Control of diatom and chrysophyte cyst dynamics by a meteorologically driven mixing regime in eutrophic Lake Zabinskie, northern Poland

Ongoing climate warming has strong moderating control over aquatic processes, such as ice cover duration, length of the growing season, vertical mixing patterns, thermal stratification, and the availability of light and nutrients. These fundamental mechanisms in turn influence the population dynamics of aquatic organisms such as diatoms and chrysophytes. In our study, over 3.5 years with contrasting meteorological conditions, we investigated eutrophic Lake Zabinskie located in northeast Poland. By combining observational data of meteorological conditions, physicochemical variables measured in the water column, and modern sedimentation, we sought to explain the relationships between changes in meteorological conditions, dynamics of total fluxes and the taxonomic composition of diatoms and chrysophyte cysts. Our results show the direct influence of meteorological conditions on physicochemical conditions and their indirect influence on total diatom and chrysophyte cyst fluxes and taxonomic succession. The former refers to the importance of air temperature and wind speed in shaping the mixing regime of Lake Zabinskie, while the latter refers to the nutrient cycling driven by changes in the mixing regime, which in turn influences the formation of biotic signals in the sediment. Our study also revealed that the biotic response to unusual meteorological conditions that occurred in 2020 (i.e., warm winter without ice cover) differed from typical years in terms of the phenology of diatom and chrysophyte cyst blooms. However, this response was not as pronounced as in lakes with lower productivity and can be attributed to the already turbid conditions and high nutrient concentrations, which alter the threshold for noticeable changes to occur. The taxonomic composition of diatoms and chrysophyte cysts also changed in response to changes in meteorological conditions. Complex interactions between physical and chemical conditions masked the direct influences of meteorological conditions on lake biota under eutrophic conditions. Nevertheless, high-resolution monitoring enabled indirect links to be recognised. Both phenology and turnover in community composition responded to changes in meteorological conditions. These results show how ongoing climate change might affect biota in eutrophic lakes in the future.