Sea urchin and seagrasses: trophic relationships in a benthic ecosystem from a coastal lagoon.

International audienceSeagrasses could constitute a major component of lagunal ecosystems. Generally, in seagrass beds, consumer control is strong. In coastal lagoons, where seagrass beds are particularly extensive, there is only scarce data on seagrass herbivory. In Mediterranean coastal lagoons, Paracentrotus lividus populations are extensive and consume Cymodocea nodosa beds. In this study, we monitored a P. lividus population during 18 months in order to analyse changes in population density and structure. On the basis of results of previous studies (i.e. biomass, density, production and nutrition parameters), we assessed the importance of P. lividus with respect to C. nodosa herbivory in a Mediterranean coastal lagoon. The results show that this sea urchin, when its density is low, is estimated to consume about 0.6-18.9% of the seagrass production. However, active movement of consumers among adjacent habitats influences nutrient fluxes. During sea urchin migration, when densities of 10-98 ind. m-2 were observed, the consumption rates in sea urchin feeding fronts temporarily exceeded seagrass production rates. The overgrazing of the C. nodosa results in patches where leaves of this plant are completely consumed, and this phenomenon may explain a part of interannual variations in the distribution pattern of this species