PROPOSAL
Hyporheic biogeochemical processes in Arctic tundra streams:
A comparison of two streams of differing geomorphologies
Morgan Johnston
Project Advisor: Breck Bowden
Degree: M.S., Natural Resources,
Aquatic Ecology and Watershed Science
BACKGROUND
Although studies over the past fifteen years have contributed greatly to the
growing knowledge base of surface-subsurface exchange and transient storage
characteristics of streams, many of the streams that have been studied are temperate or
desert (Valett et al. 1990, Valett et al. 1994) ecosystems. Information regarding
hyporheic processes in other systems is lacking. This research will focus on the need for
more complete information regarding hyporheic processes in Arctic stream ecosystems.
Initially, it was assumed that due to the presence of permafrost in the arctic
tundra, hyporheic exchange was not an important process to the functioning of the tundra
stream ecosystem. A study by Edwardson et al. (2003) was the first to consider
hyporheic processes in arctic streams and through a series of conservative tracer additions
and concurrent hyporheic and channel sampling, they found that the hyporheic zone was
indeed important to stream ecosystem functioning.
Hyporheic zones in the streams in this study served as sites for the mineralization
of organic matter, followed in most cases by rapid nitrification. Ammonium, phosphate
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and carbon dioxide were all produced in the hyporheic zone and upwelling from the
hyporheic zone to the stream channel supplied the stream with nitrate, ammonium and
carbon dioxide. In fact, they determined that the equivalent of 14-162% of the nitrogen
demands of benthic organisms was supplied to the stream by the hyporheic zone. In this
study, none of the hyporheic samples indicated anaerobic conditions (Edwardson et al.
2003).
The study proposed in this paper will follow up on the work done by Edwardson
et al. (2003). This particular project is part of a larger NSF-funded arctic hyporheic study
which is investigating the potential responses of arctic tundra streams to climate change.
The premise of the study is that changes in the geomorphology of a stream can lead to
changes in the hydrological flowpaths of a stream and surface-subsurface exchange
characteristics, which can have significant implications for the microbial metabolic
activity and biogeochemical cycling in the hyporheic and parafluvial zones (Holmes et al.
1998). Therefore, changes in stream geomorphology due to climate change may
considerably alter the hydrologic and biogeochemical functions of the hyporheic zone in
the context of the stream ecosystem.
It has been predicted that the increase in air temperature associated with global
climate change will affect the sensitive arctic ecosystem by promoting the melting of
permafrost. As a result of the melting permafrost, erosion and slumping are probable,
which will increase sediment loading into streams and lakes (Rouse et al. 1997). Along
with the sediments, the associated stored nutrients will be released into streams, with the
potential of changing aquatic ecosystem structure and dynamics from the bottom up. Of
major concern in these systems is increased phosphorus loading (Hobbie et al. 1999).
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Specifically, in the context of arctic stream hyporheic functioning, the major
concerns regarding climate change are (1) the changes in temperature, precipitation and
runoff patterns that could have an effect on stream geomorphology and (2) changes in the
shape and extent of the thaw bulb, which is the area of thawed permafrost around the
perimeter of the streambed that develops during the warm season. If the thaw bulb
increases, the extent of the hyporheic zone, which is a subset of the thaw bulb, may
increase as well, possibly changing the nature and magnitude of influence that the
hyporheic zone has on stream ecosystem functioning.
The arctic hyporheic research team is taking an interdisciplinary approach to
studying the complex processes of arctic tundra stream hyporheic zones. The following
three disciplines are represented by specialists in each area: hyporheic hydrology
[Michael Gooseff (Colorado School of Mines) and Jay Zarnetske, MS student (Utah State
University), stream geomorphology and ground penetrating radar techniques [John
Bradford and Jim McNamara (Boise State University)] and biogeochemistry [Breck
Bowden and Morgan Johnston, MS student (University of Vermont)].
The goal of my portion of this research project is to identify how transient storage
and hyporheic biogeochemical processes differ in two stream reaches of differing
geomorphology on the Arctic tundra landscape. The two reach types are an alluvial,
high-gradient, cobble bottom reach and a meandering, low-gradient, peat-bottomed reach.
In order to determine the effect of stream gradient, substrate and morphology on transient
storage, the following three questions will be addressed and compared for the two stream
types: (1) What portion of total transient storage is hyporheic storage? (2) What portion
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of the thaw bulb volume is occupied by active hyporheic processing? (3) What are the net
changes in nutrient concentrations of water in transit through the hyporheic zone?
