Changes in nutrient concentration and water level affect the microbial loop: a multi-seasonal mesocosm experiment

Eutrophication and lake depth are of key importance in structuring lake ecosystems. To elucidate the effect of contrasting nutrient concentrations and water levels on the microbial community, we manipulated water depth and nutrients in a mesocosm experiment in north temperate Estonia and followed the microbial community dynamics during a 6-month period. We used two nutrient levels crossed with two water depths, each represented by four replicates. We found treatment effects on the microbial food web structure, with nutrients having a positive and water depth a negative effect on bacterial biomass, heterotrophic nanoflagellates (HNF) and metazooplankton biomass. Nutrients and water depth had both positive impacts on phytoplankton biomass. Bacterivorous ciliates had lowest biomass in shallow and nutrient rich mesocosms, whilst predaceous ciliates had highest biomass here, influencing trophic interactions in the microbial loop. Overall, increased nutrient concentrations and decreased water level resulted in an enhanced bacterial biomass and a decrease in their main grazers. These differences appeared to reflect distinctive regulation mechanisms inside the protozoan community and in the trophic interactions in the microbial loop community. This work was supported by EU 7FP Theme 6 projects MARS (Managing Aquatic ecosystems and water Resources under multiple Stress, Contract No. 603378), Estonian Ministry of Education and Research (IUT 21-02), Estonian Research Council grant PRG709, Estonian University of Life Sciences (P190258PKKH) and Swiss Grant for Programme "Enhancing public environmental monitoring capacities". This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963. Erik Jeppesen was also supported by the TÜBITAK program BIDEB2232 (project 118C250). This work was supported by EU 7FP Theme 6 projects MARS (Managing Aquatic ecosystems and water Resources under multiple Stress, Contract No. 603378), Estonian Ministry of Education and Research (IUT 21-02), Estonian Research Council grant PRG709, Estonian University of Life Sciences (P190258PKKH) and Swiss Grant for Programme "Enhancing public environmental monitoring capacities". This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963. Erik Jeppesen was also supported by the TÜBITAK program BIDEB2232 (project 118C250).