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  The area contains many glacial meltwater streams that drain into ice covered lakes on the valley floors

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

 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)

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

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 m/s during a warm summer to no flow during cold summers  Discharge is variable – can range from ~0.5 m 3 /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

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)

 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)

 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

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 NO 3 and SRP and abundant algal mats

Synoptic Study: Von Guerard Stream  Nutrient concentrations were generally lowest in melt water and greatest in the hyporheic zone and parafluvial seeps  SRP, NH 4, and specific conductance were uniform in surface water from the main channel in the downstream direction  NO 3 + NO 2 was below the detection limit in all main channel surface water samples (as opposed to seeps)  The average concentrations of SRP, NO 3 + NO 2 and NH 4 from surface water were similar to the concentrations found in the bankside meltwater

 Mean values for SRP and NO 3 + NO 2 were significantly higher in water from the underlying hyporheic zone than in the main channel surface water  The SRP, NO 3 + NO 2, NH 4 and specific conductance were all significantly higher in surface and subsurface water from parafluvial seeps than in main channel surface water

Tracer Injection Experiment: Green Creek Hydrologic 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

 Hydrologic parameters used within the transport model to quantify hyporheic exchange include the storage zone cross-sectional area, A s, 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 A s 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 (A s /A)  High values of A s /A may be attributed to the period of minimum flow (0.3 L/s) when most of the water was within the hyporheic zone

Nutrient Additions  The concentrations of NO 3 and PO 4 were elevated at the point of injection  NO 3, NO 2, and PO 4 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  NH 4 concentrations were erratic

 Optimal simulation results for NO 3 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 NO 3 uptake occurred in the main channel, where as 6.5 to 15.5% occurred in the hyporheic zone

 Optimal simulation results for PO 4 were obtained by considering uptake in the main channel exclusively  PO 4 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

Discussion Autotrophic Uptake of Nutrients  The importance of autotrophic uptake by benthic algal communities is indicated in the comparison study by the significantly lower mean NO 3 and SRP concentrations in streams with abundant algal mats than in streams with sparse algal mats

 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

 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  NO 3 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

 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  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

 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  NO 3 uptake in the main channel can be attributed to autotrophic uptake by algal mats  NO 3 uptake in the hyporheic zone can be attributed to other microbial processes  Uptake by the algal mats in the main channel was the primary NO 3 sink in Green Creek  Uptake of PO 4 appeared to be controlled entirely by in-channel processes  The authors concluded that the algal mats assimilated PO 4 with no further loss occurring in the hyporheic zone

 High nutrient concentrations were found in the hyporheic and parafluvial zones in the synoptic study of Von Guerard Stream  These zones are sources of NO 3 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 N Transformations