Mesozooplankton Dynamics in the Aegean Sea

Abstract

Zooplankton plays a fundamental role in oceanic carbon flux as the primary biological mechanism sequestering the carbon transferred from the atmosphere into surface waters to deeper layers and higher trophic levels. Studies of the zooplanktonic organisms of the Aegean Sea have been initiated in the 1900s, whereas the number of cruises has increased since the late 1980s. This review aims at providing an updated and integrated picture of the Aegean Sea mesozooplankton in the offshore waters with emphasis on epipelagic zone, based on studies conducted during a 25-year period. The mesozooplankton standing stock, the species composition, the contribution of the main functional zooplankton groups, the mesozooplankton community structure as influenced by the major hydrological features and the role of mesozooplankton in the pelagic food web are discussed. An overview of the distribution of the mesozooplankton standing stock highlights a distinct pattern of decreasing abundance and biomass; higher abundance values were observed in early autumn than in spring whereas the opposite was true for biomass, due to the important contribution of cladocerans in early autumn, along the north–south axis whereas higher values are observed in spring-early summer than autumn. The bulk of epipelagic mesozooplankton is concentrated in the upper 100 m layer where mesozooplankton plays a major role in biological processes. Our review confirmed that copepods are by far the dominant group in the Aegean Sea and overall 257 species/genera of copepods and 6 cladoceran species are reported in this study. Different copepod species dominate the epipelagic, mesopelagic, and bathypelagic zones. In the South Aegean Sea (SAS) mesozooplankton composition in the 0–100 m layer is different from that in the North Aegean Sea (NAS), presenting a more homogeneous distribution of taxa abundances in the mesopelagic zone, while differentiation reappears in the bathypelagic zone. The compilation of data from studies in NAS revealed the strong impact of the inflowing Black Sea Water and the associated hydrological front on mesozooplankton standing stock as well as on the mesozooplankton composition. Some of the detected maxima are among the highest values observed at the surface layer (0–50 m) of Mediterranean inshore and offshore waters. The significance of small size species has been underlined not only from the Normalized Biomass Size Spectra but also by the functional group analysis coming in line with other findings in the Mediterranean Sea. A “multivorous web” seems to be common in the Aegean Sea as suggested by the great variety of feeding modes and preferences and by the significant and simultaneous grazing impact on phytoplankton and ciliates by mesozooplankton.