As a result of changes in hydrology, the former mesotrophic, shallow lake Naardermeer, has been eutrophicated during the last decades. To compensate for shortage of water, eutrophicated water with different chemical characteristics has been supplied. In order to determine the effects 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 improve water quality, particularly by reductions of turbidity and chlorophyll-a content. The significant increase in sulphate load in the water layer led to deterioriation of the water quality and an increased biomass of sessile algae by internal eutrophication, probably as a result of increased sulphate reduction in the sediment. Therefore, it seems that lowering of sulphate concentrations in the inlet water after the removal of phosphorus is necessary to improve the water chemistry. The best option, however, is to restore the former hydrological conditions after which the system will be fed only by seepage of groundwater poor in nutrients and sulphate.

The relationship between acidity and benthic invertebrate communities was investigated in two small streams in coniferous forests in central Jutland, Denmark, during 1992 and 1993. Stream pH was acid (mean pH 4.8 and 5.4) and alkalinity fluctuated greatly with mean values around zero. During rain events, water from springfed reaches upstream were mixed downstream with brown humic acid water with high aluminium content, and pH declined from circumneutral near the springs down to pH below 4 over a very short distance (0.5–1.5 km). The benthic invertebrate communities in the most acidic downstream reaches of both streams were dominated by filipalpian stoneflies, mainly Leuctra nigra (Olv.). Community composition remained very similar throughout the sampling period despite variations in pH. In the upper (neutral) reaches invertebrates intolerant of low pH such as Gammarus pulex L. were found. The concentrations of chloride and sulphate in the streams were more than double the concentrations in a nearby moorland stream indicating a strong impact of acidifying atmospheric deposition on the two forest streams. Results of this study indicate that afforestation with conifers on sandy Danish soils with low buffering capacity (about 25% of the total Danish land area) may lead to an increased acidification of surface waters and thereby also to an impoverished aquatic fauna.