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By Diane McKnight, et al. Presentation by Jean Aldrich

Inorganic N and P dynamics of Antarctic glacial meltwater streams as controlled by hyporheic exchange and benthic autotrophic communities. By Diane McKnight, et al. Presentation by Jean Aldrich. Study conducted in the McMurdo Dry Valleys of South Victoria Land, Antarctica

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By Diane McKnight, et al. Presentation by Jean Aldrich

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  1. Inorganic N and P dynamics of Antarctic glacial meltwater streams as controlled by hyporheic exchange and benthic autotrophic communities By Diane McKnight, et al. Presentation by Jean Aldrich

  2. Study conducted in the McMurdo Dry Valleys of South Victoria Land, Antarctica The area contains many glacial meltwater streams that drain into ice covered lakes on the valley floors

  3. Purpose of Study • The purpose of this study was to examine the extent to which hyporheic exchange interacts with microbial processes in benthic algal mats to influence nutrient concentrations in dry valley streams

  4. The hyporheic zone is an area of sediment adjacent to and underneath the stream • Water in the hyporheic zone flows in a downstream direction interacting with water in the main channel • (D. McKnight, et al. 2001)

  5. Study site • During winter • Continuously dark • High winds • Air temps as low as -60˚C • During summer • Continuously light • Air temps as high as 5˚C • The McMurdo Dry Valleys are the largest ice-free areas of Antarctica • There is no plant life • Permafrost at a depth of ~0.5 m • The climate is cold and dry

  6. Background • The dry valley streams are fed by meltwater from glaciers • Average summer stream flow depends upon the duration of temps above 0˚c and insolation during the summer • Discharge is variable – can range from ~0.5 m3/s during a warm summer to no flow during cold summers • Discharge can also vary as much as 10 – fold during a day • Many of the streams have abundant perennial mats of filamentous cyanobacteria which grow during streamflow in the summer and are in a dormant freeze-dried state the rest of the year

  7. Methods • Water samples were collected at the gauging stations near the outflow of the streams to the lakes • Water was collected whenever there was stream flow during the summer (mid Nov to late Jan) • The distribution of algal mats was mapped at 16 sites in 12 streams • The abundance of algal maps was characterized as: • Very sparse (undetectable) • Sparse (<5% cover) • Moderate (5% - 80% cover) • Abundant (>80% cover)

  8. On January 7, 1995 the Von Guerard stream was sampled, under low-flow conditions when hyporheic drainage influenced stream flow, for: • Specific conductance • pH • Dissolved inorganic nitrogen • Soluble reactive phosphorus (SRP)

  9. On January 13, 1995 an experimental tracer and nutrient injection was conducted in Green Creek • Water samples were collected at four sites below the injection site in areas where the channel was somewhat constrained • The injection lasted 130 minutes and samples were collected at 3 to 15 minute intervals for 30 minutes before the injection until several hours after the injection

  10. Results • Specific conductance and major ion concentrations in 12 streams in the Taylor Valley were variable, because their concentrations decreased with increasing streamflow • In general, nutrient concentrations also showed substantial variation among streams • The streams with sparse algal cover generally had the highest nutrient concentrations • The results of the statistical analysis confirmed the relationship between low concentrations for NO3 and SRP and abundant algal mats

  11. Synoptic Study: Von Guerard Stream • Nutrient concentrations were generally lowest in melt water and greatest in the hyporheic zone and parafluvial seeps • SRP, NH4, and specific conductance were uniform in surface water from the main channel in the downstream direction • NO3 + NO2 was below the detection limit in all main channel surface water samples (as opposed to seeps) • The average concentrations of SRP, NO3 + NO2 and NH4 from surface water were similar to the concentrations found in the bankside meltwater

  12. Mean values for SRP and NO3 + NO2 were significantly higher in water from the underlying hyporheic zone than in the main channel surface water • The SRP, NO3 + NO2, NH4 and specific conductance were all significantly higher in surface and subsurface water from parafluvial seeps than in main channel surface water

