Macrobenthic succession and changes in sediment biogeochemistry following marine fish farming.

Following cessation of fish production at a fish farm site in Loch Creran, Scotland, a study of the recovery of the benthic environment was undertaken. Sediment samples for macrofauna and physicochemical parameters (Redox potential; organic carbon and nitrogen; oxygen and nutrient fluxes; sulphide and pH depth profiles; particle size) were collected every 5 weeks from 3 stations at various distances from a fallowed fish farm site for 14 months. The data collected were analysed by a combination of uni- and multi-variate statistical methods. Macrobenthic communities in the 2 stations furthest from the fish cage site showed signs of recovery with time in terms of species diversity, indicator species, number of species and abundance, being moderately to slightly disturbed at the end of this study. At the station nearest to the fish cage site recovery of the macrobenthic community was also evident, although this station was still highly impacted 15 months after fish production ceased, with opportunistic species dominant. Changes in the chemical parameters were most apparent during the first 2 months. Fourteen months after fallowing, highly reduced conditions were still persistent in subsurface sediments at all stations. Bulk sediment organic carbon, redox potential and the UK Infaunal Trophic Index, although indicators of a spatial gradient, were not found to be significant indicators of recovery. Oxygen uptake appeared to be the main factor conditioning early stages of recovery, although combinations of different environmental parameters were found to be related to different stages of recovery. The seasonal timing of fallowing and the initial condition of the sediment appeared to be important in the evolution of the recovery. The exclusion of nematodes as a bulk taxon from the multivariate analyses made no difference to the conclusions