Restoration and resilience to recovery of the Lake Loosdrecht ecosystem in relation to its phosphorus flow

A reduction in external phosphorus loading since 1984 to Loosdrecht lakes system by the dephosphorization of the inlet water, yielded only minor effects in Lake Loosdrecht. This reduction measure turned out to have decreased the loading only by a factor of two. A conceptual model was constructed based on laboratory measurements to describe phosphorus flow in the lake ecosystem for the summer of 1987. The role of zooplankton and fish was more important in phosphorus recycling than diffusion at the sediment-water interface. The input and output of phosphorus of the lake were at equilibrium and therefore, further reduction in external loading was needed for recovery. The results of the conceptual model agreed well with the output of the mathematical model PCLOOS. Additional measures such as dredging, flushing, chemomanipulation, or biomanipulation would be ineffective at the present level of external loading. Only a significant further reduction in external input will restore Lake Loosdrecht's water quality over a long period of time.