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 This previously unpublished approach provides a spatially integrated, time-varying record of N transformation rates that fall well within the range of rates reported for other reservoir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 mu mol N(2)-N m(-2) h(-1) with higher rates observed early in a spring stratification event (538 mu mol N(2)-N m(-2) h(-1)) and lower rates observed later in the same stratification event (90 mu mol 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 mu mol N(2)O-N m(-2) d(-1) to 51 mu mol N(2)O-N m(-2) d(-1), averaging 26 mu mol 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.