Seasonal dynamics of phytoplankton in the surface and bottom layers of Avacha Bay (Southeastern Kamchatka) during spring–summer period of 2025

This article presents the results of hydrobiological monitoring of phytoplankton in Avacha Bay (Southeastern Kamchatka), conducted in April, May, and July 2025. The study material comprised 19 samples collected at three stations from the surface and bottom water layers. Sample processing was performed using light microscopy with a Sedgwick–Rafter counting chamber for quantitative cell enumeration. During the laboratory analysis, 51 taxa of microalgae were identified, among which diatoms constituted the dominant group in terms of species diversity (35 taxa). A clear seasonal succession of the dominant complex was established: in April–May, absolute predominance belonged to the diatoms Detonula confervacea (Cleve) Gran, Thalassiosira nordenskoldii Cleve, and Skeletonema spp., whereas in July, a monodominant bloom of the potentially toxic dinoflagellate Alexandrium tamarense (Lebour) Balech was recorded in the surface horizon, with a maximum abundance of 15.9 million cells/L, accompanied by neon bioluminescence. Significant differences in quantitative parameters between the surface and bottom horizons were revealed: in May, Skeletonema spp. reached maximum concentrations in the bottom layer (up to 7.7 million cells/L), while in July, the dominance of A. tamarense was characteristic exclusively of the surface layer, which is likely associated with differences in temperature, illumination, and nutrient content at different depths. An inverse gradient of toxic species concentration from the eutrophied inner part of the water area toward the mouth of the bay was established, which may indicate heterogeneity of hydrological and trophic conditions within the studied water area.

The high dynamism of phytoplankton in Avacha Bay necessitates enhanced monitoring of toxic dinoflagellates of the genus Alexandrium, due to their potential danger to marine organisms and humans. Regular observations will enable the prediction of bloom risks in recreationally significant areas of Kamchatka.

Objective of the study – a comparative analysis of the seasonal dynamics and structure of phytoplankton in the surface and bottom layers of Avacha Bay during spring–summer 2025.

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