Structural changes in an aquatic microbial food web caused by inorganic nutrient addition

Present theories regarding nutrient status of aquatic systems and plankton size structure, food web length and nutritional modes were tested in a microcosm experiment using water from the northern Baltic Sea. The different trophic levels included were those of pico-, nano- and microplankton, representing bacteria, flagellates, ciliates and diatoms. Nutrient enrichment resulted in a higher biomass and changed size-structure of the organisms. The nutrient-enriched microcosms changed from a picoplankton-dominated system to a microplankton-dominated system. In contrast, the nutrient-poor treatment remained at the pico- and nanoplankton levels. The increase in biomass within the enriched treatment resulted in a lengthening of the food chain to include a firmly established ciliate community. In accordance to this, no shift to inedible or toxic species was observed. In the nutrient-poor treatment the ciliates were kept at very low population densities. Furthermore, a potentially mixotrophic flagellate (Chrysochromulina sp.) was able to maintain a positive population growth in the nutrient-poor treatment, while specialised heterotrophic or autotrophic flagellates (Paraphysomonas imperforata Lucas and Plagioselmis prolonga Butcher) decreased in numbers. In the nutrient enriched system the potential mixotrophic flagellate coexisted with specialised heterotrophic and autotrophic forms. A clear succession pattern of diatoms was observed, illustrating early and late successional species within diatom populations.