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

Abstract

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 which 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.