Assemblages of planktonic cnidarians in winter and their relationship to environmental conditions in the NW Mediterranean Sea

In the present study, we addressed the spatial characterization and species assemblages of the planktonic cnidarian community (Siphonophorae, Hydromedusae, and Scyphomedusae) in winter, a period that has been the subject of few studies in the NW Mediterranean. Data were obtained on two oceanographic cruises, in February 2017 and 2018. In 2017, the early onset of spring conditions and the subsequent phytoplankton bloom favored a mixture of winter and spring species, resulting in a higher species richness but a lower abundance of cnidarians. However, the typical winter oceanographic conditions in 2018 allowed winter species populations to develop, leading to a higher abundance of cnidarians that year. The most abundant species in both winters were Lensia subtilis, Muggiaea kochii, Chelophyes appendiculata, Abylopsis tetragona (eudoxid), Aglaura hemistoma, and Velella velella rataria larvae, while Obelia spp. was particularly numerous in 2017. In both years, the cluster and redundancy analyses showed a coastal-offshore ordination in species assemblages resulting from the effect of environmental variables (particularly bathymetry) and oceanographic structures (water masses and the shelf-slope density front). The presence of submarine canyons, in which great depths are reached close to the coast, modified the circulation patterns, resulting in a mixture of coastal and offshore species in these areas. In the current scenario of global warming, our results will help to provide a baseline for identifying future changes in the structure of the planktonic cnidarian community.