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MODELING NITRATE CONCENTRATIONS IN AN ANTARCTIC GLACIAL MELTWATER STREAM UNDER FLUCTUATING HYDROLOGIC CONDITIONS AND NITRATE INPUTS
ABSTRACT
The McMurdo Dry Valleys comprise a unique polar desert ecosystem in Victoria Land, Antarctica. The hydrologic system in the Dry Valleys is often characterized as being simplified compared to temperate watersheds, due to the ability to identify physical boundaries and nutrient sources and sinks. We seek to characterize the evolution of streamflow, solutes, and nutrients along a glacial meltwater stream in the McMurdo Dry Valleys, and to understand the role of different sources and sinks under varying hydrologic conditions. The study presented here includes streamflow routing, solute modeling, and nitrate concentration modeling in Von Guerard stream, a stream with abundant algal coverage in the McMurdo Dry Valleys region of Antarctica. The streamflow model is a solution to the kinematic wave routing problem. Solute modeling addresses advection, dispersion, as well as hyporheic zone inputs, which are controlled by weathering and hyporheic exchange. Lastly, the nitrate model builds on the solute model with the addition of a gross primary production (GPP) component. Results indicate that the hyporheic source of nitrate is controlling due to rapid exchange with the main channel. GPP impacts are small due to light-‐saturated conditions for a majority of the season, but provide a consistent sink for nitrate. The role of advective and dispersive transport is highly dependent on flow conditions, with advective transport controlling at high flows and dispersive controlling at low flows.
- TABLE OF CONTENTS
LIST OF FIGURES …………………………………………………………………………………………………………………….. vi
LIST OF TABLES ……………………………………………………………………………………………………………………. viii ACKNOWLEDGEMENTS …………………………………………………………………………………………………………. ix
Chapter 1 Introduction ……………………………………………………………………………………………………………. 1
- Study Area 3
- The Hydrologic System 4
- Importance of Hyporheic Interaction 5
- Nitrate Sources in Dry Valley Streams 6
- Nitrogen Transformation in Dry Valley Streams 6
- Evidence for Hyporheic Influence on Nitrate 9
- Evidence for Autotrophic Controls on Nitrate 9
- Scope & Objectives 10
Chapter 2 Experimental Design and Data Collection …………………………………………………….. 12
- Experimental Design 12
- Data & Instrumentation 13
- Water Level/Stage 13
- Downstream Stage 14
- Electrical Conductivity 15
- Meteorological Data 15
- Nitrate Concentrations 16
- Campbell Datalogger 17
Chapter 3 Methods ………………………………………………………………………………………………………………… 18
3.1 Streamflow Routing 18
3.1.1 Numerical Methods -‐ FTBS 19
- Conceptual Model of Hyporheic Role 20
- Electrical Conductivity Modeling 21
- Numerical Methods – FTCS 21
- FTCS and Numerical Dispersion 22
- Hyporheic Discharge 22
3.4 Nitrate Modeling 24
3.4.1 Calculating GPP 24
Chapter 4 Results …………………………………………………………………………………………………………………… 26
- Flow Routing 26
- EC Modeling 27
- Constant Fraction Discharge 28
- Exponential Function Discharge 28
- GPP Model 30
- Nitrate Modeling 32
4.4.1 Relative Contributions to Concentration 35
Chapter 5 Discussion …………………………………………………………………………………………………………….. 37
- Streamflow 37
- Conductivity 37
- Nitrate 38
5.3.1 Hyporheic Versus Autotrophic Controls 38
Chapter 6 Conclusions ………………………………………………………………………………………………………….. 40 References ………………………………………………………………………………………………………………………………. 42 Appendix A Flow Duration Curve and Flow Dates …………………………………………………………. 45
Appendix B List of Equations………………………………………………………………………………………………. 47
Chapter 1
Introduction
The McMurdo Dry Valleys present a unique environment for studying both
hydrologic and biotic processes due to the lack of macrophytes or stream macroinvertebrates, lack of detritus inputs, and presence of 24-‐hour daylight during the flow season. In addition, permafrost limits the depth of subsurface exchange. Due to the high aridity and extremely low precipitation, liquid water is available in limited amounts and comes solely from glacial meltwater, eliminating the potential for regional groundwater contributions. The lack of the aforementioned sources and sinks found in most temperate streams make this simplified ecosystem an ideal location for studying the impact of hyporheic exchange on nitrogen processing because processes are more identifiable. Despite the extreme climate, biota (benthic algae and mosses) thrive in Dry Valley streams during the Austral summer, and their existence has important implications for nutrient concentrations. The ability to model downstream nutrient concentrations also has important implications for Dry Valley lakes, especially in the Lake Fryxell basin (Figure 1-‐1), where streams contribute a majority of the water, solutes, and nutrients the Lake.
