THE NITROGEN BUDGET OF A SHALLOW LAKE (GROSSER MUGGELSEE, BERLIN)

The nitrogen budget of Lake Gr. Muggelsee (mean depth 4.85 mp, area 7.12 km2) was calculated from weekly measurements of nitrogen transformations from 1978 to 1984. Common mass balances for nitrogen were completed by determination of dinitrogen fixation in order to calculate denitrification as residual budget component and resolved for distinct seasonal periods. N sedimentation was calculated from internal P balance. Influence of flushing rate on loss processes were analysed by comparison of years with very different runoff. Time series analyses and multiple comparison show a relation of hydraulic flow, nitrate load, in-lake nitrate and N elimination to the seasonal cycle of irradiance. Denitrification is the most important loss process and exceeds the effect of sedimentation. Other in- and outputs (precipitation, swimmers, volatilization) are rather stochastic and of lower quantitative importance. Nitrate leaching from the catchment and turbulent mixing within the lake are discussed as determining factors of in-lake nitrogen fluxes.