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

Spring vertical distribution of copepod communities was studied every 6 h during a 24 h cycle, using the BIONESSmultinet, to evaluate diel vertical migration (DVM)of the key species in a Central Tyrrhenian Sea station (from 0 to 2000 m). Similar abundance trends were detected for the four sampling times. Highest abundances were recorded between 20–40 and 60–80 mdepth, atmidnight and in themorning. In epipelagic layers, highest diversity occurred between 20 and 100 m in the morning and at midday, and from 100–200 m in the afternoon and 300–400 m at midnight, in mesopelagic ones. DVM involved mostly the 60 and 300 m 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 theMediterranean Sea. Lucicutia longiserrata, Gaetanus kruppi andMonacilla typica with their copepoditeswere the only species dominant below600 mdepth and they were not affected by DVM.