Impacts of marine aquaculture at large spatial scales: evidences from N and P catchment loading and phytoplankton biomass

While several studies point at off-shore aquaculture as a possible source of impacts on the local marineenvironment, very few have analysed its effects at large scales such as at the bay, gulf or basin levels.Similar analyses are hampered by the multiple sources of disturbance that may concomitantly affecta given area. The present paper addresses these issues taking the Gulf of Castellammare (SouthernTyrrhenian Sea) as an example. Nitrogen (N) and phosphorous (P) loads were calculated for the period1970e2007, and compared to chlorophyll-a concentration as measured inside and outside the Gulf overthe same period. Results indicate that N and P catchment loading has constantly decreased because ofimproved environmental management. Nevertheless, nutrient concentration in the Gulf has steadilyincreased since the establishment of aquaculture facilities in 1999. Chlorophyll-a concentration followedthis trend, showing a marked increase from 2001 onwards. In the same period, chlorophyll-a concentrationsmeasured inside and outside the Gulf have significantly diverged. As all the other possible causescan be ruled out, aquaculture remains the sole explanation for the observed situation. This paperdemonstrates for the first time ever that off-shore aquaculture may affect the marine ecosystem wellbeyond the local scale and provides an additional element of concern to be kept into consideration whenallocating oceans’ space for new fish-farming activities.