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The potentials to improve the macrophytes in polder ditches within the management area of water board “De Stichtse Rijnlanden”

By M.E. den Hartog


A number of water bodies in the management area of the Water board ‘De Stichtse Rijnlanden’ (HDSR) should meet the requirements of the Water Framework Directive (WFD). The WFD has determined that these water bodies must have a ‘good ecological status’ by 2015. To assess ecological status, the WFD requires the definition of reference conditions using biological, physical and chemical indicators and an assignment of the water bodies to one of the five quality classes. By focusing solely on the WFD water bodies, management options to increase the ecological status outside the WFD water bodies may be ignored. The water system in a polder is connected, whereby the WFD water bodies are heavily influenced by the rest of the water system. Eutrophication of polder ditches by over-fertilization and polder inlet water, with nitrogen and phosphorus, causes a shift from mainly submerged vegetation to a dominance of Lemna (Duckweed). While submerged vegetation is the desired status. In this research, the main aim was to find catchment areas which show potential for improving the macrophyte status in the management area of Water board ‘De Stichtse Rijnlanden’. Seven interesting catchment areas have been chosen to use as research areas. This is done on the basis of several criteria to have a diverse selection of areas and macrophyte statuses in order to be representative for the HDSR area. The model PcDitch calculates the ‘critical nutrient level’ of the external nutrient loads above which shifts in vegetation are likely to occur. This model is used to determine the potential for macrophytes in two steps. First the actual nutrient load in a polder is compared to the critical nutrient load to determine the present status. Second the effect of implementation of dynamic water levels on the actual and critical nutrient load was determined. To what extent this measure was considered favourable for a catchment area depended on the reduction of the critical and nutrient load. The reduced nutrient load should be lower than the adapted critical nutrient load. This is a pre-condition for submerged plants to be able to recover. As a second precondition the recovery time calculated by PcDitch needed to be within 20 years of simulation. Based on these steps the areas Zegveld, Haarrijn, Hekendorp, De Pleijt and De Koekoek show potential for improving the macrophyte status. The simulated reduction of nutrient loads in the areas Maartensdijk and Langbroekerwetering is not sufficient enough to create a shift towards dominance of submerged vegetation. It is recommended to treat the used critical nutrient loads from PcDitch with caution due to several uncertainties and model simplifications. With this in mind the output and method can be used as a basis to develop a new tool to create an overall view of the critical and actual nutrient loads within HDSR

Topics: Geowetenschappen, Ditches, PcDitch, Flexible water level management, Stichtse Rijnlanden
Year: 2013
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