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 (N 2 ) and nitrous oxide (N 2 O) production rates wherein hypolimnion gas accumulation is used to estimate N 2 and N 2 O 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 This previously unpublished approach provides a spatially integrated, time‐varying record of N transformation rates that fall well within the range of rates reported for other reservoir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 N 2 production averaged 183 μmol N 2 ‐N m −2 h −1 with higher rates observed early in a spring stratification event (538 μmol N 2 ‐N m −2 h −1 ) and lower rates observed later in the same stratification event (90 μmol N 2 ‐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 N 2 production and associated N removal. As rates of N 2 production dropped off, rates of N 2 O production increased (from 4.62 μmol N 2 O‐N m −2 d −1 to 51 μmol N 2 O‐N m −2 d −1 , averaging 26 μmol N 2 O‐N m −2 d −1 ), resulting in significant increases in N 2 O‐N : N 2 ‐N ratios as the summer progressed. Also, whereas N 2 production appeared to occur predominantly at the sediment‐water interface, N 2 O production was detected throughout the water column, suggesting a role for nitrification as a source of N 2 O. 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.