Spring copepod vertical zonation pattern and diel migration in the open Ligurian Sea (north-western Mediterranean)

Copepod community structure was investigated every 6 h during a 24-h cycle at a fixed station in the open Ligurian Sea during a BioLig cruise (May 2013), using a BIONESS multiple-net sampler. The vertical distribution and diel vertical migration in the 0-1300 m water column are described for the main copepod species. Cluster analysis identified three distinct layers within the water column, with each characterised by a different species composition. The differentiation of the species composition for the 0-200 m, 200-600 m and 600-1300 m layers indicates the epipelagic, mesopelagic and deep communities, respectively. The epipelagic community was composed of numerous weak diel vertical migration species that were characterised by low abundance. These included a few dominant coastal species, which with their high abundance, spread into the open waters of the Mediterranean Sea during the spring-summer period. The mesopelagic community was composed of the highest number of species, which mainly inhabited the intermediate waters, with a limited number that showed significant diel vertical migration. The deep community included a few mesopelagic species, which were characterised by wide vertical distribution, with the upper limits reaching into the mesopelagic layers. Among the deep community species, only Lucicutia longiserrata showed an upper limit below 800 m in depth, and thus it can be considered to be the only true deep copepod species that occurs in the Mediterranean Sea. None of the deep community species showed diel vertical migration. Our data reveal discrete layers within this water column that are characterised by different communities, and that show distinct trophic relationships and life strategies. The species vertical distribution patterns showed bimodal distribution, ontogenetic migration, and downwards seasonal migration of the overwintering Calanus helgolandicus stage C5 copepodites, as discussed with regard to the physicochemical and biotic factors.