Effect of an exceptional rainfall event on the sea urchin (Paracentrotus lividus) stock and seagrass distribution in a Mediterranean coastal lagoon

A shallow Mediterranean brackish lagoon (Urbinu, Corsica), 700 ha in surface area, characterized by low freshwater input and permanent communication with the open sea, and therefore by relatively stable salinity (usually 30–38), was subject in late 1993 to an exceptional rainfall event occurring on an average once every 50 years: 450 mm in 48 h (compared to the average annual precipitation of 650 mm). The volume of freshwater that poured into the lagoon corresponds to 36% of its volume. As a result, salinity dramatically dropped while turbidity increased. The seagrass Cymodocea nodosa and other habitats were mapped before (1990) and after (1994, 1996, 1999) the rainfall event, and the sea urchin Paracentrotus lividus stock was estimated together with its population structure. In 1994, after the rainfall event, the surface area of seagrass meadows moderately declined, but it cannot be ruled out that this loss may be within their usual inter-annual fluctuations. The sea urchin stock dropped by 50% (6–3 million individuals). Low salinity, turbidity and siltation were probably the reasons for the changes in sea urchin population in addition to variability of dynamic population parameters (e.g. recruitment, mortality). The recovery of sea urchin stock was completed within a few years (six years or less). The high population dynamics and the high recruitment potential of sea urchins may act as a mechanism to maintain sea urchin populations in this highly variable habitat. These results reflect the resilience and high adjustment stability of the system.