In recent decades anthropogenic activities have led to increased N inputs to freshwater systems and associated increases in harmful algal blooms and fish kills. Denitrification (DNF) and coupled dissimilatory nitrate reduction to ammonia/anaerobic ammonia oxidation
(DNRA/anammox) are microbially mediated anaerobic processes that reduce biologically available NO3- to N2 gas. While DNF and potentially DNRA/anammox are primary N-removal processes in freshwater systems, their temporal dynamics are poorly understood, particularly in reservoir systems. In addition, there are several N transformations, both aerobic (microbial oxidation of NH4+ to NO3-) and anaerobic (DNF), that produce N2O, a powerful greenhouse gas, as a byproduct. Environmental controls on N2O production in aquatic systems are also poorly constrained. Lacamas Lake, a small (1.3 km2) reservoir draining to the Columbia River, was sampled monthly along a vertical gradient in the deepest part of the reservoir (17m) from June 2007 to September 2008. A full factorial sediment incubation experiment was also conducted in August 2008 to test for nitrate and carbon limitation of N2O and N2 production as well as sediment NO3- uptake.
Membrane Inlet Mass Spectrometry and Gas Chromatography were used to quantify excess N2O and N2 in the water column.
Results/Conclusions
Bottom water N2O and N2 were significantly higher during stratified than mixed conditions (one-tailed paired t-test, p<0.05), increased with time across the spring-time stratification event (R2=0.89, and R2=0.68, respectively), and were negatively correlated with dissolved oxygen (R2=0.78, and R2=0.71, respectively) during this time. The sediment incubation experiment also showed that NO3-N limited NO3- removal (p<0.001). Results suggest that stratification may both isolate microbes from NO3-N and provide sufficiently anoxic conditions to support increased N removal and N2O production. We suggest that the stratification event is a biogeochemical “hot moment” in seasonally stratifiying lentic systems. Hypolimnion N2 concentrations also increase steadily during the summer (R2=0.71), suggesting that gas is accumulating below the thermocline. N removal rates quantified using this assumption account for all NO3-N present during winter mixed conditions. Significant variation in N-removal byproducts across mixing regimes, as well as evidence for disproportionately high spring N-removal rates, emphasizes the importance of considering seasonal variation when estimating annual N-removal in these systems. Future research should investigate whether increased reservoir mixing is an effective dam management strategy for reducing greenhouse gas emissions and refreshing the bottom water DIN supply for microbial removal.
