Predictions of phosphorus in Lake Rotorua following load reductions

Abstract The rate of recovery of polymictic Lake Rotorua following external load reductions is estimated by daily modelling of temperature, hypolimnion 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 and 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 occasionally in summer when the predicted stratification was incorrect, and during 1982–89 when sewage loads were high. The model predicts a noticeable reduction in lakewater TP concentration within 20 years but only a slow reduction in sediment concentration. Internal loads are predicted to remain a feature of the lake but gradually to decrease in frequency and size. It would be desirable to improve prediction of sediment layer thickness and bioavailab...