Nutrient limitation and seasonality associated with phytoplankton communities and cyanotoxin production in a large, hypereutrophic lake

Though freshwater harmful algal blooms have been described and studied for decades, several important dynamics remain uncertain, including the relationships among nutrient concentrations, phytoplankton growth, and cyanotoxin production. To identify when and where nutrients limit phytoplankton, cyanobacteria, and cyanotoxins, we conducted in situ bioassay studies. We added nitrogen (N), phosphorus (P), or N + P across various seasons in water collected from three locations across Utah Lake, one of the largest freshwater lakes in the western U.S. This shallow, hypereutrophic lake provides a powerful testbed for quantifying nutrient-growth-toxin interactions. We assessed a range of parameters over time, including photopigment concentrations, phytoplankton abundance (cell counts), cyanotoxins, and nutrient concentrations. Despite high background nutrient concentrations in lake water, phytoplankton abundance and composition were strongly affected by nutrient addition. Phosphorus limitation was more common in the spring, with N limitation and N + P limitation becoming more common in the fall. Nutrient additions were positively associated with cyanobacteria (Microcystis, Aphanocapsa, Dolichospermum, Merismopedia, Aphanizomenon spp.), eukaryotes (Aulacoseira, Desmodesmus spp.), and two taxonomical categories of phytoplankton (i.e., unicellular and colonial green algae). When detected, anatoxin-a was positively associated with Aphanizomenon and negatively associated with Microcystis spp. However, overall cyanotoxin concentrations were not associated with cyanobacterial cell density but varied seasonally. These findings highlight the importance of considering seasonal nutrient availability dynamics and provide insights into specific nutrient targets, species, and cyanotoxins that play a significant role in the health and management of similar eutrophic lake environments around the world.