Benthic recovery following cessation of fish farming: a series of successes and catastrophes

After the removal of fish cages at an intensive aquaculture site, the sedimentary environment was monitored over 23 mo for redox potential, total organic carbon and nitrogen, total phosphorus, algal pigments and macrofauna. Three sampling stations were established: one under the previous location of the cages (R-0), a second one (R-10) at 10 m distance from the edge of the cages and a control site (R-c) at >1 km distance. At both stations near the farming site the sediment was initially found to be anoxic and overlain by a highly organic black layer. Most geochemical variables at Stn R-10 attained values close to those at Stn R-c within 11 mo. Large fluctuations in the values of most variables were observed at Stn R-0 over the 23 mo, indicating that the environment had not fully recovered before the end of the observations. Similar results were obtained from the macrofaunal analysis, which revealed that after 23 mo a high proportion of benthic fauna at Stn R-0 was still composed of opportunistic species; abundance biomass and species composition showed marked successive changes in the direction of succession. This regression was attributed to a secondary disturbance due to a benthic algal bloom, caused by the seasonal release of nutrients from the farm sediment. It is concluded that the recovery process of heavily enriched benthos in a dynamic coastal environment is subject to the influence of different factors, resulting in progress and regression, and therefore the succession model proposed by Pearson & Rosenberg (1978; Oceanogr Mar Biol Annu Rev 16:229-311) may not be applicable in the early stages of succession.