Microbial diversity and freshwater ecosystems freshwater environments the influence of physicochemical conditions on microbial communities Freshwater environments the influence of physicochemical conditions on microbial communities Algae the major microbial biomass in freshwater systems Competition for light Inorganic nutrients uptake and cycling in freshwater systems Bacteria the main heterotrophic microorganisms in freshwater systems Viruses major parasites in the freshwater environment Fungi and fungallike organisms aquatic biota with a mycelial growth form Grazing activities in the freshwater environment the role of protozoa and invertebrates Eutrophication the microbial response to high nutrient levels.