Spring diel vertical distribution of copepod abundances and diversity in the open Central Tyrrhenian Sea (Western Mediterranean)

Abstract

Spring vertical distribution of copepod communities was studied every 6h during a 24hcycle, using the BIONESS multinet, to evaluate diel vertical migration (DVM) of the key species in a Central Tyrrhenian Sea station (from 0 to 2000m). Similar abundance trends were detected for the four sampling times. Highest abundances were recorded between 20–40 and 60–80m depth, at midnight and in the morning. In epipelagic layers, highest diversity occurred between 20 and 100m in the morning and at midday, and from 100–200m in the afternoon and 300–400m at midnight, in mesopelagic ones. DVM involved mostly the 60 and 300m depth interval. Epipelagic and mesopelagic copepods co-occurred in this stratum, so diel changes in species composition and diversity induced shifts in the slope of the highest k-dominance curves. The epipelagic Neocalanus gracilis and the intermediate Eucalanus elongatus copepod species showed a bimodal distribution. Only the shallower population of E. elongatus exhibited significant DVM. Few other copepod species showed significant DVM (the epipelagic Scolecithricella dentata and the subsurface Corycaeus furcifer and Pleuromamma gracilis), confirming previous information about the presence of few strong migrants in the Mediterranean Sea. Lucicutia longiserrata, Gaetanus kruppi and Monacilla typica with their copepodites were the only species dominant below 600m depth and they were not affected by DVM.