
Port Neuf (freshwater
end) Crassostrea gigas
Marennes-Oléron Bay,
Fort Lupin Crassostrea gigas
France
Les Palles Crassostrea gigas
Chassiron (marine end) Crassostrea gigas
Les Baleines (marine
end) Crassostrea gigas
Corbicula fluminea
Unknown
Freshwater?
Corbicula fluminea
Naragansett Bay,
Rhode Island, USA
Elands Bay, SW Cape
Marine
of South Africa
Estuary
Waquoit Bay,
Sage Lot Pond Estuary
Massachusetts, USA
Quashnet River Estuary
Childs River Estuary
Mouth of Shirakawa
River, Ariake Sound,
Western Kyushu,
Japan
Estuary
Whole body
10.4
11.6
Whole body
10.7
11.2
Whole body
Whole body
10.0
10.2
10.1
4.4
Muscle
4.8
Stomach
3.3
Whole body
Whole body
Whole body
Whole body
Whole body
Whole body
10.4
10.1
10.9
18.7
19.6
~18
Whole body
Whole body
Whole body
Whole body
11.5
10
9
8.3
Whole body
Whole body?
Whole body?
Chromytilus meridionalis ?
Geukensia demissa
Geukensia demissa
Geukensia demissa
Mactra veneriformis
(juvenile)
Ruditapes philippinarum
(juvenile)
Low intra-annual and minor interannual
variability. Significant spatial trend
Pruell et al. (2006)
(depleted oceanward) that likely relates
10.9
to sewage inputs
11.0
Dominance of deposit feeding (vs.
suspension feeding) inferred
Raikow and Hamilton
(2001)
Elevated tissue δ15N in Westerschelde
Estuary, which is more affected by
urbanization. Utilization of
Riera et al. (2000)
riverine/terrestrial material is low while
utilization of 15N-enriched (from
wastewater) benthic algae is high
POM more enriched seaward, yet
oysters show opposite trend. Suggests
that oysters at head of estuary at higher
Riera (1998)
trophic level (not feeding directly on
terrestrial detritus but perhaps on
bacterial intermediary).
8.8
6.7 Tissue preservation (formalin, EtOH) on
Sarakinos et al. (2002)
6.3 archived specimens found to not
produce δ15N shift > 0.5 ‰
8.1
8.9
8.46
Sealy et al. (1987)
15
Whole body
Whole body
Whole body
Both bivalve and POM δ N track the
7.4
level of anthropogenic N
8.4
(Childs>Quashnet>Sage)
9.3
Whole body
8.2
Whole body
8.5
Yelenik et al. (1996)
Yokoyama et al. (2005)
Las Isletas
Fig. 1. Study area. (a) Regional map of Baja, Mexico and Gulf of
California. (b) Colorado River Delta and Upper Gulf of California, with
bivalve collection sites identified by arrows. Adapted from Rodriguez et al.
(2001a).
Ground &
homogenized for
whole-shell δ15N
average
(Umbo)
Summer growth
bands
Ventral margin
(most recent
growth)
Fig. 2. Sampling scheme for whole-shell, margin, and
summer shell powder
Spot samples
for δ18O
measurement
Table 2. Rooftop conditions during diagenesis experiment.
Max temp Min temp Avg temp Max light
(oC)
(oC)
(oC)
Month
(Lux)
April (19th-)
58
7
28
231468
May
65
10
34
231468
June
67
15
39
220446
July
67
20
37
209424
August
63
21
37
165334
Sept (-29th)
62
15
34
159823
Table 3. Mean whole-shell δ15N of Corbicula
collected from the Lower Colorado R. drainage
Collection site
Mitry Lake
Imperial Lake
Island Lake
Lake Mead
Yuma, AZ
Walter's Camp
Mean
Location
ID
A
F
L
M
Y
W
δ15N (‰)
4.88
10.93
9.88
12.51
9.60
9.52
9.55
Table 4. All δ15N values obtained from the shells of bivalves collected from the Colorado River estuary and the lower Colorado River drainage basin
Lower Colorado River
Lower Colorado River
Coribcula fluminea
Unionid mussels
15
Shell ID
Shell area δ N (‰)
Shell ID
Shell area δ15N (‰)
margin
2.73, 3.43 S1
whole
5.77
A1
whole
4.81 U1
whole
5.59
margin
5.18
A2
whole
3.99
margin
3.15
A3
whole
4.81
margin
2.12
A4
whole
4.58
AX1-control whole
4.96
AX2-control whole
6.11
FX1-control whole
10.90
FX2-control whole
10.97
L1
whole
9.03
L2
whole
10.73
W1
whole
9.83
W2
whole
9.22
Y1
whole
9.48
Y2
