Short-term changes in microbial communities in the water column around the fish farm in the Bay of Piran

A multidisciplinary approach was used to study the impact of fish farming on coastal bacterial communities in the inner part of the Bay of Piran (northern Adriatic). Differences in bacterial abundance, production and the occurrence of selected bacterial groups were studied in the water column around the cage and at different distances from the centre of the fish cage towards the open water, i.e., reference marine station. We also examined the effect of fish feeding on the surrounding system in a short-term in situ experiment based on the simultaneous collection of seawater samples from different locations around the fish cage before and after feeding of fish. Our study suggests that fish feeding has a moderate short-term effect on water column parameters, including bacterial abundance and production, only at a limited distance from the fish cages. The nitrifying, ammonia-oxidizing bacterial groups, as determined by the fluorescent in situ hybridization method, were represented at a higher percentage in the seawater samples in the middle and around the fish cages. β- Proteobacteria, γ-Proteobacteria and the Cytophaga-Flavobacterium group were represented to a higher percentage at sampling sites in the middle of the Bay of Piran and at the reference marine station. The Vibrio group was detected at all sampling sites. The accumulation of organically enriched fish food and waste products releasedinto the seawater during the short-term experiment resulted in a significant increase in particulate matter, orthophosphate and ammonium. In response to the increase in inorganic nutrients, we observed a significant increase in bacterial production, whileno significant differences were observed in bacterial abundance in such short time.