A dynamic lake model for trophic state prediction

Abstract

Primary production per unit lake area, ΣP, is selected as a measure of the trophic state of a lake. The relationship between phosphorus loading and ΣP is simulated by means of a dynamic, one-dimensional vertical diffusion model for phosphate [OP] and particulate phosphorus [PP]. ΣP is described by a non-linear function of [OP] and [PP] which takes into account Michaelis Menten saturation and self shadowing by algae. Included in the model are lake morphometry, hydraulic loading, respiration rate, sedimentation, vertical eddy diffusion, depth of thermocline, and exchange of phosphorus at the sediment—water interface. Application of the model to two Swiss lakes, Alpnachersee and Greifensee, as well as to Lake Washington (U.S.A.) shows a good agreement between observation and calculation of the phosphorus variation as a function of time and depth. The calibrated model is used to predict the effectiveness of different eutrophication control measures, such as reduction of phosphorus loading, discharge of hypolimnic water, aeration of the hypolimnion, and destratification. For Greifensee (mean residence time of water 1.2 years) it is shown that a reduction in P-loading to 20% of its present value is needed to achieve permanent aerobic conditions throughout the lake. Other measures result only in minor improvements. For Lake Washington, the model simulates well the observed recovery of the lake after sewage diversion between 1963 and 1967.