LAUSR

Abstract Changes in the composition and biomass distribution of deep-living zooplankton over wide gradients of depth (0–1300 m) have been analyzed in the Central Ligurian Basin (Northwestern Mediterranean Sea), seeking the environmental variables responsible for these changes. Spring vertical distribution (early May 2013) and diel vertical migration (DVM) of some key species of macroplankton and micronekton communities were studied every 6 h during a 24 h cycle. The hauls, performed with the multiple opening/closing 230 µm BIONESS net system, made possible a detailed description of the vertical distribution of twenty-three species: one siphonophore, one pyrosomid, one salp, three pteropods, four amphipods, one mysid, eight euphausiids, two decapods, two fishes. Cluster analysis performed on major species of macroplankton and micronekton community assemblage mainly highlighted the presence of four well defined assemblages, based on the depth range in the water column: 0–60 m, 60–100 m, 100–600 m and from 600 to 1300 m. During daytime, vertical distributions of biomass exhibited similar profiles, with maxima in the 0–60 m layer. Highest abundances were recorded between 20–40 and 40–60 m depth, at midnight and in the morning. The highest values of both biodiversity and number of species were found in a deeper layer comprised between 400 and 600 m. Most of the species showed similar distributions throughout the sampled area, either with typical non-diel-migrating characters such as the euphausiid Stylocheiron longicorne and the fishes Cyclothone braueri and C. pygmaea, or with clear diel-migrant behaviour (e.g. the siphonophore Chelophyes appendiculata and the euphausiid Euphausia krohni). Some species (e.g. Vibilia armata, Phronima sedentaria, Scina crassicornis, Salpa fusiformis, Cavolinia inflexa, Gennadas elegans) exhibited a bimodal vertical distribution that could be attributed to different causes able to influence and modify their vertical migration. The lower values of chlorophylla concentrations found in the present studyinthe month of May corresponds to the transition period from a mesotrophic system to a rather oligotrophic one. Temperature and chlorophyll were the variables that principally influenced the distinction between shallow (0–60 m) and deep communities. Below 60 m depth, the dissolved oxygen and salinity become the most important variables affecting the zooplankton community.