Zooplankton vertical-distribution in relation to the hydrology in the nw levantine and the se aegean seas (spring 1986)

Zooplankton composition and vertical distribution, and its relation to the hydrological regime are studied in the NW Levantine and SE Aegean Seas. The area is characterized by intense sub-basin and mesoscale variability. Various gyres and eddies as well as currents and jets affect the vertical and horizontal transport of the identified water masses. Furthermore, in late winter LIW is generated exclusively in this region of the Eastern Mediterranean, mainly in the Rhodes cyclonic gyre and to a lesser extent in the Cretan Sea. Generally, zooplankton density and taxa numbers decreased with depth. Four groups of copepod species were distinguished according to their vertical distribution: epipelagic, epi-mesopelagic, mesopelagic and bathypelagic. From 0-500 m, the community was dominated by small cyclopoids (Oncaea, Oithona, Corycaeus, Corycella), while the calanoid Eucalanus monachus was extremely dominant below 1000m. Herbivores were dominant in the upper layers, normally decreasing with depth. At the station located in the anticyclonic flow region close to the Asia Minor coast, their percentages remain high, even down to deeper layers, where some cladoceran, siphonophor and appendicularian specimens were present. These phenomena can be attributed to the downwelling which occurs within the eddy field that transports coastal waters, rich in detritus, to the deeper layers. Conversely, downwelling processes in the offshore oligotrophic region lead to zooplankton paucity in deeper layers. Hierarchical clustering and MDS grouped the samples collected in the upper layer of all stations according to the flow of the Asia Minor Current in the whole area. Zooplankton vertical distribution seems to be more influenced by temperature limits than by the thermocline or halocline position.