Integrated modelling for nutrient loading and ecology of lakes in The Netherlands

Many shallow lakes in the lower parts of The Netherlands are hypertrophic and dominated by algae blooms. The present ecological situation reflects the result of past and present nutrient loading-a combination of point and diffuse loading from different sources. The difference in catchments and hydrology between lakes leads to a difference in nutrient loading and water residence time. Management options to reduce the trophic state have to be analysed. The LakeLoad model presented here calculates the load of nutrients for 41 polder lakes in The Netherlands. Detailed hydrological information on the complex water system of the polders is used to determine the lake catchment. A lake may loaded as a result of run-off and leaching in the catchment, atmospheric deposition, point-source emissions and the inlet of water from outside the polder. These input fluxes are modelled separately and the input is retrieved from other models and databases. The output of this model is input to the ecological model, PCLake, which calculates the growth of algae, fish and plants in the lake. Using these models it is possible to calculate the effects of differences in agricultural practice or the reduction in point sources on the ecological state of the lake. The effect of water management options, such as phosphorus removal from inlet water. or reducing inlet water by allowing flexible water levels, can also be modelled. The LakeLoad model is validated by comparing the estimated input concentrations with loads for 15 lakes gained from the literature. The results obtained using the LakeLoad-PCLake combination were compared with in-lake concentrations of total nitrogen, total phosphorus, chlorophyll and transparency (Secchi depth) for 34 lakes. In general, the model simulations fit reasonably well with the observations made in these lakes. The models seem a useful tool to classify the lakes in either mesotrophic or eutrophic within acceptable margins. (C) 2004 Elsevier B.V. All rights reserved.