  13. Tracer Injection Experiment: Green CreekHydrologic Characterization • All of the flow in Green Creek comes from glacial meltwater that enters the stream above the injection site • Cl concentrations increased at all 4 sampling locations as a result of the 130 min LiCl injection, but plateau concentrations were obtained at only the most upstream site • Cl concentrations at the remaining sites were affected by mixing processes that delayed and attenuated the tracer pulse • Authors concluded that the 130 min injection period was not of sufficient duration to obtain plateau concentrations at the down stream sites • Conservative solute transport simulations reproduced the general features of the observed Cl profiles at all sampling locations

  14. Hydrologic parameters used within the transport model to quantify hyporheic exchange include the storage zone cross-sectional area, As, and the exchange rate coefficient,  • The presence of a large hyporheic zone within Green Creek was indicated by the lack of a steady state plateau for Cl at 226, 327, and 497m (the three downstream sites) • Hyporheic-zone waters did not become fully saturated with tracer-enriched water during the 130 min injection period - this was supported by the large estimates of As in reaches 2 and 3 • The relative size of the hyporheic zone may be quantified using the ratio of storage zone and main channel cross-sectional areas (As/A) • High values of As/A may be attributed to the period of minimum flow (0.3 L/s) when most of the water was within the hyporheic zone

  15. Nutrient Additions • The concentrations of NO3 and PO4 were elevated at the point of injection • NO3, NO2, and PO4 concentrations increased at the upper three sampling locations (50, 226, & 327 m) • Concentrations were near or below the detection limit at the most downstream site • NH4 concentrations were erratic

  16. Optimal simulation results for NO3 were obtained by considering uptake in both the main channel and the hyporheic zone • Estimates of mass loss indicate that ~84.5 to 93.5% of the observed NO3 uptake occurred in the main channel, where as 6.5 to 15.5% occurred in the hyporheic zone

  17. Optimal simulation results for PO4 were obtained by considering uptake in the main channel exclusively • PO4 simulations that included hyporheic uptake in addition to main channel uptake were indistinguishable from those that relied solely on main-channel uptake • Simulations that considered only hyporheic-zone uptake did not reproduce the features of the observed data

  18. DiscussionAutotrophic Uptake of Nutrients • The importance of autotrophic uptake by benthic algal communities is indicated in the comparison study by the significantly lower mean NO3 and SRP concentrations in streams with abundant algal mats than in streams with sparse algal mats

  19. The release of nutrients through weathering reactions or dissolution of aerosols occurs in the hyporheic zone of all dry valley streams • Therefore the absence of algal mats because of unsuitable habitat causes the nutrients to remain in solution when hyporheic exchange brings these solutes into the main channel

  20. The results of the Von Guerard Stream study indicate that nutrient concentrations varied laterally along a gradient from the main channel to the hyporheic zone • NO3 and SRP concentrations were significantly lower in the main channel than in the underlying hyporheic zone or the parafluvial seeps • The results were interpreted as showing that nutrient uptake by the benthic algal communities matches the nutrient flux from hyporheic exchange, preventing accumulation of nutrients in the main channel

  21. The nutrient uptake occurring in both the main channel and the hyporheic zone in the simulations of the nutrient enrichment experiment in Green Creek was taken into consideration • The results may have been influenced by nutrient concentrations in the first two reaches that exceeded those observed in the stream itself • Mulholland et al. 1990, 2000,Dodds et al. 2002 • No increase in nutrient concentrations was detected at the furthest downstream site during the experiment, suggesting that saturation of nutrient uptake did not have a large influence • Dodds et al. 2002

  22. The solute transport model coupled with non-linear regression provided an objective means of quantifying the processes within the main channel and the hyporheic zone • NO3 uptake in the main channel can be attributed to autotrophic uptake by algal mats • NO3 uptake in the hyporheic zone can be attributed to other microbial processes • Uptake by the algal mats in the main channel was the primary NO3 sink in Green Creek • Uptake of PO4 appeared to be controlled entirely by in-channel processes • The authors concluded that the algal mats assimilated PO4 with no further loss occurring in the hyporheic zone

  23. N Transformations • High nutrient concentrations were found in the hyporheic and parafluvial zones in the synoptic study of Von Guerard Stream • These zones are sources of NO3 and SRP to the benthic algal mats in the streams because of hydrologic exchange processes • Therefore hyporheic exchange in dry valley streams appears to influence nutrient cycling in a manner similar to that observed in a temperate desert stream • Valett et al. 1990,1994, Grimm et al. 1991, Holmes et al. 1994, Jones et al. 1995

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