Inorganic N and P dynamics of Inorganic N and P dynamics of Antarctic glacial meltwater streams as Antarctic glacial meltwater streams as controlled by hyporheic exchange and controlled by hyporheic exchange and

benthic autotrophic communitiesbenthic autotrophic communities

By Diane McKnight, et al.By Diane McKnight, et al.

Presentation by Jean AldrichPresentation by Jean Aldrich

Study conducted in the McMurdo Dry Study conducted in the McMurdo Dry Valleys of South Victoria Land, Valleys of South Victoria Land, AntarcticaAntarctica

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

PurposePurpose of Study of Study

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

The hyporheic zone is an area of sediment The hyporheic zone is an area of sediment adjacent to and underneath the streamadjacent to and underneath the stream

Water in the hyporheic zone flows in a Water in the hyporheic zone flows in a downstream direction interacting with downstream direction interacting with water in the main channelwater in the main channel

(D. McKnight, et al. 2001)(D. McKnight, et al. 2001)

Study siteStudy site

During winterDuring winter Continuously darkContinuously dark High windsHigh winds Air temps as low as -60˚CAir temps as low as -60˚C

During summerDuring summer Continuously lightContinuously light Air temps as high as 5˚CAir temps as high as 5˚C

The McMurdo Dry Valleys are the The McMurdo Dry Valleys are the largest ice-free areas of Antarcticalargest ice-free areas of Antarctica

There is no plant lifeThere is no plant life Permafrost at a depth of ~0.5 mPermafrost at a depth of ~0.5 m The climate is cold and dryThe climate is cold and dry

BackgroundBackground The dry valley streams are fed by meltwater from glaciersThe dry valley streams are fed by meltwater from glaciers Average summer stream flow depends upon the duration Average summer stream flow depends upon the duration

of temps above 0˚c and insolation during the summerof temps above 0˚c and insolation during the summer Discharge is variable – can range from ~0.5 mDischarge is variable – can range from ~0.5 m3/s during a /s during a

warm summer to no flow during cold summerswarm summer to no flow during cold summers Discharge can also vary as much as 10 – fold during a dayDischarge can also vary as much as 10 – fold during a day Many of the streams have abundant perennial mats of Many of the streams have abundant perennial mats of

filamentous cyanobacteria which grow during streamflow filamentous cyanobacteria which grow during streamflow in the summer and are in a dormant freeze-dried state in the summer and are in a dormant freeze-dried state the rest of the yearthe rest of the year

MethodsMethods

Water samples were collected at the gauging stations Water samples were collected at the gauging stations near the outflow of the streams to the lakesnear the outflow of the streams to the lakes

Water was collected whenever there was stream flow Water was collected whenever there was stream flow during the summer (mid Nov to late Jan)during the summer (mid Nov to late Jan)

The distribution of algal mats was mapped at 16 sites The distribution of algal mats was mapped at 16 sites in 12 streamsin 12 streams

The abundance of algal maps was characterized as:The abundance of algal maps was characterized as: Very sparse (undetectable)Very sparse (undetectable) Sparse (<5% cover)Sparse (<5% cover) Moderate (5% - 80% cover)Moderate (5% - 80% cover) Abundant (>80% cover)Abundant (>80% cover)

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

On January 13, 1995 an experimental tracer and On January 13, 1995 an experimental tracer and nutrient injection was conducted in Green Creeknutrient injection was conducted in Green Creek

Water samples were collected at four sites below the Water samples were collected at four sites below the injection site in areas where the channel was injection site in areas where the channel was somewhat constrainedsomewhat constrained

The injection lasted 130 minutes and samples were The injection lasted 130 minutes and samples were collected at 3 to 15 minute intervals for 30 minutes collected at 3 to 15 minute intervals for 30 minutes before the injection until several hours after the before the injection until several hours after the injectioninjection

ResultsResults Specific conductance and major ion concentrations in Specific conductance and major ion concentrations in

12 streams in the Taylor Valley were variable, because 12 streams in the Taylor Valley were variable, because their concentrations decreased with increasing their concentrations decreased with increasing streamflowstreamflow

In general, nutrient concentrations also showed In general, nutrient concentrations also showed substantial variation among streamssubstantial variation among streams

The streams with sparse algal cover generally had the The streams with sparse algal cover generally had the highest nutrient concentrationshighest nutrient concentrations

The results of the statistical analysis confirmed the The results of the statistical analysis confirmed the relationship between low concentrations for NOrelationship between low concentrations for NO33 and and SRP and abundant algal matsSRP and abundant algal mats

Synoptic Study: Von Guerard StreamSynoptic Study: Von Guerard Stream Nutrient concentrations Nutrient concentrations

were generally lowest in were generally lowest in melt water and greatest in melt water and greatest in the hyporheic zone and the hyporheic zone and parafluvial seepsparafluvial seeps

