Mesozooplankton dynamics in relation to distinct water masses in the Aegean Sea, Eastern Mediterranean

Hydrographic structure and water mass circulation influence the mesozooplankton communities, particularly in oligotrophic regions such as the Eastern Mediterranean Sea. In the Aegean Sea, complex thermohaline dynamics, including the interaction of Black Sea Water, Levantine Surface Water, Levantine Intermediate Water, and deep water masses, create pronounced vertical and horizontal environmental gradients that shape pelagic ecosystem functioning. This study investigates the relationship between mesozooplankton community structure and distinct water masses in the Aegean Sea during winter conditions. An oceanographic survey was conducted in March 2017 in the deep basins of the Aegean Sea while mesozooplankton samples were collected along the water column and analyzed in terms of abundance, biomass and functional composition. Multivariate analyses were applied to evaluate community similarities and their association with hydrographic characteristics. Results revealed strong vertical structuring of mesozooplankton communities linked to water mass identity and depth. Surface layers influenced by Black Sea Water exhibited higher abundance and biomass, dominated by herbivorous and omnivorous copepods associated with enhanced trophic conditions. In contrast, intermediate and deep layers were characterized by lower biomass and increased contributions of carnivorous, omnivorous, and detritus-associated taxa. Reproductive strategies also varied with depth, with broadcast spawners dominating productive surface layers and sac-spawning taxa prevailing in deeper oligotrophic environments. Although this study included a single-month survey with limited station coverage, the results suggest that hydrographic structure provides an important ecological framework for understanding the vertical organization of mesozooplankton in the Aegean Sea, particularly during periods of strong winter hydrographic variability.