PREDICTIONS OF PHOSPHORUS IN LAKE-ROTORUA FOLLOWING LOAD REDUCTIONS

The rate of recovery of polymictic Lake Rotorua following external loa d reductions is estimated by daily modelling of temperature, hypolimni on dissolved oxygen (DO), lakewater, and sediment total phosphorus (TP ). Four coefficients of the TP model were successfully ''calibrated'' by matching observations from 1977, the only period when sediment TP a nd stream loads were measured simultaneously but a period when sewage loads were low. A fifth coefficient, sediment layer thickness, had to be specified a priori. Model performance was satisfactory except occas ionally in summer when the predicted stratification was incorrect, and during 1982-89 when sewage loads were high. The model predicts a noti ceable reduction in lakewater TP concentration within 20 years but onl y a slow reduction in sediment concentration. Internal loads are predi cted to remain a feature of the lake but gradually to decrease in freq uency and size. It would be desirable to improve prediction of sedimen t layer thickness and bioavailability, and to apply the model to nitro gen.