MEPS 493:31-42 (2013)  -  DOI: https://doi.org/10.3354/meps10520

ABSTRACT: Plankton community composition changes in response to nutrient enrichment were examined at 2 sites in the eastern Mediterranean. Samples from bacteria up to mesozooplankton were taken upstream and downstream of finfish farms in the north Aegean (Lesvos, Greece) and on the southern coast of Cyprus. The pattern of change appeared to be similar, albeit of different magnitude, in both areas. In Cyprus, results showed an increase in bacteria and a decrease in diatom abundance close to the cages, response to effluents was limited, and consequent growth was controlled by microzooplankton. In Lesvos, results showed increased abundance of bacteria, nanoflagellates and mesozooplankton, a decrease in diatoms and an increase in larger dinoflagellates and ciliates downstream of the farm. A shift towards a more diverse microplankton community was observed, consistent with the intermediate disturbance hypothesis. The community consisted of larger-sized microzooplankton, and changes persisted throughout the downstream stations. Results showed that the plankton response to nutrient enrichment is more pronounced at intermediate distances from the farm. The community composition changes observed in Lesvos indicate that even in exposed sites with high current velocities there is an influence of farm cages. Changes manifest in terms of size for some groups and abundance for others. Although fish farms have been associated with degradation of the environment in which they are located, this was not apparent in the present study. Indeed, the shifts induced by farm effluents can help us identify drivers of change and assess community responses to perturbations in a dynamic environment.

KEY WORDS: Fish farm · Impact · Oligotrophic conditions · Picoplankton · Microplankton · Mesozooplankton · Intermediate disturbance hypothesis · IDH · Mediterranean Sea