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Global climate change is altering the physical environment within lakes including summer stratification strength, mixing regime, and winter ice cover. Variability in these physical drivers structure water chemistry and, in turn, could affect phytoplankton dynamics. Shifts in phytoplankton phenology and cyanotoxin production have become more common in recent decades threatening water quality, recreation, and energy transfer up the food web. Antecedent conditions are hypothesized to impact subsequent phytoplankton phenology and many of these events occur during the “shoulder seasons”. However, most studies focus on summer months, limiting our ability to predict the timing in shifts of phytoplankton biomass and cyanotoxin production.��

Here, we analyze monthly, year-round phytoplankton biomass, represented by chlorophyll a (chl a), cyanotoxin concentration, and biogeochemical data from eight lakes with a gradient of ice cover, mixing regimes, and trophic states. In a single year, these lakes span a wide range of ice cover durations (0 to 6 months) and concentrations of total N and P (TN 130 - 1530 μg/L, TP 3 - 247 μg/L), chl a (0.05 - 263 μg/L), and cyanotoxins (0 - 1.74 μg/L). Intra-year variability in phytoplankton biomass and toxicity can be linked to ice cover, mixing regime, and water chemistry. Our results demonstrate the importance of moving beyond lake averages and trends to examine intra- and inter-year variability. This is the first year of a three-year study aimed at understanding the implication of shorter winters on the future of freshwater health and human use.