STUDY SITE
As shown in Figure 5, Toolik Lake (68ºN, 149ºW) is located on the North Slope
of the Brooks Range, Alaska. The lake is located about 180 km south of the Arctic
Ocean (Hobbie et al. 1999). The two clear water streams in this study are a part of a
series of lakes and connecting streams that flow into Toolik Lake and originate from
springs in the foothills of the Brooks Range (Figure 5). Although in very close proximity
to each other on the landscape, the two streams have very different physical
characteristics. The I-8 Inlet is an alluvial, high-gradient, cobble-bottom stream with a
slope of 2%, while the Peat Inlet is a meandering, beaded, low-gradient, peat-bottom
reach with a slope of 0.07%.
A.
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B.
Toolik Lake
N
↑
Figure 5. (A) Location of Toolik Field Station, Alaska and (B) Infrared image of Toolik
Lake and the Inlet series of smaller lakes and streams flowing northward into it.
(Images: Andrew W. Balser)
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Research carried out by the Arctic Hyporheic Team in the 2004 field season
suggests that these two streams exhibit quite different patterns of transient storage. The
I-8 Inlet has been shown to have developed a larger thaw bulb during the warm season
than the Peat Inlet. This is likely due to the insulation provided by the peat substrate of
the stream. Additionally, stream tracer additions and corresponding models carried out in
the 2004 season indicate that the solute transport patterns in I-8 Inlet are best described
by a model with a power-law residence time distribution, while the patterns in the Peat
Inlet are best described by a model with an exponential residence time distribution
(Figure 6).
Figure 6. Results of 2004 stream tracer additions to I-8 Inlet (top panels) and the Peat
Inlet (bottom panels) model with an exponential residence time distribution (left panels)
and with a power-law residence time distribution (right panels) (Zarnetske et al. 2004).
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JUSTIFICATION OF KEY RESEARCH QUESTIONS
Q1. What portion of total transient storage is hyporheic storage? Transient storage in a
typical stream reach has an open-channel and a hyporheic component. Traditional field
methods and models used to assess transient storage typically do not distinguish between
these two components. To accurately assess the potential for nutrient processing and
regeneration from the hyporheic zone it is necessary to distinguish between that
component of the residence time distribution that is attributable to true hyporheic storage
from the component that is simply open-channel storage in eddies and backwaters.
Q2. What portion of the thaw bulb volume is occupied by active hyporheic processing?
The hyporheic zone can only develop in thawed sediments. However, the volume of
thawed sediments (thaw bulb) may be greater than the volume of the hyporheic zone that
actively exchanges with the open-channel flow. To better understand the nature of
hyporheic exchange in Arctic streams it is important to know what portion of the thaw
bulb is active hyporheic zone.
Q3. What are the net changes in nutrient concentrations of water in transit through
the hyporheic zone?
A portion of open channel flow typically enters the hyporheic zone at riffle heads,
traverses riffles and upwells at riffle tails. The flow velocity along this path is slow
relevant to the flow velocity of average open channel flow (including storage in eddies
and backwaters) and extensive contact with biofilms on hyporheic sediments provides an
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excellent opportunity for biogeochemical transformations in water in transit through the
hyporheic zone. Thus, the hyporheic zone can be a significant source for some nutrients
or sink for others.
Q4. What effects do stream substrate and gradient have on hyporheic zone structure
and processes?
There are a variety of different stream reach types on the North Slope of Arctic Alaska.
High-gradient, alluvial, cobble-bottom reaches and low-gradient, meandering, peatbottomed streams are two fundamentally different, contrasting stream reach types that are
common. General physical hydraulic principles suggest that hyporheic processing should
be deeper and more extensive in the high-gradient reaches than in the low-gradient
reaches. This hypothesis is supported by early results from our hyporheic project and
further suggests that net nutrient exchange is likely to differ in these two contrasting
stream types.
GENERAL APPROACH
This research project has two main interrelated components. The first of these is
hydrology and the second is biogeochemistry. These two components will be addressed
in two field campaigns during the 2005 field season. Because the biogeochemical
cycling in the hyporheic zone is so dependent on the hydrology of the system, the
hydrologic aspects of the system will be addressed first in the June field campaign.
The focus of the August campaign will be on biogeochemical dynamics.
Hyporheic nutrient sampling in the August campaign will focus on only those regions of
the thaw bulb that are found to be actively exchanging with the stream in the June
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campaign. Our previous work suggests that the thaw bulb develops quite quickly (prior
to June) and so waiting until August to do the more intensive biogeochemical work
should not be a problem.
METHODS
Physical characterization of transient storage and hyporheic exchange
The first two questions will be addressed through a combination of solute tracer
injections and inverse solute transport modeling administered by Gooseff and Zarnetske
(as described by the Stream Solute Workshop 1990) and concurrent physical sampling of
hyporheic and channel transient storage zones. The conservative tracer used in these
injections will be rhodamine WT (RWT), an organic, red dye whose concentrations in
water samples can be determined by fluorescence at a wide range of concentrations.
