Résumé
Coral reefs of Martinique have undergone an excessive growth of macroalgae over the past two decades, leading to a decrease in coral cover and health status. Increasing urbanization, agricultural and industrial activities over the South coast of Martinique have raised land-based nutrients discharges. In this study we used macrophytes ä15N signatures to point out the influence of anthropogenic nutrient enrichment on macroalgal communities' development. Field experiments were conducted at 10 stations located along the Southern reef of Martinique. Seven sites were chosen close to anthropogenic nutrient discharges and 3 offshore sites were selected as reference sites. Three macroalgae indicator species were collected at each site and two depths to assess spatial variability among shallow subtidal (5m) and mid-depht (25-30m) waters. Dried tissues were used for stable nitrogen isotope (ä15N) analysis. Water-column dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and chlorophyll a were measured. The sampling was performed during the wet and dry season for seasonality changes in N source availability. We expect that concentrations of dissolved inorganic nitrogen, soluble reactive phosphorus, and chlorophyll a increase at sites directly impacted by sewage discharges (outfalls, rivers, bays...). The offshore reference sites should be more nutrient limited than the nearshore ones. We assume that macroalgae in nearshore waters have elevated ä15N values characteristic of sewage enrichment and agriculture fertilizers. Furthermore ä15N values are presumed to be significantly highest over shallow reefs compare to deep water. These results will suggest that regional-scale agricultural runoff from the mainland as well as local sewage discharges from urbanization and economic activities are both significant nitrogen sources supporting eutrophication and macroalgal invasion in coral reef communities.