A preliminary study into the influence of filtration on phytoplankton dynamics in an oligotrophic marine fish farm environment

Fish farms play a crucial role in meeting the escalating demand for fish in human diets, yet their nutrient releases pose potential environmental risks. This study explores the influence of a fish farm in the eastern Aegean Sea on local phytoplankton dynamics, serving as an indicator of nutrient abundance. Designing a phytoplankton bioassay near the fish farm, natural phytoplankton communities were incubated within dialysis membrane bags, creating a confined environment for accessing farm-released nutrients before dispersing into surrounding seawater. Consequently, higher growth rates within the bags were anticipated compared to the ambient seawater. However, natural interactions within phytoplankton communities involve predator-prey dynamics, influencing the net growth rates of phytoplankton. To investigate different grazing pressures on the incubated phytoplankton, five experimental groups were established. Four of these groups involved filtering seawater through various mesh sizes (40 µm, 56 µm, 100 µm, and 150 µm) and then filling the dialysis membrane bags with the filtered water. The fifth group contained seawater without any filtration. Despite the oligotrophic nature of the ambient seawater, a remarkable increase in phytoplankton growth was observed inside the bags. Variable growth rates were observed among the groups, with unfiltered and 150 µm mesh-filtered bags exhibiting the highest growth rates, suggesting copepod absence may contribute. Although the species composition within the bags differed from that of the ambient seawater, the overall species diversity remained limited. A total of 33 phytoplankton taxa were identified in the seawater samples taken from the study site, comprising 17 diatom and 16 dinoflagellate species. Pronoctiluca spinifera (Lohmann) Schiller 1932 was documented for the first time along the Aegean Sea coast of Türkiye. This study enhances our understanding of how fish farming can impact phytoplankton communities and underscores the necessity for further investigations into the complex interactions between aquaculture and marine ecosystems in oligotrophic environments.