EFFECTS OF ALKALINITY AND EXTERNAL SULFATE AND PHOSPHORUS LOAD ON WATER CHEMISTRY IN ENCLOSURES IN AN EUTROPHIC SHALLOW LAKE

As a result of changes in hydrology, the former mesotrophic, shallow l ake Naardermeer, has been eutrophicated during the last decades. To co mpensate for shortage of water, eutrophicated water with different che mical characteristics has been supplied. In order to determine the eff ects of alkalinity, acidity and external sulphate and phosphorus load on eutrophication processes, developments in water chemistry have been studied in enclosures. A decrease of the phosphorus load of the lake did not improve water quality on the short term. This observation was confirmed by this enclosure experiment. Reduction of alkalinity did im prove water quality, particularly by reductions of turbidity and chlor ophyll-a content. The significant increase in sulphate load in the wat er layer led to deterioriation of the water quality and an increased b iomass of sessile algae by internal eutrophication, probably as a resu lt of increased sulphate reduction in the sediment. Therefore, it seem s that lowering of sulphate concentrations in the inlet water after th e removal of phosphorus is necessary to improve the water chemistry. T he best option, however, is to restore the former hydrological conditi ons after which the system will be fed only by seepage of groundwater poor in nutrients and sulphate.