Budget calculations in the shallow lakes Dümmer and Schlei, F.R.G., and in limnocorral experiments carried out in Lake Schlei between 1981 and 1983 showed large seasonal fluctuations of internal phosphorus loading. These fluctuations were related to the load of the electron acceptors nitrate and sulfate from the tributaries and the internal metabolic processes. Based on lake budget studies, phosphorus release occurs predominantly during late spring and early summer (two months with maximum release rates of up to 200 mgP/m2·d).In siturelease rates correlated poorly with water column pH and with results of laboratory experiments carried out on those sediments. The phosphorus released was correlated with iron-bound phosphorus in the sediment (phosphorus measured as extractable by hydrogen sulfate treatment); however, down to a depth of microbial activation in the sediment. Algal development and nitrate depletion in the water had a strong triggering effect on phosphorus release from sediment. The mechanisms of internal phosphorus recycling became evident by monitoring the concentrations of iron, phosphorus, sulfate, and nitrate in the interstitial solutions. Management implications were derived from these results.