Evaluating ecosystem functioning of a long-term dumping site in the Bay of Seine (English Channel)
Résumé
The impact of 70 years of dumping harbour dredged material on ecosystem functioning was compared to a control site (non-impacted zone). A combination of indices (Shannon-Wiener diversity, Pielou evenness, BO2A, AMBI benthic indices) and modelling (Ecopath models) generated Ecological Network Analysis indices were used to detect the dumping operations effects. ENA indices showed that dumping operations appear to have a relatively limited impact on the structure and flow pattern of the local food web. In fact, only small variations between the two sites are revealed by a comparison of ENA indices between the impacted and non-impacted zone. This result could be explained by the fact that the Bay of Seine is historically characterized by a high level of human activities, which have led to an increase in resilience through time to face these multiple pressures. AMBI and BO2A indices (of the sediment fauna) did not allow a clear distinction between impacted and nonimpacted zone while species richness, abundance, biomass and Shannon-Wiener diversity index values were significantly higher in the non-impacted zone which is in accordance with the small variations of ENA indices between the two zones and revealed a local and limited impact. Our results suggested that a combination of several tools should be used to detect the true impacts and to conduct diagnosis of the ecological status of dumping deposit in a coastal area permanently stressed under natural and anthropogenic forcing.