Impact of an industrial and urban sewage off a coral fringing reef at Mauritius (Indian Ocean): modeling of plumes, distribution of trace metals in sediments and effects of the eutrophisation on coral reef communities

The impact of industrial and urban sewage outflows off tropical high islands is an important environmental problem. This problem is yet more acute when there are coral reefs. For Mauritius Island the effects of the main sewer outflow was studied. Dispersion of the plume was modelized using a bidimensional code. Seawater physical and chemical classic parameters, plus fecal bacteriological ones, were also recorded and used for the modeling of oceanic waters and reefal ones. Sediment parameters of the inner reef flat were analyzed for grain-size indices and silt ratio, living and detrital organic matter contents, and then trace metals composition. ACP statistical analyzes performed allow to map the sewage outflow influences, showing a clear decreasing gradient from this point. Qualitative analyzes of the distribution of macrobenthic species living of the reef mainly those known in the SW Indian Ocean as biologic indicators of some edaphic conditions, were also done. The main edaphic facies induced by the sewage outflow plume was the growth of a high and dense carpet of the green algae creating anoxic conditions at the sediment-wafer interface. High densities of sea urchins seem more the consequences of an overfishing on the reef flat, destroying the carnivorous fishes. Nevertheless these sea urchins outbreaks are characteristic of degraded stages of the coral reef communities as it is well known in the SW Indian Ocean.