Interactions between microbial species may have a central role in shaping microbial communities. However, the role of such biotic factors in relation to abiotic parameters such as nutrients, temperature and light availability are still unclear. In this study, we performed an incubation experiment where the strength of plankton species interactions was reduced through a dilution series under two nutrient conditions (nitrogen limited and Redfield ratio). The experiment was conducted in July 2017 using seawater from the Baltic Sea proper and was incubated in situ, ensuring natural seawater temperature and illumination variations. The phytoplankton community was typical to Baltic Sea summer conditions and included filamentous nitrogen fixing cyanobacteria. The dilution gradient was verified using bacterial abundances which responded to both nutrient treatments while phytoplankton growth was significantly higher in the nitrogen limited treatment compared to the Redfield treatment. Less competition led to higher bacterial production rates while primary production followed the dilution gradient at the end of the experiment. The phytoplankton and bacterial community composition will be further characterized using next generation amplicon sequencing. The experiment will provide insights into the role of abiotic and biotic factors in shaping microbial communities.