Knowing the variations of planktonic cnidarians under climate change conditions is of importance due to the key role of carnivorous gelatinous zooplankton in the pelagic ecosystem. We investigated the abundance, spatial distribution, species richness and community structure of planktonic cnidarians in a temperate area, the NW Mediterranean, during the extremely warm summer of 2003 and the average summer of 2004. The surveys covered a broad and heterogeneous area. During the warm summer, the cnidarian community was half as abundant as during 2004, whereas its diversity was higher. The mesoscale spatial distribution of several species varied between the years. The structure of the community had a coastal-offshore ordination in the typical summer, but a north–south one in the warm summer when the temperature gradient prevailed instead of the bathymetry. Only hydromedusae were more abundant during the warm summer. The dominant siphonophore, Muggiaea atlantica, exhibited one of the greatest abundance decreases in 2003. Our results emphasize that exceptionally warm summer conditions could reduce gelatinous zooplankton abundance and change their latitudinal distribution and community composition. We stress the importance of mesoscale plankton surveys, including detailed taxonomic identification, as a critical tool for better understanding marine ecosystem responses to climatic variability

This study was partially supported by the European Union Project VECTORS (FP7 OCEAN-2010, 266445; Food, Agriculture and Fisheries, Biotechnology) and the Spanish project FISHJELLY (MAR_CTM2010-18875)

19 pages, 7 figures, 3 tables

Peer Reviewed