RWT is particularly useful as a tracer because its presence can be detected at very low
concentrations (as low as 0.1µg L-1) (Sutton et al. 2001).
Based on methods used by Gücker and Boëchat (2004), I will use stream velocity
data to identify in-stream transient storage zones. Prior to the study, I will survey each of
the stream reaches, recording a channel cross-sectional depth profile at transects spaced
ten meters apart. At each of these transects, the relative cross-sectional areas of freeflowing water (A’) and lentic water (ASS) zones will be estimated based on whether or
not velocity is detectable with a velocity meter. With this information, I will be able to
estimate the in-stream transient storage area relative to stream area (ASS/A’) and compare
that value to the total transient storage to stream area ratio (AS/A) in order to help
distinguish hyporheic and in-stream transient storage.
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Additionally, the reach will also be qualitatively assessed to determine likely
transient storage zones in the channel. Approximately ten to fifteen representative zones
will be selected to be monitored during the solute injection. These will likely include
eddies and the edge and bottom waters of pools. The pre-selected in-stream storage
zones will be characterized in more detail by noting their approximate shape and volume
and by qualitatively describing the patterns of water flow within each zone.
During the conservative tracer additions administered by Gooseff and Zarnetske,
the concentration of tracer in the pre-selected zones will be monitored by taking multiple
grab samples from the stream bank. The zones will be sampled frequently on the rising
and falling limbs of the solute tracer curve and less intensively during the tracer plateau.
In addition, hyporheic samples will be taken throughout the tracer plateau and falling
limb. Both the in-stream transient storage and hyporheic samples will be returned to the
laboratory where rhodamine WT concentration will be determined using a fluorometer
(Turner Designs, 10-AU). By knowing the residence time of tracer and approximate
volume of various types of transient storage zones, the residence time distribution can be
compared to the transient storage parameters obtained from the transport model.
Hyporheic water samples will be obtained through the implementation of
inexpensive, lightweight minipiezometers inserted into the streambed to known depths.
At the beginning of the June campaign, these hyporheic samplers will be installed in one
riffle-pool sequence in each study reach. Depending on ease of installation, more than
one riffle per reach may be outfitted. Samplers will be installed at each of the following
locations in each reach: riffle head (downwelling zone), riffle tail (upwelling zone) and
pool. The samplers will be installed in nested bundles of four minipiezometers arranged
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at staggered depths using a steel insertion tool with a one inch diameter. The design of
this insertion tool is very similar to that described in Baxter et al. (2003).
Each individual minipiezometer will consist of a rigid delrin rod (1/4”OD,
1/8”ID) in which a series of holes have been pre-drilled in the bottom 10 cm. The bottom
of the rod will be left open and the drilled region will be covered with a geotextile sleeve
to prevent sediments from entering the piezometer. The four piezometers will be
arranged at the desired depth intervals and affixed together with electrical tape before
being installed. This arrangement will depend on the depth of resistance at each point of
installation.
The insertion tool will be pounded into the streambed to a depth of resistance at
each of the three locations within the reach using a fence post driver. The inner steel rod
will then be removed from the insertion tool and the minipiezometer bundle will be
dropped into the outer casing. Before they are inserted, the piezometer bundles will be
arranged so that the deepest piezometer will be installed at the deepest accessible point,
the shallowest piezometer will be at approximately 30 cm depth, and the two piezometers
at intermediate depths will be spaced at intervals more closely weighted toward the
shallow depths. The outer casing will then be removed from the streambed using a jack
and the sediments will collapse around the minipiezometer bundle, securing it in place at
the desired depth.
Hyporheic samples will be drawn from the installed piezometers by applying
suction via 60-ml BD syringe that is connected to the delrin rod with a 3-way stopcock.
The hyporheic zone will be sampled periodically in order to note the travel time of the
tracer from the point of solute addition to the riffle-head piezometer location. The
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piezometers at the riffle-tail position will be monitored to determine travel time through
the sediments from the riffle-head position. All tracer samples will be taken to the
laboratory and analyzed for rhodamine WT concentration using a fluorometer. The
presence or absence of tracer at each of the depths will be noted and those data will be
used to address Question 2.
Nutrient Sampling and Characterization of Biogeochemical Dynamics
The August campaign of the field season will be devoted to studying the
biogeochemical dynamics of the streams’ hyporheic zones. Using the tracer data
obtained in the June campaign, I will have determined the extent of the hyporheic zone
and the relative contributions of in-stream and hyporheic storage to overall transient
storage. Based on this information, hyporheic piezometers will be installed more densely
in only the areas of the reach exhibiting active hyporheic exchange.
