Nitrogen dynamics and fixation control cyanobacterial abundance, diversity, and toxicity in Lake of the Woods (USA, Canada)

Our understanding of drivers of cyanobacterial harmful algal blooms (cHABs) is evolving, but it is apparent that not all lakes are created equal. Nitrogen (N) is an important component of all cHABs and is crucial for cyanotoxin production. It is generally assumed that external nitrogen inputs are the primary N source for cHABs. However, in northern lakes, nitrogen inputs are typically low, and suggests that internal nitrogen cycling, through heterotrophic organic matter decomposition or nitrogen fixation, may play a significant role in cHAB development and sustainment. Using Lake of the Woods as a testbed, we quantified nutrients, cyanotoxins, nitrogen fixation, and the microbial community in the southern extent of the lake. During our temporal study, inorganic nitrogen species (NO3-+NO2- and NH4+) were either at very low concentrations or below detection, while phosphorus was in excess. These conditions resulted in nitrogen-deficient growth and thereby favored nitrogen fixing cyanobacterial species. In response, nitrogen fixation rates increased exponentially throughout the summer and coincided with the Aphanizomenon sp. bloom. Despite nitrogen limitation, microcystin, anatoxin, saxitoxin, and cylindrospermopsin were all detected, with microcystin being the most abundant cyanotoxin detected. Microcystin concentrations were highest when free nitrogen was available and coincided with an increase in Microcystis. Together, our work suggests that internal nitrogen dynamics are responsible for the dominance of nitrogen fixing cyanobacteria and that additions of nitrogen may increase the likelihood of other cyanobacterial species, currently at low abundance, to increase growth and cyanotoxin production. Statement of Significance This study is the first assessment of nitrogen fixation rates and water column 16S rRNA gene amplicon sequencing in Lake of the Woods during a harmful algal bloom season. The aim of this study is to better understand nitrogen dynamics and the microbial ecology of cyanobacterial harmful algal blooms on Lake of the Woods. Result from this study reveal that internal nitrogen cycling via nitrogen fixation may alleviate nitrogen deficiencies, and structure and control the cyanobacterial community and cyanotoxin production. Molecular analysis reveals that cyanotoxins in Lake of the Woods are produced by less abundant cyanobacteria that are limited by nitrogen. This study has significant management implication as agencies continue to mitigate toxic blooms on Lake of the Woods, the largest shoreline lake in the United States. Our work is an important initial assessment and jumping off point for further research on Lake of the Woods when assessing how nitrogen plays a role in bloom formation and toxicity. Submitting to L&O, we believe would allow for the greatest outreach and access to an audience that will continue to build upon our findings. Additionally, submitting with L&O our work will reach beyond the scientific audience, but also reach other parties participating in the mitigation of harmful algal blooms.