Drivers of seasonal variability in sediment composition and accumulation rate in varve-forming Lake Gorzyńskie, central Europe

Biogenic varves provide valuable annually resolved archives of environmental change. Yet their palaeoenvironmental interpretation requires a detailed understanding of the processes that control seasonal sedimentation. We integrated 3 years (2021–2024) of limnological monitoring, hydrochemical measurements, phytoplankton analyses, and sediment-trap observations to investigate the mechanisms of modern calcite–diatom varve formation in Lake Gorzyńskie (western Poland), a eutrophic hardwater lake preserving an almost continuous varved sediment record spanning approximately the last 13,250 cal yr BP. Despite some yearly fluctuations, the overall sediment deposition shows a pronounced seasonal trend, with approximately 93% of the annual sediment flux occurring during spring and summer. Spring sedimentation was dominated by centric diatoms and large idiomorphic calcite crystals formed during phytoplankton blooms. Elevated spring phosphate levels delayed carbonate precipitation despite calcite supersaturation, indicating that nutrient availability plays a key role in controlling seasonal varve composition. The summer deposition featured fine-grained calcite and a significant presence of Phacotus lenticularis loricae, indicating increased carbonate deposition in warm, highly productive conditions. During autumn and winter, only small quantities of organic matter and biogenic SiO 2 from diatoms were deposited. Despite prolonged winter mixing and bottom waters remaining oxygenated for up to six to seven months annually, sediment lamination remained uninterrupted, indicating that biogenic varves in Lake Gorzyńskie remain resilient to bottom-water oxygenation. However, increasingly mild winters may negatively affect the preservation of sediment lamination not only in this lake but also in other temperate hardwater lakes.