Fish farming effects on benthic community structure in coastal sediments: analysis of the meiofaunal recovery

To study the impact of organic enrichment and high biodeposition caused by intensive fish farming in coastal sediments, meiofaunal samples were collected on a monthly basis between March and October 1997 at two stations of the Gaeta Gulf: one under the Farmocean cage (2000 m(3)), while the other (the control) was located at about 1 km from an area not impacted by fish farming. The fish farm contained about 120 000 Dicentrarchus labrax (density 18 kg m(-3)). The most evident changes in the benthic habitat under the cage were a large accumulation of primary organic material (phytopigment concentrations up to 44 mug g(-1)), changes in sedimentary organic matter composition (increased lipid levels related to the composition of the fish diet), and a strong reduction of redox potential values, which resulted in a significant reduction of meiofaunal penetration depth into the sediments. Organic loads had a clear impact on meiofaunal densities, which were 50% lower under the cage than at the control site (1112 +/- 118 and 2160 +/- 339 ind. 10 cm(-2), respectively). Compared with the control, farm sediments also showed an increased importance of copepods, which dominated the meiofauna together with nematodes (both 39% of the total density) and polychaetes (17%). The removal of the fish farm from the site on July 1997 allowed an analysis to be made of the initial short and medium-term recovery of the assemblages. After two months, meiofaunal densities were closer to the control (about 30% recovery). Community structure recovered only partially to the characteristics typical of the study area, with increased importance of nematodes (about 70% of total density) and reduced copepod contribution, but it was still characterized by a lower number of taxa after four months. These data suggest that meiofaunal recovery after fish farm disturbance is rapid, but far from complete after four months.