Nutrient studies conducted in McMurdo Dry Valley streams can provide
insight into processes occurring in many headwater streams, in the sense that they provide complexities not present in laboratory experiments, but with a stripped down landscape. In-‐stream processing of inorganic nitrogen by means of nitrification, denitrification, sorption, and uptake has been found to play a significant role in determining export of NH4+ and NO3-‐ from small watersheds (Peterson et al., 2001). Due to the high rates of nitrogen transformation and removal in headwater streams from elevated biological activity and sediment-‐water contact
time (Peterson et al., 2001), the role played by these streams in processing nitrate and other forms of organic and labile nitrogen is vital. In many temperate headwater streams, the roles played by different sources and sinks are difficult to identify. Many temperate watersheds suffer from high nitrate concentrations, due in large part to anthropogenic inputs, such as agricultural runoff and atmospheric deposition. Specifically, 47% of streams assessed in the U.S. in 2004 were either impaired or threatened, with nutrients being the fifth highest cause of impairment (U.S. Environmental Protection Agency, 2009). Therefore, it is important to understand how and where these processes occur in the stream environment, and
what determines the rates at which processing occurs.
Previous studies have laid the foundation for understanding how nutrients
are processed in Dry Valley streams, but we believe our methods can improve our
understanding in the following ways:
- Continuous, in-‐situ records of nitrate concentrations as collected for this study throughout the season, help characterize seasonal dynamics under the
widely varying conditions seen in the Dry Valleys.
- Continuous measurements also provide an opportunity for modeling nutrient concentrations under diel fluctuations in hydrologic and nutrient conditions, over multiple These fluctuations are an important
signature of hydrology in the region.
- We are able to gain an understanding of contributions to nutrient processes without adding nutrients to the Nutrient addition experiments intentionally elevate concentrations to levels outside of the normal range. Biota may respond to the increased stimuli by altering uptake rates (Thouin et. al., 2009), potentially leading to erroneous conclusions about ambient nutrient processing.
2.1 Study Area
The McMurdo Dry Valleys comprise the largest ice-‐free area on the Antarctic
continent. Taylor Valley, the site of the present study, as well as many long-‐term studies in the region, spans about 33km east to west between McMurdo Sound at
Explorers’ Cove and Taylor Glacier on the western edge of Lake Bonney, as seen in Figure 1-‐1. Between 1987 and 2000, the Lake Fryxell basin experienced an average mean annual temperature of -‐20°C, with a maximum recorded temperature of 9°C
and minimum of -‐60°C (Doran et. al., 2002). Less than 10 cm of precipitation falls annually, all in the form of snow, and mostly during the austral winter (Doran et. al., 2002).
[Figure 1-‐1] A site map showing the extent of Taylor Valley, including Von Guerard
Stream, the location of the present study. The Valley is bordered by the Asgard
Range to the north, McMurdo Sound to the east, Kukri hills to the south, and Taylor
Glacier to the west. Lake Fryxell is the easternmost lake shown above.
2.1.1 The Hydrologic System
Despite such extreme conditions, ephemeral streams flow for 4-‐12 weeks
during the austral summer (depending both on the stream, and weather conditions) through permanent channels into the valley’s 3 endorheic lakes. Thirteen of the streams have streamflow gauges that are currently monitored by the McMurdo Dry Valleys Long Term Ecological Research (MCM-‐LTER) group. Major ion, nutrient, and dissolved organic carbon samples are collected by team members approximately weekly throughout the flow season to be analyzed in the lab. Cyanobacteria mats, consisting largely of Phormidium and Nostoc communities (Howard-‐Williams & Vincent, 1989), exist in varying abundances throughout the Valley, surviving the austral winter in a “freeze-‐dried” state (Howard-‐Williams C. , Vincent, Broady, &
Vincent, 1986 and McKnight et. al., 1999).
