A model of nutrient dynamics in shallow lakes in relation to multiple stable states

There is increasing evidence that, within a range of nutrientloadings, shallow lakes may have two alternative stablestates. One is dominated by phytoplankton and the other one bysubmerged macrophytes as the main primary producer. Thequestion arises at what level of nutrient loading a transitionmay occur between the two states. This question was addressedby means of the integrated lake model PCLake. The modeldescribes the competition between phytoplankton andmacrophytes, within the framework of closed nutrient cycles inthe lake system, including the upper sediment. Top-downeffects via the food web were regarded as well. The model wasrun for a hypothetical shallow lake, representative for thesituation in The Netherlands. Long-term simulations werecarried out for a realistic range of nutrient loadings andstarting from different initial conditions. The results showeda highly non-linear response, which also showed hysteresis:the loading level at which a transition occurs turned out tobe dependent on the initial conditions. The results werecompared with empirically derived chlorophyll a tophosphorus relations. Factors influencing the ’criticalnutrient level‘ were the lake dimensions and the netsedimentation rate. The model was also used to evaluate therole of food web management in lake restoration. The resultssuggest that a long-term effect of additional management ispossible only if combined with a decrease in nutrient loading.