Long term effects of shore arrangements on fine-sand macrofauna at two stations of Prado's bay (North-western Mediterranean Sea - Gulf of Marseilles)

Since 30 years, the macrofauna of subtidal fine sands has been affected by important shore arrangements, in the Prado bay (Marseille gulf, North-Western Mediterranean Sea). The perturbation is firstly related to pier constructions and land reclaiming, and secondly to diversion of both sewage and freshwater effluents out of the bay. A comparison of quantitative sampling performed over a 30 year period, at 2-stations, showed that biodiversity was maintained even improved during that period, but that density and biomass were severely depressed. It is suggested that the improvement in the water quality of the bay has a positive effect on biodiversity, but that the drastic reduction of freshwater inputs has a negative effect on density and biomass levels of the macrobenthos. This last point leads to an increase in macrofauna instability especially for bivalve populations.