Data was collected for the present study in Von Guerard Stream in the Lake
Fryxell basin, one of the three major closed-‐basin lake systems in Taylor Valley. Von Guerard is considered to be a long stream, and has abundant algal mats for much of is approximately 5-‐kilometer length. Von Guerard Stream flows into Lake Fryxell’s southeast corner, with a median discharge of 3.3 L/s. The average flow season for Von Guerard Stream starts on December 16 and ends on January 30. Refer to
Appendix A for flow duration curves of Von Guerard and other long streams, as well as a chart of average flow dates for all gauged streams.
2.2 Importance of Hyporheic Interaction
Sophocleous (2002) describes exchange between stream water and the
hyporheic zone as “the key to evaluating the ecological structure of stream systems and their management”. The nature of hyporheic flowpaths is that exchange occurs rapidly, and water may exchange between the stream and hyporheic zone several times along a single reach, making the biogeochemical signature of hyporheic water an important factor in stream water quality (Harvey & Wagner, 2000).
There is very limited transport of solutes and nutrients to streams from the
surrounding landscape due to the limited availability of liquid water in the Dry Valleys. Melt water from Taylor Valley frozen-‐based glaciers is also a poor source of weatherable minerals, and as a result water-‐rock interaction in the hyporheic zone is a primary source of certain cations (Nezat et. al., 2001). The hyporheic zone has been found to be an important source of ions that are not abundant in marine aerosols, such as Si and K, as a result of weathering of silicate materials (Gooseff et. al., 2002). Maurice et. al. (2002) observed mineral precipitates and etch pits on muscovite mica that was buried in Green Creek, and attributed these features to chemical weathering processes in the hyporheic zone. Nezat et. al. (2001) and Gooseff et. al. (2002) both identified stream discharge as a primary control on chemical weathering rates. We propose that the hyporheic zone may also be an important source of other solutes and nutrients, such as nitrate.
2.3 Nitrate Sources in Dry Valley Streams
When compared with other flowing waters of the world, nitrate
concentrations in Dry Valley streams have been found to be considerably low (Green, et al., 1989). Due to the absence of overland flow and regional groundwater influence, much of the dissolved inorganic nitrogen (DIN) occurring in dry valley streams comes from wet-‐deposited nitrate on glacier surfaces and in streambeds (Witherow, et al., 2006). Only a small amount of nitrogen is available in stream sediments (Howard-‐Williams, Priscu, & Vincent, 1989). The lack of N sources during the flow season means that the highest concentrations are often during first flows and at low flow (Howard-‐Williams, Priscu, & Vincent, 1989).
2.4 Nitrogen Transformation in Dry Valley Streams
Several studies have previously addressed nutrient dynamics in Dry Valley streams. Early studies focused on periodically collecting water samples throughout the flow season, sometimes at several locations along a stream reach, at ambient conditions. One such study found that inorganic and labile organic nitrogen sources are removed by stream biota, and that uptake lengths are comparable to temperate stream environments (Howard-‐Williams, Priscu, & Vincent, 1989). An analysis of water samples taken along a stream reach, coupled with a model of primary
productivity in Canada Stream confirmed that microbial communities transform inorganic nitrogen and urea (Moorhead, McKnight, & Tate, 1998). A water quality study on the Onyx River in Wright Valley did not find significantly different concentrations of ammonium, dissolved reactive phosphorus (DRP), or nitrate
entering Lake Vanda at different discharges, but did find lower nitrate concentrations downstream of reaches with visible microbial activity, such as the Boulder Pavement (Howard-‐Williams, Hawes, Schwarz, & Hall, 1997). Figure 1-‐2 shows a map of the Onyx River study reach. The Boulder Pavement was earlier identified as an area of active nutrient transformation (Howard-‐Williams C. ,
Vincent, Broady, & Vincent, 1986).