SRP, NHSRP, NH44, and specific , and specific conductance were uniform conductance were uniform in surface water from the in surface water from the main channel in the main channel in the downstream directiondownstream direction

NONO33 + NO + NO22 was below the was below the detection limit in all main detection limit in all main channel surface water channel surface water samples (as opposed to samples (as opposed to seeps) seeps)

The average The average concentrations of SRP, NOconcentrations of SRP, NO33 + NO+ NO22 and NH and NH4 4 from surface from surface water were similar to the water were similar to the concentrations found in the concentrations found in the bankside meltwaterbankside meltwater

Mean values for SRP and NOMean values for SRP and NO33 + NO + NO22 were significantly were significantly higher in water from the underlying hyporheic zone than higher in water from the underlying hyporheic zone than in the main channel surface waterin the main channel surface water

The SRP, NOThe SRP, NO33 + NO + NO22, NH, NH44 and specific conductance were and specific conductance were all significantly higher in surface and subsurface water all significantly higher in surface and subsurface water from parafluvial seeps than in main channel surface from parafluvial seeps than in main channel surface waterwater

Tracer Injection Experiment: Green CreekTracer Injection Experiment: Green CreekHydrologic CharacterizationHydrologic Characterization

All of the flow in Green Creek comes from glacial meltwater All of the flow in Green Creek comes from glacial meltwater that enters the stream above the injection sitethat enters the stream above the injection site

Cl concentrations increased at all 4 sampling locations as a Cl concentrations increased at all 4 sampling locations as a result of the 130 min LiCl injection, but plateau concentrations result of the 130 min LiCl injection, but plateau concentrations were obtained at only the most upstream sitewere obtained at only the most upstream site

Cl concentrations at the remaining sites were affected by Cl concentrations at the remaining sites were affected by mixing processes that delayed and attenuated the tracer pulsemixing processes that delayed and attenuated the tracer pulse

Authors concluded that the 130 min injection period was not Authors concluded that the 130 min injection period was not of sufficient duration to obtain plateau concentrations at the of sufficient duration to obtain plateau concentrations at the down stream sitesdown stream sites

Conservative solute transport simulations reproduced the Conservative solute transport simulations reproduced the general features of the observed Cl profiles at all sampling general features of the observed Cl profiles at all sampling locationslocations

Hydrologic parameters used within the transport model to quantify Hydrologic parameters used within the transport model to quantify hyporheic exchange include the storage zone cross-sectional area, hyporheic exchange include the storage zone cross-sectional area, AAss, and the exchange rate coefficient, , and the exchange rate coefficient,

The presence of a large hyporheic zone within Green Creek was 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, indicated by the lack of a steady state plateau for Cl at 226, 327, and 497m (the three downstream sites)and 497m (the three downstream sites)

Hyporheic-zone waters did not become fully saturated with tracer-Hyporheic-zone waters did not become fully saturated with tracer-enriched water during the 130 min injection period - this was enriched water during the 130 min injection period - this was supported by the large estimates of Asupported by the large estimates of Ass in reaches 2 and 3 in reaches 2 and 3

The relative size of the hyporheic zone may be quantified using the The relative size of the hyporheic zone may be quantified using the ratio of storage zone and main channel cross-sectional areas (Aratio of storage zone and main channel cross-sectional areas (Ass/A)/A)

High values of AHigh values of Ass/A may be attributed to the period of minimum /A may be attributed to the period of minimum flow (0.3 L/s) when most of the water was within the hyporheic zoneflow (0.3 L/s) when most of the water was within the hyporheic zone

Nutrient AdditionsNutrient Additions The concentrations of NOThe concentrations of NO33 and PO and PO44 were elevated at the point of injection were elevated at the point of injection NONO33, NO, NO22, and PO, and PO44 concentrations increased at the upper three sampling concentrations increased at the upper three sampling

locations (50, 226, & 327 m)locations (50, 226, & 327 m) Concentrations were near or below the detection limit at the most Concentrations were near or below the detection limit at the most

downstream sitedownstream site NHNH44 concentrations were erratic concentrations were erratic

Optimal simulation results for NOOptimal simulation results for NO33 were obtained by were obtained by considering uptake in both the main channel and the considering uptake in both the main channel and the hyporheic zonehyporheic zone

Estimates of mass loss indicate that ~84.5 to 93.5% of the Estimates of mass loss indicate that ~84.5 to 93.5% of the observed NOobserved NO33 uptake occurred in the main channel, where uptake occurred in the main channel, where as 6.5 to 15.5% occurred in the hyporheic zoneas 6.5 to 15.5% occurred in the hyporheic zone

Optimal simulation results for POOptimal simulation results for PO44 were were obtained by considering uptake in the main obtained by considering uptake in the main channel exclusivelychannel exclusively

POPO4 4 simulations that included hyporheic simulations that included hyporheic uptake in addition to main channel uptake uptake in addition to main channel uptake were indistinguishable from those that relied were indistinguishable from those that relied solely on main-channel uptakesolely on main-channel uptake

