Showing papers by "Diane M. McKnight published in 2004"

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

Abstract: The McMurdo Dry Valleys of South Victoria Land, Antarctica, contain numerous glacial meltwater streams that drain into lakes on the valley floors. Many of the streams have abundant perennial mats of filamentous cyanobacteria. The algal mats grow during streamflow in the austral summer and are in a dormant freeze-dried state during the rest of the year. NO3 and soluble reactive P (SRP) concentrations were lower in streams with abundant algal mats than in streams with sparse algal mats. NO3 and SRP concentrations were higher in the hyporheic zone of a stream with abundant algal mats than in the stream itself. An experimental injection of LiCl, NaNO3 ,a nd K 3PO4 was con- ducted in Green Creek, which has abundant algal mats. Substantial hyporheic exchange occurred. The NO3 and PO4 concentrations at 50 m below the injection were 55 m Ma nd 18mM, respectively, during the experiment. NO3 and PO4 concentrations were below the detection limit of 1 to 2 m Ma t a site 497 m below the injection during the Cl tracer arrival, indicating a high capacity for nutrient uptake by algal communities. NO2 and NH4 were present at sites 226 and 327 m below the injection, indicating that, in addition to denitrification and algal uptake, dissimilatory NO3 reduction to NO2 and NH4 may be a NO3 sink during transport. Transport modelling with nutrient uptake represented as a1 st-order process yielded reach-scale parameters of 4.3 3 1025 to 3.9 3 1024/s and 1.4 3 1024 to 3.8 3 1024/s for uptake of NO3 and PO4, respectively. The best match with the observed data was a model in which PO4 uptake occurred only in the main channel and NO3 uptake occurred in the main channel and in the hyporheic zone. Hyporheic NO3 uptake was 7 to 16% of the total uptake for the different stream reaches. These results demonstrate that nutrient flux to the lakes is controlled by hyporheic exchange and nutrient uptake by algal mats in dry valley streams. Streams without algal mats contribute more nutrients to the lakes than streams with algal mats.

The Landscape Continuum: A Model for High-Elevation Ecosystems

Abstract: Interactions between climate and ecosystems with complex topographic gradients generate unique source and sink habitats for water and nutrients as a result of precipitation, energy, and chemical redistribution. We examined these phenomena for a high-elevation site in the Colorado Front Range. Current changes in climate and atmospheric deposition of nitrogen to these systems are causing rapid changes in some portions of this system but not in others. Using a conceptual model that links terrestrial ecosystems to each other and to aquatic ecosystems, we report how atmospheric inputs and endogenous resources can be amplified or attenuated by transport processes. High-elevation lakes and the alpine tundra–forest ecotone are expected to receive the brunt of anthropogenic inputs obtained from (a) the redistribution of exogenous materials from the regional environment and (b) endogenous sources originating in other montane areas.

Denitrification and hydrologic transient storage in a glacial meltwater stream, McMurdo Dry Valleys, Antarctica

Abstract: In extreme environments, retention of nutrients within stream ecosystems contributes to the persistence of aquatic biota and continuity of ecosystem function. In the McMurdo Dry Valleys, Antarctica, many glacial meltwater streams flow for only 5‐12 weeks a year and yet support extensive benthic microbial communities. We investigated NO 3 2

Conservative and reactive solute transport in constructed wetlands

Abstract: [1] The transport of bromide, a conservative tracer, and rhodamine WT (RWT), a photodegrading tracer, was evaluated in three wastewater-dependent wetlands near Phoenix, Arizona, using a solute transport model with transient storage. Coupled sodium bromide and RWT tracer tests were performed to establish conservative transport and reactive parameters in constructed wetlands with water losses ranging from (1) relatively impermeable (15%), (2) moderately leaky (45%), and (3) significantly leaky (76%). RWT first-order photolysis rates and sorption coefficients were determined from independent field and laboratory experiments. Individual wetland hydraulic profiles influenced the extent of transient storage interaction in stagnant water areas and consequently RWT removal. Solute mixing and transient storage interaction occurred in the impermeable wetland, resulting in 21% RWT mass loss from main channel and storage zone photolysis (10%) and sorption (11%) reactions. Advection and dispersion governed solute transport in the leaky wetland, limiting RWT photolysis removal (1.2%) and favoring main channel sorption (3.6%). The moderately leaky wetland contained islands parallel to flow, producing channel flow and minimizing RWT losses (1.6%). INDEX TERMS: 1890 Hydrology: Wetlands; 1871 Hydrology: Surface water quality; 3230 Mathematical Geophysics: Numerical solutions; KEYWORDS: constructed wetlands, OTIS, transient storage

