Microbial dinitrogen and nitrous oxide production in a small eutrophic reservoir: An in situ approach to quantifying hypolimnetic process rates

Nitrogen (N) dynamics within the hypolimnion of a thermally stratified reservoir were examined to test an in situ approach to measuring dinitrogen (N2) and nitrous oxide (N2O) production rates wherein hypolimnion gas accumulation is used to estimate N2 and N2O production. This previously unpublished approach provides a spatially integrated, time‐varying record of N transformation rates that fall well within the range of rates reported for other reservoir systems using other methods. Hypolimnion N2 production averaged 183 μmol N2‐N m−2 h−1 with higher rates observed early in a spring stratification event (538 μmol N2‐N m−2 h−1) and lower rates observed later in the same stratification event (90 μmol N2‐N m−2 h−1). Sediment incubation experiments and hypolimnion nitrate (NO−3) data show that, over the course of the summer, progressive NO−3 depletion at the sediment‐water interface limited N2 production and associated N removal. As rates of N2 production dropped off, rates of N2O production increased (from 4.62 μmol N2O‐N m−2 d−1 to 51 μmol N2O‐N m−2 d−1, averaging 26 μmol N2O‐N m−2 d−1), resulting in significant increases in N2O‐N : N2‐N ratios as the summer progressed. Also, whereas N2 production appeared to occur predominantly at the sediment‐water interface, N2O production was detected throughout the water column, suggesting a role for nitrification as a source of N2O. The use of hypolimnion accumulation to quantify N transformation rates can thus offer new insights into spatial and seasonal N transformation patterns in stratified or otherwise capped aquatic systems.