Recurrent summertime algal blooms, often dominated by cyanobacteria capable of producing toxins, are a widespread perturbation on the landscape. In temperate regions, the paradigm of algal bloom ecology asserts that blooms proliferate in excessive nutrient and warm water conditions, which are conducive to cyanobacterial growth. As water temperatures cool in the autumn, algal growth slows, and blooms typically begin to dissipate. Recently, there have been increasing reports of algal blooms during the late autumn or early winter in temperate regions. These temperate autumnal cold-water algal blooms (autumn blooms) appear to be composed of similar toxin producing cyanobacteria as blooms that occur during warmer months. There is currently little mechanistic understanding of how or why these blooms form, although warmer temperatures in the autumn and climate-related changes to nutrient availability likely play a role.

Fall and winter visitors to National Parks that experience these autumnal blooms may be at risk of exposure to cyanotoxins, which are prevalent in surface waters throughout the summer months. Although cyanotoxins are detectable during autumn bloom events, there are currently no published studies that have systematically investigated the persistence and magnitude of cyanotoxins within autumn blooms.

Few studies examine algal blooms beyond the early autumn, leading to a dearth of baseline information on autumn blooms that can be used to make predictions on how large these blooms could become, or whether they are likely to produce toxins. We seek to determine whether park visitors are at risk of cyanotoxin exposure in the fall by collecting baseline data on the biomass, composition, and toxicity of autumnal cold-water algal blooms in two Upper Midwest National Park Units (SLBE and VOYA) over two autumns and comparing toxin measurements to established exposure guidelines. We will use these baseline data in concert with physiochemical measurements to assess the risk of park visitors to cyanotoxins during autumn months.