Delay in lake recovery caused by internal loading

A simplified model of the lake-sediment system is proposed, which aims to predict the long term evolution and the recovery time of eutrophied lakes. The model represents the state of lake and sediment each by means of a single variable. Having recourse to a well-defined set of assumptions as for sediment processes, and adopting time independent coefficients, the model balance equations can be solved analytically, and the evolution of the state variables towards steady state conditions, can be discussed in terms of characteristic times of the considered system. Particular attention is devoted to the consequences of P saturation of the solid phase in surface sediment layers, i.e. to a situation which is frequently present in eutrophied lakes. The model is applied to the case of the highly eutrophied Lake Varese (Northern Italy), in which internal P loadings from sediments appear to be substantial. Taking into account the coupling between lake and sediments, the time needed to attain the steady state P concentration compatible with the present P loadings is evaluated as ∼ 5 times the water renewal time of the lake. It is also shown to what extent the assumptions on sediment dynamics affect the model predictions.