Simulations that considered only hyporheic-Simulations that considered only hyporheic-zone uptake did not reproduce the features of zone uptake did not reproduce the features of the observed datathe observed data

DiscussionDiscussionAutotrophic Uptake of NutrientsAutotrophic Uptake of Nutrients

The importance of autotrophic uptake by benthic algal The importance of autotrophic uptake by benthic algal communities is indicated in the comparison study by the communities is indicated in the comparison study by the significantly lower mean NOsignificantly lower mean NO33 and SRP concentrations in and SRP concentrations in streams with abundant algal mats than in streams with streams with abundant algal mats than in streams with sparse algal matssparse algal mats

The release of nutrients through weathering reactions The release of nutrients through weathering reactions or dissolution of aerosols occurs in the hyporheic zone or dissolution of aerosols occurs in the hyporheic zone of all dry valley streamsof all dry valley streams

Therefore the absence of algal mats because of Therefore the absence of algal mats because of unsuitable habitat causes the nutrients to remain in unsuitable habitat causes the nutrients to remain in solution when hyporheic exchange brings these solution when hyporheic exchange brings these solutes into the main channelsolutes into the main channel

The results of the Von Guerard Stream study indicate The results of the Von Guerard Stream study indicate that nutrient concentrations varied laterally along a that nutrient concentrations varied laterally along a gradient from the main channel to the hyporheic zonegradient from the main channel to the hyporheic zone

NONO33 and SRP concentrations were significantly lower in and SRP concentrations were significantly lower in the main channel than in the underlying hyporheic the main channel than in the underlying hyporheic zone or the parafluvial seepszone or the parafluvial seeps

The results were interpreted as showing that nutrient The results were interpreted as showing that nutrient uptake by the benthic algal communities matches the uptake by the benthic algal communities matches the nutrient flux from hyporheic exchange, preventing nutrient flux from hyporheic exchange, preventing accumulation of nutrients in the main channelaccumulation of nutrients in the main channel

The nutrient uptake occurring in both the main The nutrient uptake occurring in both the main channel and the hyporheic zone in the simulations of channel and the hyporheic zone in the simulations of the nutrient enrichment experiment in Green Creek the nutrient enrichment experiment in Green Creek was taken into considerationwas taken into consideration

The results may have been influenced by nutrient The results may have been influenced by nutrient concentrations in the first two reaches that exceeded concentrations in the first two reaches that exceeded those observed in the stream itselfthose observed in the stream itself

Mulholland et al. 1990, 2000,Dodds et al. 2002Mulholland et al. 1990, 2000,Dodds et al. 2002

No increase in nutrient concentrations was detected at No increase in nutrient concentrations was detected at the furthest downstream site during the experiment, the furthest downstream site during the experiment, suggesting that saturation of nutrient uptake did not suggesting that saturation of nutrient uptake did not have a large influencehave a large influence

Dodds et al. 2002Dodds et al. 2002

The solute transport model coupled with non-The solute transport model coupled with non-linear regression provided an objective means of linear regression provided an objective means of quantifying the processes within the main quantifying the processes within the main channel and the hyporheic zonechannel and the hyporheic zone

NONO33 uptake in the main channel can be attributed uptake in the main channel can be attributed to autotrophic uptake by algal matsto autotrophic uptake by algal mats

NONO33 uptake in the hyporheic zone can be uptake in the hyporheic zone can be attributed to other microbial processesattributed to other microbial processes

Uptake by the algal mats in the main channel was Uptake by the algal mats in the main channel was the primary NOthe primary NO33 sink in Green Creek sink in Green Creek

Uptake of POUptake of PO44 appeared to be controlled entirely appeared to be controlled entirely by in-channel processesby in-channel processes

The authors concluded that the algal mats The authors concluded that the algal mats assimilated POassimilated PO44 with no further loss occurring in with no further loss occurring in the hyporheic zonethe hyporheic zone

High nutrient concentrations were found in High nutrient concentrations were found in the hyporheic and parafluvial zones in the the hyporheic and parafluvial zones in the synoptic study of Von Guerard Streamsynoptic study of Von Guerard Stream

These zones are sources of NOThese zones are sources of NO33 and SRP to and SRP to the benthic algal mats in the streams the benthic algal mats in the streams because of hydrologic exchange processesbecause of hydrologic exchange processes

Therefore hyporheic exchange in dry Therefore hyporheic exchange in dry valley streams appears to influence valley streams appears to influence nutrient cycling in a manner similar to that nutrient cycling in a manner similar to that observed in a temperate desert streamobserved in a temperate desert stream

Valett et al. 1990,1994, Grimm et al. 1991, Holmes et al. 1994, Jones et al. 1995Valett et al. 1990,1994, Grimm et al. 1991, Holmes et al. 1994, Jones et al. 1995

N TransformationsN Transformations