[Figure 1-‐2] The Onyx River is located in Wright Valley, which is adjacent to Taylor Valley to the north. An early study characterized how nutrient concentrations changed along the River. Lake Vanda and the Boulder Pavement are indicated. Figure from Howard-‐Williams et. al. (1997).
More recent studies have been conducted using stream tracer experiments to understand uptake processes. McKnight et al. (2004) conducted a nutrient addition experiment in Green Creek in order to characterize nitrate and phosphate transformation processes along a reach, attributing nitrate losses to denitrification in the hyporheic zone. Gooseff et al. (2004) considered nutrient additions, microbial mat assays, and transient storage modeling to focus more specifically on the role of the hyporheic zone in nutrient processing, and found that biotic assimilation and mat transient storage were important nitrate sinks. Most recently, Koch et al. (2010) conducted a nitrate enrichment experiment in Huey Creek, and found that reactivity of nitrogen species may be primarily controlled by exchange between surface water and hyporheic sediments.
2.4.1 Evidence for Hyporheic Influence on Nitrate
Previous nutrient studies in Taylor Valley streams, as well as some temperate stream studies, have attributed nutrient transformation processes to
cycling in the hyporheic zone. Nitrate losses in the return flow of a tracer experiment in Huey Creek were attributed to increased nitrate reduction in the hyporheic zone of the anabranching reach (Koch et. al., 2010). Nitrate reduction by means of denitrification in the hyporheic zone was also identified during a tracer injection and transient storage analysis in Green Creek (Gooseff et. al., 2004), which is corroborated by the earlier identification of nitrate-‐reducing bacteria in hyporheic sediments (Maurice et. al., 2002). Hyporheic nitrate uptake in Green Creek was estimated to be between 7 and 16%, depending on the reach (McKnight
- al., 2004).
2.5 Evidence for Autotrophic Controls on Nitrate
Many recent studies in temperate streams have attempted to isolate the impacts that primary producers have on stream nitrate, and found that uptake rates were higher when GPP was higher (Hall and Tank 2003, Mulholland et. al., 2006,
Hoellein et. al., 2007, Roberts and Mulholland 2007, Heffernan and Cohen 2010). Some studies found that community respiration explained less of the variation in nitrate uptake than did primary productivity (Hall and Tank 2003, Roberts and Mulholland 2007), and several found correlations between stream temperature and uptake (Mulholland et al 2006, Heffernan and Cohen 2010). Nitrate concentrations
had varying impacts on uptake, from none at all (Hoellein et. al., 2007) to having a primary influence (Thouin et. al., 2009). Persistent illumination during the austral summer necessitates the
development of alternative methods for studying primary production processes in Antarctic streams, as methods in all of the above studies relied on the principle that autotrophic activity ceases during nighttime hours. We propose that autotrophic uptake may control nitrate concentrations in Dry Valley streams, especially under low-‐flow conditions.
2.6 Scope & Objectives
The scope of the proposed work is to create and explore a model for nitrate concentrations in single-‐source glacial meltwater streams, in the Dry Valleys of Antarctica, that accounts for variable flow and sources and sinks of nitrate along a reach. The model will consider both physical and biological controls on nitrate concentrations in the main channel and the hyporheic zone, including advection, dispersion, net hyporheic/respiration effects, and autotrophic activity. The model was informed and evaluated mainly by data collected during the 2011-‐2012 austral summer in Von Guerard Stream, but also by values reported from previous studies in the McMurdo Dry Valleys. The role of the hyporheic zone will be evaluated by using a model for electrical conductivity, which was developed for use in the present study. More specifically, we propose two potential controls on nitrate concentrations in dry valley streams and seek to quantify their contributions under
fluctuating discharge and meltwater concentrations:
- We propose that nitrate uptake is positively associated with gross primary production (GPP) (assimilatory uptake by benthic algae), which depends on photosynthetically active radiation (PAR).
- Nitrate is controlled by processes occurring in the hyporheic zone, which
depend on discharge. This could include:
- Regeneration through nitrification
- Mineralization
- Ecosystem respiration
- Denitrification
MODELING NITRATE CONCENTRATIONS IN AN ANTARCTIC GLACIAL MELTWATER STREAM UNDER FLUCTUATING HYDROLOGIC CONDITIONS AND NITRATE INPUTS
