Comparing seasonal heterogeneity of phytoplankton habitat and community in northern lakes with low to moderate but historically variable DOC concentrations

Dissolved organic matter (DOM) influences several physical and chemical drivers of phytoplankton habitat. Increasing variability in lakewater dissolved organic carbon (DOC) concentrations in recent decades has raised uncertainty over temporal responses of phytoplankton communities to changing DOM. We conducted limnological surveys in four lakes with low (~ 2 mg l⁻¹) to moderate (~ 4 mg l⁻¹) DOC in Maine, USA, from February to November 2020 to assess variability in phytoplankton metrics and habitat gradients. Low-DOC lakes had similar thermal structure patterns compared to moderate-DOC lakes, but more stable euphotic depths. While moderate-DOC lakes had greater seasonal heterogeneity of biomass and vertical distribution of chlorophyll-a, the low-DOC lake had greater phytoplankton community turnover (Morisita-Horn Dissimilarity Index), underscored by a shift in dominance from autotrophs in the winter under ice to mixotrophs throughout the open water season. Long-term trends showed DOC concentrations generally increased for one decade beginning in 1995, followed by decreases in all four lakes for 12–15 years until 2020 by rates of 0.02–0.15 mg l⁻¹ year⁻¹, during which time water clarity increased in one lake. Small differences in or changes to DOC elicit greater variability of phytoplankton in lakes with low to moderate DOC concentrations.