A reactive nitrogen budget for Lake Michigan

The reactive (fixed) nitrogen (Nr) budget for Lake Michigan was estimated, making use of recent estimates of watershed and atmospheric nitrogen loads. Reactive N is considered to include nitrate, nitrite, ammonium, and organic N. The updated Nr load to Lake Michigan was approximately double the previous estimate from the Lake Michigan Mass Balance study for two reasons: 1) recent estimates of watershed loads were greater than previous estimates and 2) estimated atmospheric dry deposition and deposition of organic N were included in our budget. Atmospheric and watershed Nr loads were nearly equal. The estimated loss due to denitrification at the sediment surface was at least equal to, and possibly much greater than, the combined loss due to outflow and net sediment accumulation. Within the considerable uncertainty of the denitrification estimate, the budget was nearly balanced, which was consistent with the slow rate of accumulation of nitrate in Lake Michigan (~ 1%/yr). The updated loads were used to force the LM3-PP biogeochemical water quality model. Simulated water column concentrations of nitrate and organic nitrogen in the calibrated model were consistent with available observational data when denitrification was included at the sediment surface at a rate that is consistent with literature values. The model simulation confirmed that the estimated denitrification rate does not exceed the availability of settling organic N mass. Simulated increase (decrease) in nitrate concentration was sensitive to model parameters controlling supply of sediment organic N, highlighting the importance of internal processes, not only loads, in controlling accumulation of N.