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Abstract Cyanobacteria harmful algal blooms (cyanoHABs) are a complex threat to water quality. Most research to date on the drivers of cyanoHABs focuses on environmental factors in the typical “growing season” despite evidence that cyanobacteria overwintering dynamics may have substantial effects on cyanobacteria seasonal succession and bloom formation. Additionally, the growing season is now beginning earlier and ending later in many parts of the world. Here, we examine the impacts of light, temperature and nutrients on the magnitude and timing of cyanobacteria recruitment from sediments in two hypereutrophic reservoirs in the Midwestern USA in the early spring season via microcosm recruitment experiments. We observed that recruitment was greatest at the first sampling point (Day 3), then declined throughout the rest of the 18-day experiment for both reservoirs. Further, increasing light and temperature significantly promoted recruitment in both systems, while nutrient additions were only a significant driver of recruitment in one lake. The recruited cyanobacteria community identity was similar in both lakes, with Planktothrix, Raphidiopsis and Pseudanabaena being most abundant. This study highlights the complex, interactive effects of environmental variables on cyanobacteria recruitment. 

Abstract Freshwater ecosystems are increasingly at risk of experiencing toxin-producing cyanobacterial blooms during the winter due to anthropogenic nutrient loading and climate change. However, understanding how increased light, temperature and nutrient levels impact cyanotoxin production during the winter is limited, as most research has historically focused on blooms during the summer and fall. We conducted 2 × 2 × 2 incubation experiments in February and March to test the individual and interactive effects of light intensity (50 and 150 μmol m−2 s−1 PAR), elevated temperature (+3°C), and nitrogen and phosphorus enrichment on microcystin concentrations in a Planktothrix agardhii-dominated community sampled from Grand Lake Saint Mary’s, a hypereutrophic Ohio reservoir. Microcystin concentration significantly increased with elevated temperature in both months. In February, low light also promoted higher microcystin concentrations, particularly when combined with elevated temperature and nutrient enrichment. In March, nutrient enrichment had individual and interactive effects with temperature that caused higher microcystin concentrations. These results demonstrate that toxin-producing cyanobacteria are active in winter and that climate-driven changes in environmental conditions can interactively increase total toxin concentrations in the water column, even in the non-growing season.