whole
9.72
M1
whole
11.18
M2
whole
13.50
M3
whole
12.58
12.11a
whole
M4
13.18b
whole
MEAN
MEAN§
MEAN$
margin
whole
whole
3.38 MEAN
8.61
9.55
a = Value from buried shell valve
b = Value from exposed shell valve
* = Value based on cross-sectional slice
§ = Based on all whole-shell values
$ = Based on location means (A, F, L, P, W, Y, M)
whole
Colorado River Delta
Chione fluctifraga (modern)
Shell ID
Shell area δ15N (‰)
margin
9.48
B1
whole
14.91*
B2
whole
14.26*
margin
9.39, 10.37
B3
whole
15.03*
margin
5.96
B4
whole
14.47*
summer
13.52
BX1-control whole
15.43*
BX2-control whole
14.43
BX3-control whole
15.08*
5.68 MEAN
MEAN
MEAN
margin
whole
summer
Colorado River Delta
Chione fluctifraga (pre-dam)
Shell ID Shell area δ15N (‰)
whole
12.41
G1
early summer
11.87
whole
15.39
G2
early summer
15.59
whole
12.84
G3
early summer
12.26
G4
whole
14.90
G5
whole
14.72
G6
whole
16.00
G7
whole
16.10
8.44 MEAN
14.77 MEAN
13.52
whole
early summer
Colorado River Delta
Mulinia coloradoensis (modern)
Shell ID
Shell area δ15N (‰)
margin
11.46
C1
whole
11.49
margin
11.84
C2
whole
12.32
margin
9.08, 10.94
C3
whole
13.40
margin
13.28
C4
whole
13.56
summer
13.22
CX1-control whole
14.36
CX2-control whole
13.31
CX3-control whole
14.71
14.62 MEAN
13.24 MEAN
MEAN
margin
whole
summer
Colorado River Delta
Mulinia coloradoensis (pre-dam)
Shell ID
Shell area
δ15N (‰)
D1
whole
14.11
D5
whole
14.31
D6
whole
13.08
D7
whole
12.20
D8
whole
13.07
D9
whole
12.82
whole
12.95
D10
early summer
8.76
7.74*,
whole
D11
7.89*, 9.63
early summer
9.04
whole
7.96*
D12
early summer 9.21, 9.71
11.58 MEAN
13.87 MEAN
13.22
whole
early summer
12.14
9.09
Table 5. δ18O values obtained from shell carbonate. Gray = shell growth during the early summer flood pulse (predam); Yellow = summer shell growth (modern)
Pre-dam Mulinia
Drill sample
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
Early summer 1 (predicted)
Early summer 1 (sampled)
Early summer 2 (predicted)
Early summer 2 (sampled)
Shell D10 Shell D11 Shell D12
δ18O
δ18O
δ18O
-1.68
-1.68
-3.90
-3.11
-3.65
-4.19
-3.63
-4.44
-4.55
-3.49
-4.96
-3.79
-3.23
-4.30
-1.29
-3.50
-4.20
-1.39
-3.74
-2.84
-1.19
-3.42
-3.91
-2.33
-3.92
-1.88
-3.28
-4.67
-1.91
-3.55
-4.27
-1.90
-5.38
-4.79
-1.70
-5.02
-4.53
-2.45
-3.16
-3.43
-2.28
no data
-3.62
-2.91
-3.26
-3.12
-3.39
-3.29
-2.39
-3.04
-2.38
-2.02
-4.08
-2.50
-2.37
-1.00
-2.56
-0.76
-4.15
-4.13
-4.31
-4.73
-4.11
-4.13
-4.08
-4.28
Modern
Modern
Pre-dam Chione
Mulinia
Chione
Shell C4 Shell G1 Shell G2 Shell G3 Shell B4
δ18O
δ18O
δ18O
δ18O
δ18O
-1.43
0.03
0.48
0.86
-0.71
-1.24
0.13
0.05
0.56
-1.38
-1.56
-0.34
0.43
0.24
-1.52
-1.65
-0.43
0.61
0.56
-1.72
-0.37
-0.70
0.65
0.48
-1.68
1.89
-2.28
no data
0.04
-2.14
1.21
-2.72
no data
-0.05
-2.24
1.74
-3.43
0.78
0.18
-2.15
1.69
-2.89
0.35
-0.65
-2.30
1.19
-4.78
0.30
-2.23
-2.47
0.87
-5.18
0.23
-3.18
-2.16
0.30
-3.18
-0.75
-3.03
-0.41
-1.58
-3.43
-4.44
-1.08
-1.41
-2.01
-4.16
-1.64
-1.09
0.83
-2.94
-0.70
-0.75
-2.32
-0.20
-2.24
-0.78
0.12
-3.43
0.44
-1.19
-2.60
0.68
-1.29
-2.53
0.70
-1.43
-4.86
-5.32
-3.79
-1.88
-1.40
-1.47
-4.01
-2.70
-3.19
-2.26
-1.64
-3.62
-2.85
-3.53
-2.10
-4.66
NS
Mulinia
Shell D10
2.00
Shell G1
2.00
1.00
1.00
0.00
0.00
-1.00
-1.00
-2.00
-2.00
-3.00
-3.00
-4.00
-4.00
-5.00
-5.00
-6.00
-6.00
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Shell D11
2.00
0
2.00
1.00
2
4
6
8
10
12
14
16
18
20
22
24
26
28
8
10
12
14
16
18
20
22
24
26
28
8
10
12
14
16
18
20
22
24
26
28
8
10
12
14
16
18
20
22
24
26
28
Shell G2
1.00
0.00
0.00
-1.00
-1.00
-2.00
-2.00
-3.00
-3.00
-4.00
-4.00
-5.00
-5.00
-6.00
Pre-dam
Fig. 3. δ18O
values (y-axis)
obtained from
shell carbonate.
