The paper presents results of comprehensive hydrological, hydrochemical and hydrobiological investigations of the coastal waters off Koreiz(Southern Coast of Crimea, Black Sea). In 2023–2024, a total of six seasonal sampling sessions were conducted within the area near a mussel and oyster farm, thus ensuring comprehensive coverage of all seasons. In the surface layer(0–0.5 m), 61 samples were collected, and 720 hydrochemical analyses were performed. Phytoplankton was sampled at stations 1–5 in the surface layer. Meroplankton samples were collected with a Juday net(entry ring diameter 36 cm, gas size no. 49) in the 0–10 m layer at coastal stations 1, 5 and 7. It was shown that the temperature and salinity levels of the surface layer during the study period exhibited a consistent increase compared to the long-term means for the Yalta area. However, these parameters remained within the range 7–25°С and 17–20‰, which is conducive for cultivation of bivalve molluscs. The dissolved oxygen concentrations in all seasons remained substantially above the threshold values specified by fisheries water quality standards. The values of five-day biochemical oxygen demand(BOD5) and the pollution coefficient Kz corresponded to the category of “unpolluted” waters. Concentrations of inorganic forms of nitrogen and phosphorus were in most cases low and spatially homogeneous. The local hotspots of elevated ammonium, nitrate and organic nitrogen and phosphorus were identified in the mussel and oyster farm area and a coastal station influenced by domestic wastewater. In the phytoplankton, 77 microalgal species were identified; diatoms dominated numerically throughout the year, while in the cold and spring seasons a substantial part of the biomass was formed by taxa considered food for mussels and oysters. The meroplankton community was found to comprise larvae of 43 benthic invertebrate species; larvae of the cultivated bivalves Mytilus galloprovincialis and Magallana gigas occurred within the temperature ranges characteristic of these species. The obtained data indicate that the coastal waters off Koreiz can be regarded as an environmentally favourable area for the operation of a mussel and oyster farm.
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