Fish farming, metals and antibiotics in the eastern Mediterranean Sea: Is there a threat to sediment wildlife?

Chemical residues released from aquaculture farms may persist in the environment and may pose adverse ecological effects. The aim of this study was to assess the pollution status of marine sediments underneath or close to aquaculture farms in the Eastern Mediterranean Sea, the factors that affect the elemental distribution, and the exceedance of environmental quality standards and factors. To this end, surface sediment samples were collected (underneath fish cages, at 25 m distance and from a reference station) from 48 fish farms in Greece with variable environmental and geochemical characteristics. The content of 29 metals and major and trace elements, and three antibiotics (oxytetracycline, florfenicol, and flumequine), was determined in the sediment samples. Most of the elements found in the sediments originated from geological sources and their concentrations were close to Earth's Crust content. Below and close to fish farm cages, the sediment was enriched with P, Cu, Zn, Mo, and Cd mainly due to the aquaculture biosolid deposition, and changes in environmental conditions (increased organic matter, low redox potential). Cr and As were found to exceed twice the upper threshold limit of the available sediment quality guidelines in 13.5% and 7.3% of sampling stations, respectively. The potential ecological risk of V, Cr, Mn, Co, Ni, Cu, Zn, As and Pb was found to be low in the sediment close to fish cages except for Cd, which may pose a moderate to considerable risk in 12.5% of sampled fish farms. However, the estimated risks for this metal may be influenced by the high background levels found in the investigated areas. This study also shows that the occurrence of antibiotics in sediments below fish farms in the Eastern Mediterranean Sea is very sparse (i.e., only flumequine was found in two farms) and concentration levels are relatively low.