Sample points (xaxis) were taken
by drill at the
outer shell
surface along a
short transect
between the
umbo and ventral
margin. Boxed
areas represent
summertime shell
deposition and
were
subsequently
sampled for δ15N
measurement.
Chione
-6.00
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Shell D12
2.00
0
2.00
1.00
2
4
6
Shell G3
1.00
0.00
0.00
-1.00
-1.00
-2.00
-2.00
-3.00
-3.00
-4.00
-4.00
-5.00
-5.00
-6.00
-6.00
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
0
Shell C4
2.00
2
4
6
Shell B4
2.00
1.00
1.00
0.00
-1.00
-1.00
-2.00
-2.00
-3.00
-3.00
-4.00
-4.00
-5.00
-5.00
Modern
0.00
-6.00
-6.00
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
0
2
4
6
Table 6. Differences between whole-shell and summer-shell δ15N values
Species
Pre-dam shells
D10
D11
D12
G1
G2
G3
Modern shells
C4
B4
M. coloradoensis
Whole-shell Summer-shell
δ15N
δ15N
M. coloradoensis
Ch. fluctifraga
Ch. fluctifraga
Ch. fluctifraga
12.95
7.74, 7.89,
9.63
7.96
12.41
15.39
12.84
M. coloradoensis
Ch. fluctifraga
13.56
14.47
M. coloradoensis
Difference
8.76
4.19
9.04
-1.30 to 0.59
9.21, 9.71
11.87
15.59
12.26
-1.25 to -1.75
0.54
-0.20
0.58
13.22
13.52
0.33
0.95
Table 7. δ15N values obtained in diagenesis experiment.
Shell δ15N (‰)
Shell ID
A1X
A2X
F1X
F2X
C1X
C2X
C3X
B1X
B2X
B3X
Control
4.96
6.11
10.90
10.97
14.36
13.31
14.71
15.23
14.43
15.08
Roof
3.48
6.71
10.68
10.87
14.60
13.91
14.38
14.26
14.79
14.60
Oven
3.93
5.44
10.29
10.50
14.46
12.64
13.66
14.78
14.03
13.49
Roof δ15N shift (‰) Oven δ15N shift (‰)
Difference
Difference
Direction
Direction
(control(controlof change
of change
roof)
oven)
-1.48
-1.03
0.60
+
-0.67
-0.22
-0.61
-0.10
-0.47
0.24
+
0.10
+
0.60
+
-0.67
-0.33
-1.05
-0.97
-0.45
0.36
+
-0.40
-0.48
-1.59
-
1.00
Oven-control
0.50
0.00
δ15N (‰)
-
-0.50
-1.00
-1.50
-2.00
Fig. 4. Diagenesis experiment: Difference in δ15N between control and experimental groups
Roof-control
Table 8. Turnover rates for bivalve tissues.
Species
Freshwater stream
Foot
Stomach
Gut contents
Turnover time
Source
(days)
357
Raikow and Hamilton
78
(2001)
85
Whitsand Bay,
Southwest England
Marine
Whole body
250-345
Hawkins (1985)
North Carolina
Freshwater streams Hemolymph
113
Gustafson et al. (2007)
Naragansett Bay,
Rhode Island
Salt marshes
Collection location
12 species of Unionid Eagle Creek,
mussels
Southern Michigan
Mytilus edulis
Elliptio complanata
Geukensia demissa
Environment
Tissue
Whole body
(30 mm animal)
Whole body
(50 mm animal)
206
McKinney et al. (2001)
397