Phytoplankton nutrient limitation persists into the winter in a productive Midwestern US reservoir

While phytoplankton dynamics in the growing season are well studied, winter remains understudied despite recent evidence of year-round activity. This study examines seasonal phytoplankton dynamics in Acton Lake, a hypereutrophic reservoir in southwest Ohio, USA, focusing on nutrient limitation and shifts in nitrogen (N) versus phosphorus (P) limitation. Year-long sampling revealed persistent nutrient limitation, with strong P limitation in summer, likely due to high nutrient demand, depleted nutrients, and elevated lake water N:P ratios. N limitation emerged in late summer and persisted into early winter, likely driven by reduced N inputs and internal nutrient cycling. Winter phytoplankton activity remained high, with chlorophyll <i>a</i> levels comparable to summer until extreme cold (&lt;4 °C) suppressed biological activity. Spring runoff elevated N:P ratios, favoring P limitation, while summer drought and internal recycling may have promoted N limitation. Cyanobacteria and diatoms exhibited the strongest nutrient limitation responses, reinforcing their dominant roles in seasonal succession. These findings highlight the need for year-round lake monitoring to predict and inform lake management in a changing climate, particularly in reservoirs where hydrology strongly influences nutrient dynamics.