Abstract
This study examines the species composition and abundance of phytoplankton in Petropavlovskaya Guba (Avacha Bay, southeastern Kamchatka) during the winter period of 2025-2026. The relevance of the work is results from the limited knowledge of phytoplankton in winter, whereas spring, summer, and autumn seasons have been studied much more comprehensively. Samples were collected using an Apstein plankton net from the bottom to the surface, which allowed for a detailed characterization of the community's taxonomic structure under winter conditions. The article presents data on species diversity and quantitative characteristics of phytoplankton. For the first time in Avacha Bay, the presence of colonies of the haptophyte microalga Phaeocystis sp. and the diatom Hyalodiscus sp. was recorded in March.