Bloom and bust: Historical trends of harmful algal blooms in Muskegon Lake, Michigan, a Great Lakes estuary

Around the world, freshwater lakes are increasingly suffering from harmful algal blooms (HABs) as a result of anthropogenic eutrophication. Muskegon Lake (Muskegon, Michigan, USA), a drowned river mouth, Great Lakes estuary, was declared an Area of Concern by the Environmental Protection Agency in 1987 with nuisance algal blooms cited as a beneficial-use impairment. The objective of this study was to quantify changes in HAB prevalence and assemblage composition in Muskegon Lake over 14 y (2003-2016) and explore the environmental factors potentially driving the changes. We performed univariate and multivariate analyses on long-term monitoring data, including environmental and phytoplankton data, to understand their relationships and temporal variation. Using linear regressions, we assessed changes in environmental variables over time and relationships between cyanobacteria biovolume and environmental variables. We used non-metric multidimensional scaling to visualize variation in annual cyanobacteria assemblage composition and association with environmental variables. Analyses revealed that, despite generally rising water temperatures, which would be expected to increase HABs, a reduction in nutrient concentrations caused by restoration efforts has likely led to substantial decreases in HAB abundance over time. Additionally, HAB assemblage composition appears to be driven by temperature and nutrient form and amount, with Microcystis (Lemmermann, 1907) often dominating. HABs in freshwater bodies should be closely monitored into the future as we enter an era of uncertain climatic conditions.