Consequences of a gelatinous dominated ecosystem: Preliminary observations from a Mediterranean lagoon.

In recent years, many coastal environments by the Mediterranean Sea (Varano, Etang de thau, Empuriabrava, Bizerte lagoon) were invaded by carnivorous gelatinous organisms like jellyfish and ctenophores. Jellyfish, which are voracious consumers, may cause significant alterations in zooplankton abundance and dynamics. A confined environment like Varano Lagoon represents a suitable habitat for invasion and establishment of the scyphomedusa Aurelia sp., which has successfully resided there since 2000 and represents our case study. The role of this gelatinous top predator was investigated by analyzing spatial and temporal dynamics of the meso- and microzooplankton community and jellyfish abundance. We also studied the composition and abundance of prey in the jellyfish stomach contents. Comparison between 2 years of sampling showed that high jellyfish abundance in the second year (3 ind/m3) exerted a top – down control on zooplankton community. The peaks of phytoplankton, mesozooplankton and gelatinous predator biomass distributions occurred sequentially as a result of prey-predator interaction, visible also spatially in the four areas analyzed. The selectivity index C calculated from medusa diet and prey availability showed null selection toward most prey, suggesting that Aurelia sp. medusae feed on zooplankton prey mostly non-selectively. Ongoing activities on jellyfish trophic ecology (e.g. stable isotopes) will clarify their predatory impact on zooplankton community and the consequences for the fish harvested in the lagoon (sea bass, sea bream, sand smelt, gray mullet), which now must compete with jellyfish as top predators from April to December. Zooplankton depletion due to the gelatinous explosion should be considered as a cause of for possible decreasing fish yields in the lagoon ecosystem.