Integrated modelling for nutrient loading of polder lakes

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 the present nutrient loading − a combination of point and diffuse loading from different sources. The difference in catchment 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. A model is presented that calculates the load of nutrients for 35 polder lakes in the Netherlands separated for different years and emission routes. Detailed hydrological information on the complex water system of the polders is used to determine the lake catchment. The loading of a lake is the result of runoff 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 sequence from agricultural practice and reduction in point source up to ecological effects in the lake. The effect of water management options, such as phosphorus removing from inlet water or reducing inlet water by allowing flexible water levels, can also be modelled. Estimated input concentrations for the nineties are compared with in lake measurements. They are with a few exceptions in line with the assumption that they have to be higher because of the in lake retention of nutrients. Comparison with a few available observations of input concentrations shows a good agreement for nitrogen. For phosphorus estimates agree with more recent observations and lie below observations made in the eighties when point sources were more important.