A MODEL OF NUTRIENT DYNAMICS IN SHALLOW LAKES IN RELATION TO MULTIPLESTABLE STATES

There is increasing evidence that, within a range of nutrient loadings , shallow lakes may have two alternative stable states. One is dominat ed by phytoplankton and the other one by submerged macrophytes as the main primary producer. The question arises at what level of nutrient l oading a transition may occur between the two states. This question wa s addressed by means of the integrated lake model PCLake. The model de scribes the competition between phytoplankton and macrophytes, within the framework of closed nutrient cycles in the lake system, including the upper sediment. Top-down effects via the food web were regarded as well. The model was run for a hypothetical shallow lake, representati ve for the situation in The Netherlands. Long-term simulations were ca rried out for a realistic range of nutrient loadings and starting from different initial conditions. The results showed a highly non-linear response, which also showed hysteresis: the loading level at which a t ransition occurs turned out to be dependent on the initial conditions. The results were compared with empirically derived chlorophyll a to p hosphorus relations. Factors influencing the 'critical nutrient level' were the lake dimensions and the net sedimentation rate. The model wa s also used to evaluate the role of food web management in lake restor ation. The results suggest that a long-term effect of additional manag ement is possible only if combined with a decrease in nutrient loading .