Evidence for wastewater influence in a low impacted area throughout stable isotope analyses of the limpet Patella caerulea and epilithic biofilms

Eutrophication is considered to be one of the main threats to marine environments leading to changes in trophic status of ecosystems and alteration of biological diversity. Carbon and Nitrogen stable isotopes analyses were performed on the limpet Patella caerulea and one of its food sources (epilithic biofilms) to assess their potential as early indicators of eutrophication. Samples were seasonally collected in 2005-2006 on five locations gradually exposed to urban sewage in the Calvi Bay and in the Marseilles harbour. Stable isotope signatures of Patella caerulea muscles exhibited steady site - specific values over seasons. In contrast to this time – integrated signal, wide variations in biofilm values show that either composition or isotopic ratios of food sources may vary greatly in time and space. Elevated δ15N values of limpets and biofilms, typical of wastewater influence but unrelated to nitrogen loads, indicate the biological availability of sewage-derived nitrogen in the Calvi Bay and the Marseilles harbour. A reference level of δ15N values is rapidly reached with increasing depth that indicates the limited vertical extent of pollution in the Calvi Bay.