Mesozooplankton community structure in the Eastern Mediterranean Sea

Mesozooplankton composition including functional groups in relation to hydrological features is studied in the oligotrophic pelagic ecosystem of the eastern Mediterranean Sea. Combining classic/standard microscopy and advanced digital imaging approach copepod growth rates, ingestion and production rates were estimated using allometric relationships. For this purpose, mesozooplankton samples were taken in the epipelagic zone (0–200 m) in June 2016, over a grid of 16 stations in the Cretan Sea, the Cretan Passage and the eastern and western Cretan Straits including one station in the Mirtoan Sea. The sampling area is influenced by the presence of the permanent Rhodes Cyclonic Gyre east of Crete and a series of smaller permanent or recurrent gyres south of Crete that influence the anti-estuarine pathway of the Atlantic Water from the Ionian Sea towards the Levantine Basin at surface and subsurface layers. Mesozooplankton abundance exhibited almost similar values (mean value 312 ind m−3 ± 100 SD) among transects with a higher value at the western Straits (up to 562 ind m−3). Copepods were the dominant group at all stations with almost the same relative contributions among stations (76%) but slightly higher values in the Cretan Passage. The contribution of copepod functional groups in the mesozooplankton community is also highlighted. The importance of small sized copepods was underlined by small ambush feeding carnivores, prevailing at all stations and demonstrating a high degree of opportunistic feeding of copepods in these ultra-oligotrophic waters, with chlorophyll a ranging from 0.05 to 0.20 μg L−1. Copepod vital rates (production, ingestion and respiration) estimated by a growth rate model were generally low, with higher values in the Cretan Sea.