Summer Subregional Patterns in Selected Holoplankton and Meroplankton Distribution Along the Turkish Mediterranean Epipelagic, with Diel Vertical Migration (DVM) Observation at a Single Station

This study examined the horizontal distribution and environmental drivers of selected epipelagic mesozooplankton along the Turkish Levantine coast during summer 2019, together with diel vertical migration (DVM) observations conducted at a single offshore station near Anamur. Zooplankton were collected from 64 stations with a 300 µm WP-2 net using stratified coastal-offshore sampling design, and relationships with hydrographic variables were tested using multivariate analyses. Among the investigated groups, Appendicularia, Siphonophora, Pteropoda, Doliolida, Foraminifera, and meroplanktonic larvae dominated assemblages. Holoplankton and meroplankton showed regional and inshore–offshore variability. Offshore waters influenced by the Asia Minor Current and rim-current structures had higher abundances of oligotrophic taxa, including Siphonophora, Pteropoda, Foraminifera, and Euphausiacea, whereas coastal and semi-enclosed areas supported Bivalvia, Gastropoda, Decapoda, and Cirripedia larvae, consistent with larval retention and reproduction. Noctiluca spp. showed localized maxima near anthropogenically influenced coasts. At the single offshore station near Anamur, DVM differed among taxa and was associated with the local thermocline structure, chlorophyll-a maxima, light intensity, and suspended particulate matter. Most taxa occurred within the upper 100 m, showing nocturnal upward migration or thermocline-associated aggregation. Appendicularia, Pteropoda, Doliolida, and Foraminifera aggregated near subsurface chlorophyll maxima, whereas Hyperiid Amphipoda and Euphausiacea occurred deeper, potentially reflecting predator avoidance and trophic optimization. Fish larvae were thermocline-associated. Temperature, sea-surface density, optical variables, chlorophyll-a, pH, and distance from the coast were the main environmental drivers. Overall, circulation, stratification, mesoscale hydrodynamics, and anthropogenic influences shaped distribution and DVM.