This study examines the importance of disturbances of the seafloor which result in local mortality of resident populations, and the following recolonization sequences. The role of defaunation by pollution and anoxia in the ecological succession was examined by in situ experiments in Dunkerque Harbour. Samples of defaunated mud coming from the inner part of the harbour were placed in experimental containers (0.10 m², 10 dm³) 50 cm above the seafloor at 8 m depth, near the entrance of the harbour, in order to simulate a local disaster. Containers were sequentially recuperated and analyzed. The experiment was repeated at five different periods of the year between April 1985 and July 1986, in order to describe different possible seasonal scenarios of recolonization. On the whole, the recolonization sequences are described from the examination of 67 containers. Simultaneously surrounding grab sample surveys were made with a Smith-McIntyre grab, for comparative purposes. Changes in the experimental community were considered in relation to heavy metal content (Pb, Cd, Cu, Zn, Fe, Mn), organic matter, Eh and pH variations. In the surrounding medium, these values vary during the year. In the experimental boxes, a release of the pollutant content (specially Pb, Cd, Cu, Zn) has been first observed, but incidentally the metal contents increased during the experimental period, probably due to new deposits from the surrounding water masses. The experimental community shows a rapid development (faster than in the Mediterranean, probably due to adaptation of this fauna to perturbed conditions), after a latent phase during which sediments release pollutants. Recovery of the infauna occurs form benthic species of the surrounding environment (Abra alba community). Number of individuals, total biomass, number of species and diversity index increase until a maximum value, reached after 20 to 24 weeks, according to the season. Diversity and number of species reached the value of the surrounding community, whereas biomass and number of individuals did not. The final proportion of the species is also different. Fauna recruitment was highest in spring and early summer. The arrival of opportunistic species was observed first, specially Polychaetes and Crustaceans, and finally Molluscs in proportion varying according to the season. The main colonizing species are Capitella capitata, Scoloplos armiger, Cirratulus cirratus, Spio filicomis, Polydora ciliata, Gammarus zaddachi, Microprotopus maculatus, Corophium sextonae, Orchomene nana, Abra alba, Tellina fabula, Macoma balthica, Mya truncata. Polychaeta and Mollusca seemed to colonize more as larvae and juveniles, while Crustacea did so principally as juveniles and adults. The zoobenthic community establishment and succession follow certain patterns which are governed and modified by the seasonal availability of recruits, food supply and faunal interactions. Experimental containers allowed, after 19 to 33 weeks of immersion, the development of a community, qualitatively but not quantitatively, similar to the one observed in the surrounding area. |
