Phytoplankton community dynamics and vertical nutrient fluxes during the winter-to-spring transition in a monomictic temperate reservoir

Winter phytoplankton blooms have been documented in several temperate lakes, yet the mechanisms triggering them remain poorly understood. Understanding the ecology of these blooms is key for predicting succession patterns, especially given the impact of climate change on winter conditions. To elucidate the relationships between exponential algal growth, abiotic conditions, and vertical nutrient transport, we investigated the phytoplankton community in a deep temperate reservoir during the winter-to-spring transition. We collected biweekly physical, chemical, and phytoplankton data, along with monthly sedimenting material data. We found that our study system can support the exponential growth of diatom species in late winter during circulation, 6–10 weeks before stratification onset, and once photosynthetically active radiation had reached 1.3 mol m⁻² d⁻¹. The phytoplankton community was mainly dominated by diatoms, motile species, and Planktothrix rubescens. These species displayed distinct growth patterns and ecological strategies, likely as an adaptation to the varying abiotic conditions during this period. Specifically, P. rubescens was evenly distributed during circulation and formed a deep chlorophyll layer at ~ 11 m depth immediately after stratification onset. Additionally, a few of these dominant phytoplankton species predominantly influenced vertical nutrient transport through sedimentation. Our results demonstrate that exponential phytoplankton growth can occur in temperate lakes before stratification onset under low light conditions if there is sufficient time before grazing begins and sinking losses increase. Our findings emphasize winter's significant role in shaping species distribution in subsequent seasons and underscores the need for an annual perspective in limnology.