Changes in fulvic acid redox state through the oxycline of a permanently ice-covered Antarctic lake

Abstract: The McMurdo Dry Valleys of Antarctica contain many permanently ice-covered lakes that support populations of algae and bacteria in the water column. In these lakes the concentration of dissolved organic carbon (DOC) is typically greatest at depth. In Lake Fryxell, the DOC concentration is 25 mg C/L at 18 m and 5 mg C/L at 5 m, just below the ice-cover. Dissolved humic substances account for about 20–24% of the DOC in the lake water. The DOC sources to the photic zone of this lake are streamflow, extracellular release by phytoplankton and benthic algal mats and upward diffusion across the oxycline at 9.5 m. Experiments with fulvic acids isolated from four depths show that these humic substances have the capacity to act as electron acceptors in the anoxic degradation of acetate by an iron-and humic-reducing microorganism. We used fluorescence spectroscopy to characterize the abundance and redox state of the quinone functional groups in the oxidized and reduced fulvic acids in these experiments and in filtered whole water samples from the lake. The fluorescence intensity of fulvic acid was greater in the oxycline and bottomwaters than in the photic zone. This result suggests that incorporation of quinone functional groups into humic substances may be enhanced in zones of high bacterial activity. Statistical analysis of the excitation emission matrices (EEMs) was used to evaluate trends in the fulvic acid redox state with depth. The results indicate that fulvic acid in the upper photic zone was in an oxidized state and that fulvic acid in the bottomwaters was in a reduced state. The shift in the EEMs indicating a more reduced state occurred in the vicinity of the oxycline (8 to 11 m). The shift in the EEMs began in the zone from 8 and 9 m, where dissolved oxygen concentrations range from to 5 and 10 mg L–1, suggesting that fulvic acid was oxidized upon upward diffusion from the oxycline. This oxidation may be an abiotic process in which reduced humic substances interact with ferric iron generated in this zone.

Reach-scale cation exchange controls on major ion chemistry of an Antarctic glacial meltwater stream

Abstract: McMurdo dry valleys of Antarctica represent the largest of the ice-free areas on the Antarctic continent, containing glaciers, meltwater streams, and closed basin lakes. Previous geochemical studies of dry valley streams and lakes have addressed chemical weathering reactions of hyporheic substrate and geochemical evolution of dry valley surface waters. We examine cation transport and exchange reactions during a stream tracer experiment in a dry valley glacial meltwater stream. The injection solution was composed of dissolved Li+, Na+, K+, and Cl-. Chloride behaved conservatively in this stream, but Li+, Na+, and K+ were reactive to varying degrees. Mass balance analysis indicates that relative to Cl-, Li+ and K+ were taken up in downstream transport and Na+ was released. Simulations of conservative and reactive (first-order uptake or generation) solute transport were made with the OTIS (one-dimensional solute transport with inflow and storage) model. Among the four experimental reaches of Green Creek, solute transport simulations reveal that Li+ was removed from stream water in all four reaches, K+ was released in two reaches, taken up in one reach, and Na+ was released in all four reaches. Hyporheic sediments appear to be variable with uptake of Li+ in two reaches, uptake of K+ in one reach, release of K+ in two reaches, and uptake of Na+ in one reach. Mass balances of the conservative and reactive simulations show that from 1.05 to 2.19 moles of Li+ was adsorbed per reach, but less than 0.3 moles of K+ and less than 0.9 moles of Na+ were released per reach. This suggests that either (1) exchange of another ion which was not analyzed in this experiment or (2) that both ion exchange and sorption control inorganic solute transport. The elevated cation concentrations introduced during the experiment are typical of initial flows in each flow season, which flush accumulated dry salts from the streambed. We propose that the bed sediments (which compose the hyporheic zone) modulate the flushing of these salts during initial flows each season, due to ion exchange and sorption reactions.

Reply to Comment on ``Abandoned Mines, Mountain Sports, and Climate Variability: Implications for the Colorado Tourism Economy''

Abstract: Our article focused on the complex interactions among climate variability, hydrology, chemical weathering reactions, and stream ecology that influence water resource availability for recreation in watersheds of the Colorado Rocky Mountains. In responding to our article, our colleagues at Hydrosphere Resource Consultants provide additional detailed information about snow-making approaches at ski resorts. However, they make other assertions that warrant comment and clarification. We disagree with the statement that the ski industry may not be the appropriate tourism sector for illustrating the impacts of climate variations. The success of the ski industry hinges on a variety of climate-related variables (for example, temperature, precipitation quantity, precipitation as snow versus rain) that are expected to change in an uncertain climatic future.