Cyanobacterial and Microcystin Response to Nutrient Additions at Lake Fayetteville throughout the 2021 Growing Season

Harmful Algal Blooms (HABs) are becoming a global concern due to their increasing distribution, frequency, intensity, and the occurrence of toxins. While it is known that eutrophication influences algal blooms, there is less known about what triggers these HABs to produce toxins, especially microcystin. In this study, we conducted 21 community bioassays at Lake Fayetteville, a hypereutrophic reservoir in Fayetteville, Arkansas, from April-November 2021 to examine how the addition of phosphorous and nitrogen influence cyanobacteria concentrations, microcystin concentrations, and microcystin toxin production. These experiments included a control, nitrogen (1.0 mg/L as KNO3), low phosphorus (0.025 mg/L as K2HPO4), high phosphorus (0.100 mg/L) and nitrogen plus low and high phosphorus treatments with four replicates, where the treatments were incubated in a chamber at temperature representing lake conditions below the surface and a light intensity of 140 μmol/L with 14 hours light and 10 dark. These bioassays took place throughout the nitrate decline and well into the growing season when dissolved nutrients are not readily available in the surface water. We found that cyanobacterial and algal growth were limited by nutrients, but the relative importance of N and/or P varied over time. Additionally, we found that seasonal variations in cyanobacteria and nutrient supply influenced microcystin concentration and production. Finally, while we did show that it was possible for nutrients to stimulate toxin production, for the majority of the bioassays we only stimulated algal growth and did not increase toxin production on a cellular basis