Phytoplankton development in a floodplain water body during winter and ice-free periods under different winter phenology conditions (Lake Lunskoe, Middle Volga basin, Russia)

Abstract

Climate change is transforming the winter regime of water bodies–reduced ice cover and increased thaws and winter floods require a reassessment of the role of the under-ice period in phytoplankton life. Floodplain water bodies remain especially poorly studied, as their response to winter phenological anomalies can be unpredictable. This study analyzes the seasonal dynamics of phytoplankton in the small eutrophic oxbow Lake Lunskoye (Middle Volga basin) in years with different winter phenology (2023 and 2025–2026). The lake experiences significant anthropogenic pressure and regular outbreaks of cyanobacterial blooms. Winter is demonstrated to be an active period for phytoplankton development. During the cold, snowy winter of 2023, an under-ice bloom (biomass up to 11.8 g/m³) of the non-heterocyst cyanobacteria, Planktothrix agardhii (Gom.) Anagn. et Kom. (up to 86% of total values) and Limnothrix redekei (Van Goor) Meffert (up to 55%), was recorded, remaining dominant throughout the summer. These algal blooms occurred both in the under-ice water and within the ice (especially in the crystalline part), exceeding the growth rates in the under-ice layer by up to twice. During the abnormally warm “Atlantic” winter of 2025, mixotrophic cryptomonads (Cryptomonas spp. and Komma caudata (Geitler) D.R.A. Hill) dominated, creating a winter flood biomass peak of up to 21 g/m³, comparable to summer data. In the winter of 2026, which was similar in phenology to 2023, under-ice phytoplankton was diatom-dominated. Algal development was weak in the ice but more active in the upper snow fraction, which periodically melted during thaws. A seasonal succession was revealed in the ice structure: from a heterogeneous “predecessor” community to monodominance of green algae in white ice, a cryptophyte complex in crystal ice, or complete dominance of diatoms.The obtained results refute the simplified idea that any warming stimulates cyanobacteria and demonstrate that the “atlantization” of winter can lead to the replacement of cyanobacteria by cryptophytes.