Mesozooplankton vertical patterns along an east-west transect in the oligotrophic Mediterranean sea during early summer

Mesozooplankton vertical pattern was studied down to 2000 m along an east-west transect in the oligotrophic Mediterranean Sea in June 1999. Depth-stratified samples were collected by a WP-2 net (200 µm mesh size). In addition, the copepod grazing impact on autotrophs was assessed in the epipelagic zone by measurement of small and large copepod gut content by day-night sampling. An east-west increasing trend of integrated total biomass and abundance over the 0–1000 m was detected. Similar vertical pattern of standing stock was found along the transect, with subtle differences at stations positioned over cyclonic and anti-cyclonic gyres. No significant differences were found between stations in the epipelagic and upper mesopelagic zone, except at the westernmost station (NW Mediterranean); discrimination of assemblages between the eastern and the western basins were revealed in the lower mesopelagic and bathypelagic zones. Estimated copepod grazing impact on primary production was found overall higher in the western than in the eastern basin. The entire Mediterranean Sea was dominated by copepod functional groups characterized by reproduction strategies (mostly sac spawners), trophic preferences (mainly omnivores and detritivores) and feeding modes (ambush and cruise feeding), facilitating their adaptation and efficiency in the oligotrophic to very oligotrophic environment of the Mediterranean Sea.