ISC participants investigated the occurring organism diversity and abiotic, physico-chemical factors of three lakes in southern Lower Saxony using numerous modern measuring instruments and methods as well as some classical procedures. They first dealt with basic ecosystem processes in deep, temperature-stratified and shallow, unstratified lakes and then addressed the question of how to explain the large differences between the lakes in terms of nutrient ratios and the resulting algal growth with its consequences for the distinct vertical profiles of abiotic factors. It turned out that the very different nutrient ratios of the lakes could not be explained solely classically by different ratios of lake size, water volume, depth and size of the catchment area, as is the case for undisturbed lakes, but that land use in the catchment area has a decisive influence. Own measurements supplemented with literature studies showed: Intensive agriculture with high N and P inputs leads to strong eutrophication and excessive algal growth, dominated by potentially toxic blue-green algae in warm summers. This explains the increase in blue-green algal blooms observed in recent decades as the climate warms. The thinning of forests due to mainly climate-induced spruce dieback in the Harz Mountains leads mainly to significantly increased N input as a result of increased mineralization, but this cannot lead to increased algal growth because the much less mobile P remains at naturally low levels.