Monday, August 4, 2008PS 4-68: Seasonal variation in denitrification in a small eutrophic reservoir: Lacamas Lake, Camas, Washington
Bridget R. Deemer and John A. Harrison. Washington State University Vancouver
Background/Question/Methods
In recent decades anthropogenic activities have more than doubled the rate of terrestrial nitrogen (N) fixation, leading to increased N inputs to freshwater and coastal systems and associated increases in eutrophication, harmful algal blooms, fish kills, and community composition shifts. Denitrification (DNF), the microbially mediated process whereby biologically available nitrate is reduced to N2 gas, is a primary pathway by which N is removed from aquatic ecosystems. DNF is thought to occur under anoxic conditions where there is adequate dissolved organic carbon, nitrate, and phosphorous. Despite a body of research identifying factors that can influence DNF rates, the spatial and temporal dynamics of DNF are poorly understood. In particular, DNF is understudied in reservoir systems. Lacamas Lake, a small (1.3 km2) monomictic reservoir draining to the Columbia River, was sampled monthly from June 2007 to February 2008 at 1m intervals along a vertical transect in the deepest part of the lake (17m). A membrane-inlet mass spectrometer was used to determine dissolved N2 and Ar concentrations. Expected N2:Ar ratios were compared to measured ratios in order to ascertain supersaturation of N2, a proxy for DNF.
Results/Conclusions
During winter mixed conditions, the reservoir was supersaturated with N2 throughout the water column whereas during summer stratified conditions water below 13m was consistently supersaturated with N2. A general linear model showed significant interaction between season and depth of N2 supersaturation (p<0.05) and significantly higher N2:Ar below 13m than above 13m (P<0.05). These results indicate that DNF plays an important role in reservoir N removal. They also suggest that controls on DNF differ seasonally. Future work aims to quantify an annual DNF flux in order to identify time periods that may be particularly important with regard to reservoir N removal. These findings are likely to have important implications for modeling N transport at larger scales and for dam management strategies.
In recent decades anthropogenic activities have more than doubled the rate of terrestrial nitrogen (N) fixation, leading to increased N inputs to freshwater and coastal systems and associated increases in eutrophication, harmful algal blooms, fish kills, and community composition shifts. Denitrification (DNF), the microbially mediated process whereby biologically available nitrate is reduced to N2 gas, is a primary pathway by which N is removed from aquatic ecosystems. DNF is thought to occur under anoxic conditions where there is adequate dissolved organic carbon, nitrate, and phosphorous. Despite a body of research identifying factors that can influence DNF rates, the spatial and temporal dynamics of DNF are poorly understood. In particular, DNF is understudied in reservoir systems. Lacamas Lake, a small (1.3 km2) monomictic reservoir draining to the Columbia River, was sampled monthly from June 2007 to February 2008 at 1m intervals along a vertical transect in the deepest part of the lake (17m). A membrane-inlet mass spectrometer was used to determine dissolved N2 and Ar concentrations. Expected N2:Ar ratios were compared to measured ratios in order to ascertain supersaturation of N2, a proxy for DNF.
Results/Conclusions
During winter mixed conditions, the reservoir was supersaturated with N2 throughout the water column whereas during summer stratified conditions water below 13m was consistently supersaturated with N2. A general linear model showed significant interaction between season and depth of N2 supersaturation (p<0.05) and significantly higher N2:Ar below 13m than above 13m (P<0.05). These results indicate that DNF plays an important role in reservoir N removal. They also suggest that controls on DNF differ seasonally. Future work aims to quantify an annual DNF flux in order to identify time periods that may be particularly important with regard to reservoir N removal. These findings are likely to have important implications for modeling N transport at larger scales and for dam management strategies.
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See more of The 93rd ESA Annual Meeting (August 3 -- August 8, 2008)