The piezometers installed at the beginning of the August campaign will be
constructed of larger diameter (3/4” in) PVC pipe. Rather than gathering samples from
various distinct depths, these piezometers will enable me to collect one composite sample
from each sampling location that includes the range of depths at which hyporheic
exchange is active. Individual piezometers will be installed more densely at each rifflepool sequence and additional riffle-pool sequences within the reach will be outfitted.
Hyporheic samples will be taken for analysis of phosphate, ammonium, nitrate,
dissolved oxygen (DO) and dissolved organic carbon (DOC) concentrations. Water
samples will be collected in the field, filtered through a 25-mm diameter 0.45µm syringe
filter and stored on ice until they are brought back to the laboratory for analysis. DO will
be measured immediately in the field by connecting Tygon tubing from the three-way
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stopcock into a glass jar with a stopper. Another piece of Tygon tubing will extend from
the glass jar to a hand pump and a third hole in the stopper will accommodate a DO
probe. Pressure will be applied via the hand pump and water will be drawn into the jar
without coming in contact with the atmosphere. DO will be measured with the probe.
Ammonium and phosphate analyses will be carried out at the Toolik Field Station
laboratory. Nitrate and DOC samples will be frozen and shipped to the University of
Vermont for analysis.
The ammonium analysis will be performed using the OPA
(orthophthaldialdehyde) method. In this method, fluorescence is produced when
ammonium is combined with OPA. Using the OPA method, ammonium samples are
combined with a working reagent containing OPA (active ingredient), sodium sulfite and
a borate buffer and the fluorescence of the sample is then measured with a fluorometer.
Along with the field samples, a series of ammonium standards is also prepared and
measured in order to develop a standard curve from which to determine the ammonium
concentrations of field samples based on the fluorometer readings. This method can be
used with relatively small-volume samples (2.5 ml) and a wide range of ammonium
concentrations. Holmes et al. (1999) document the details of this methodology.
Analysis of phosphate concentration will be determined through the molybdate
blue method, in which ammonium molybdate and antimony potassium tartrate react in an
acid medium in the presence of orthophosphate. This reaction forms antimony-phospomolybdate complex, which is reduced by ascorbic acid to form a blue-colored complex.
This solution can then be analyzed colorimetrically to determine phosphate concentration
(www.nemi.gov). Analysis of nitrate will be performed by the cadmium reduction
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technique. In this method, the nitrate in a sample is reduced to nitrite in the presence of
cadmium. The nitrite is then diazotized with sulfanilamide and coupled with N-(1napthyl)-ethylenediamine dihydrochloride. This forms an azo dye that can be detected
colorimetrically. The range of detection for this method is 0.5 to 10 mg-N L-1
(www.nemi.gov).
DOC samples will be filtered (0.45 µm) and frozen and shipped to the University
of New Hampshire for analysis with a Total Organic Carbon (TOC) Analyzer. The
method of analysis will be the UV/Persulfate method, in which sodium persulfate is used
in combination with UV light to oxidize the organic matter in the sample. Initially, the
sample will be acidified and purged to eliminate inorganic carbon. The carbon dioxide
produced in the wet oxidation process will be measured by infrared spectrometry in order
to determine the total organic carbon in the sample. The detection limit for this method is
0.1 mg/L (www.nemi.gov).
Both the nutrient and rhodamine sample data will be analyzed using a MANOVA,
in which the five response variables will be the concentrations of nitrate, ammonium,
phosphate, dissolved oxygen and dissolved organic carbon. The predictor variables will
be the stream (alluvial or peat), the position in the reach (riffle head, mid-riffle, riffle tail
or pool) and the sample depth (30, 60, 100 cm, etc.).
Based on the information presented in the Hyporheic Biogeochemistry section of
the literature review and the observed patterns of nitrate, ammonium, phosphate, DO and
DOC concentrations seen as water moves through the hyporheic zone, inferences can be
made regarding the dominant microbial biogeochemical processes in the hyporheic zone.
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EXPECTED OUTCOMES
Based on the data already collected from these two streams, it is likely that they
will exhibit very different transient storage characteristics. Due to their geomorphic
structure, gradient and substrate type, I would expect that the transient storage of the
alluvial stream would be dominated by hyporheic storage and that of the peat stream
would be dominated by in-stream storage. Because hyporheic storage tends to produce
longer residence times than in-stream storage, I expect that the water in the hyporheic
zone of the alluvial stream will exhibit more notable nutrient transformations than that in
the peat stream. I would also expect that hyporheic exchange would occur more deeply
beneath the alluvial stream than the peat stream. Although the thaw bulb of the alluvial
stream is likely to be more extensive than the peat stream, I believe that the majority of
hyporheic exchange will be contained to approximately the top 30-60 cm of the thaw
bulb and perhaps even more shallowly in the well-insulated peat stream.
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