Wastewater discharge, seagrass decline and algal proliferation on the côte dʹAzur

The green alga, Caulerpa taxifolia, has recently proliferated in areas of the N-W Mediterranean that were previously dominated by the seagrass Posidonia oceanica. Sediments in areas of proliferation had high exoenzyme activities, NH+4 concentrations and production rates, but very low capacities to transform organic-N into NH+4 relative to the quantity of organic material available. All sediments supporting C. taxifolia contained large quantities of precipitated phosphorus and exhibited extremely low capacities to transform NH+4 into NO−2 and NO−3. Low transformative capacities and high phosphate concentrations are characteristic of sediments that have been polluted by urban wastewater. Where C. taxifolia did not exist, or where growth had stabilized, sediments had exoenzyme activities, NH+4 concentrations and NH+4 production rates that were proportional to organic-N transformative potential. All sediments contained significant concentrations of non-crystalline sulphide. Caulerpa taxifolia proliferation may be linked to enrichment of substrata by urban wastewater and dead or dying seagrass vegetation.