Zooplankton in the zones of transformation of river runoff in the north-western Black Sea region and the Mediterranean Sea in 2017-2020

Estuary zones play a key role in the regulation of the impact of continental processes on marine ecosystems. The zone "river-sea" is a unique area with specific conditions, geochemical and biological processes. They belong to the so called "ecological hot spots" and that is why their research is especially needed to understand possible changes in the ecosystems. In the present study we described the spatial distribution of zooplankton in the zones of transformation of river runoff in the Ukrainian part of the north-western Black Sea region and the Turkish coast of the Mediterranean Sea in 2017-2020. In the Black Sea, samples were collected from the coast of the Odesa Marine Region, Danube Region and coastal waters between Dnipro-Bug estuary and Yuzhne town. On the Turkish Mediterranean Sea coasts, samples were collected between Antalya city and Patara village (Mugla province). A total of 64 samples of the zooplankton were studied. We did not establish a direct dependence between zooplankton biomass and annual volume of river runoff (correlation between these metrics is from - 0.25 to -0.80). 20 taxa of zooplankton in the Odesa Marine Region, 21 taxa in the Danube Region, 11 taxa in the Dnieper Region, 23 taxa in the Turkish coastal zone of the Mediterranean Sea were registered. Species of forage zooplankton were dominant over non-forage ones in all seasons and all investigated aquatoria. The percentage of Noctiluca scintillans (Macartney) Kofoid & Swezy, 1921 and jellyfish did not exceed 5% of the total biomass of zooplankton. In summer in the Black Sea, the majority of zooplankton by number and biomass was formed by organisms of meroplankton, species of Cyclopoida and Harpacticoida, in autumn - species of the genusAcartia, Oithona davisae Ferrari F. D. & Orsi,1984, larvae of Cirripedia and Penilia avirostris Dana, 1849. The number and biomass of adults and Copepodid Stages of Acartia species increased from spring to autumn in all years. In populations of Acartia species Copepodid stages predominated over adults by number and biomass in all seasons. In the Mediterranian Sea, the majority of zooplankton by number and biomass was formed by larvae of benthic invertebrates and Copepoda. The abundance and biomass of zooplankton at a distance of 300 m from the river mouth were greater than directly at the mouth. The hydro-front was expressed near all investigated rivers, the maximum values of zooplankton number and biomass were observed here and they decreased in direction both towards the mouth of the river and towards the open sea.