Fish farming impact on sediments and macrofauna associated with seagrass meadows in the Mediterranean

The fish farming impact on sediment and macrofauna associated with Posidonia oceanica meadows has been studied in three coastal areas (Spain, Italy and Greece) of the Mediterranean Sea. The stations were characterized by coarse sediment with a low percentage of silt/clay and oxic conditions. Chlorophyll a and organic carbon were occasionally enhanced in the vicinity of the cages and nitrogen remained almost constant between the stations. Phosphorus consistently showed a decreasing trend towards the reference station. Macrofaunal biomass peaked at an intermediate distance from the fish cages. Diversity indices were quite high at all stations, implying that the specific sites were not under severe stress due to aquaculture. A tenuous clustering of stations with distance from the farm was encountered, which was mainly attributed to abundance shifts rather than presence/absence of typical species. It is suggested that oxic conditions induced by intense currents and supported by coarse sediments at the study sites allow the existence of diversified communities with high abundance and biomass.