Blooms of toxic Raphidiopsis raciborskii in Laguna del Sauce (Uruguay): environmental drivers and impacts

Laguna del Sauce is a eutrophic shallow lake and the second drinking water source in Uruguay. This ecosystem suffers recurrent blooms of potentially toxic nitrogen-fixing cyanobacteria, mostly during summertime. Here we analyze environmental drivers and potential impacts of blooms of the toxic cyanobacteria Raphidiopsis raciborskii in Laguna del Sauce from 2003 to 2020, and assess its effects on governance and biomonitoring. We registered three blooms of R. raciborskii, in 2004, 2015, and 2020, the last two expressed saxitoxin (lack of toxicity information for 2004). The 2015 bloom occurred in autumn, reaching an absolute biovolume of 104 mm3 l&minus1, and had high saxitoxin concentrations (up to 9.8 &mug l&minus1) which continued to be detected even after the bloom collapsed. This was the first saxitoxin register in this ecosystem and drastically affected the drinking water supply. After this massive bloom, a monitoring program was established which continues up to date. The 2020 bloom also occurred in autumn and lasted until winter it was of lower magnitude (up to 6.3 mm3 l&minus1) and had lower saxitoxin concentration (up to 0.9 &mug l&minus1) compared to the 2015 bloom. Blooms of R. raciborskii were associated with a combination of persistent low water level, low water color, and high water transparency, conditions that rarely coincide in this ecosystem. Hence, the programs which started after the 2015 bloom, allowed better characterizing the environmental drivers of R. raciborskii blooms in Laguna del Sauce. Furthermore, these implied adapting the ecosystem management and water treatment strategies, and strengthening of academia&mdashstakeholders&rsquo interactions.