Beneath the ice: exploring winter's impact on nutrient and phytoplankton dynamics in a shallow eutrophic lake

As global climate patterns change and winters decrease in duration it becomes increasingly important to understand the seasonal factors influencing nutrient concentrations in lake ecosystems. Winter ice cover can influence both nutrient cycling and phytoplankton dynamics. This study focused on a small shallow hypereutrophic lake in south-central Québec, Canada. We will assess ice-on vs ice-free seasonal differences in nutrient concentrations, and evolving chemical changes through winter, as they link to climatic conditions and ice phenology. The accumulation of dissolved inorganic nitrogen during the ice-on period over a decadal timestep was significantly correlated with the duration of ice cover (ranging from 102 to 166 days), meaning that long winters led to much greater nitrate concentrations at the onset of spring. Notable shifts in phytoplankton composition between ice-on and ice-free periods were observed, accompanied by a significant overall (annual) long-term decline in species richness. Crucially, nitrate-nitrite accumulation during the ice-covered period was positively correlated with the relative proportion of cyanobacteria in total phytoplankton in the subsequent growing season. This study provides evidence for the importance of understanding winter limnological characteristics relative to nutrient cycling and the seasonal dynamics of phytoplankton, particularly amid ongoing climate changes.