The N and P budget of the shallow, flushed lake Muggelsee:: retention, external and internal load

The shallow (mean depth 4.9 m), polymictic and eutrophic lake Muggelsee was highly loaded with phosphorus (6 gP m(-2) a(-1)) and nitrogen (170 gN m(-2) a(-1)) by the river Spree up to the end of the 1980s. Annual load declined by 40-50% during the last years (1991-97). Phosphorus retention fluctuated strongly during the seasonal cycle between -200 and +100 kgP d(-1) and from year to year between -44% and + 26% of the P import. At the end of the eighties, the P retention capacity of the sediment was exceeded and Muggelsee became a source of phosphorus. The lake regained its ability to retain P in the sediments after external load reduction in the 1990s. However, the internal load of P reached the level of the external one. The release of P during summer was strongly related to the import of nitrate. On long-term average (1979-1997), less than 1% of the P input was retained in Muggelsee. About 24% of the nitrogen load were removed in the lake on annual mean. This rate decreased during the last years.