Changes in sediment phosphorus as a result of eutrophication and oligotrophication in Lake Veluwe, The Netherlands

Since the creation of Lake Veluwe, The Netherlands, in 1956, a period of eutrophication has been followed by a period of oligotrophication. Around 1970, the lake switched from clear to turbid and recently from turbid to clear. The effect of eutrophication and oligotrophication has been studied with a conceptual model, based on the mass balance of total phosphorus in the top layer of the sediment. The model has been applied to the shallow part and the deeper part of Lake Veluwe. Whereas drastic changes in the external phosphorus loading, and concomitantly the phosphorus concentration in the lake water, occurred, the phosphorus pool in the sediment top layer only responds gradually to changed boundary conditions. The phosphorus content in the sediment top layer of the deeper part of Lake Veluwe doubled between 1956 and 1979. After 1979 it decreased. Losses were due to infiltration of dissolved phosphorus and burial of solid phosphorus caused by net sedimentation with material from the shallow part of the lake. Sensitivity analyses revealed that sediment phosphorus is also primary affected by these two factors. Based on the model predictions it is expected that in 2006 the phosphorus content of the sediment will be below the phosphorus content at creation of Lake Veluwe. Changes in flushing of the lake will probably have little effect on sediment phosphorus, while development of macrophytes will cause higher phosphorus contents in the shallow areas. However, with the current boundary conditions further ecological improvement of Lake Veluwe may be expected. The phosphorus pool in the sediment top layer is an important determinant in the long-term development of shallow lake ecosystems and, therefore, the modeling concept may also be applicable to other shallow lakes.