Nitrogen limitation, toxin synthesis potential, and toxicity of cyanobacterial populations in Lake Okeechobee and the St. Lucie River Estuary, Florida, during the 2016 state of emergency event

• Published in: PloS one Vol. 13; no. 5; p. e0196278
• Main Authors: Kramer, Benjamin J, Davis, Timothy W, Meyer, Kevin A, Rosen, Barry H, Goleski, Jennifer A, Dick, Gregory J, Oh, Genesok, Gobler, Christopher J
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 23.05.2018; Public Library of Science (PLoS)
• Subjects: Algae; Analysis; Anthropogenic factors; Aphanizomenon; Aquatic ecosystems; Atmospheric sciences; Biology and Life Sciences; Chlorophyll; Cyanobacteria; Earth Sciences; Ecology and Environmental Sciences; Environmental monitoring; Environmental science; Estuaries; Estuarine environments; Eutrophication; Experiments; Flooding; Freshwater ecosystems; Gene expression; Lakes; Medicine and Health Sciences; Microcystis; Microcystis aeruginosa; Nitrogen; Nitrogen (Nutrient); Phosphorus; Physical Sciences; Phytoplankton; Plankton; Planktothrix; Rivers; Salinity; Salinity effects; Saxitoxin; Synthesis; Toxicity; Toxins; Canada; Florida; Ann Arbor Michigan; United States > US; Lucie River; Lake Okeechobee
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0196278

Lake Okeechobee, FL, USA, has been subjected to intensifying cyanobacterial blooms that can spread to the adjacent St. Lucie River and Estuary via natural and anthropogenically-induced flooding events. In July 2016, a large, toxic cyanobacterial bloom occurred in Lake Okeechobee and throughout the St. Lucie River and Estuary, leading Florida to declare a state of emergency. This study reports on measurements and nutrient amendment experiments performed in this freshwater-estuarine ecosystem (salinity 0-25 PSU) during and after the bloom. In July, all sites along the bloom exhibited dissolved inorganic nitrogen-to-phosphorus ratios < 6, while Microcystis dominated (> 95%) phytoplankton inventories from the lake to the central part of the estuary. Chlorophyll a and microcystin concentrations peaked (100 and 34 μg L-1, respectively) within Lake Okeechobee and decreased eastwards. Metagenomic analyses indicated that genes associated with the production of microcystin (mcyE) and the algal neurotoxin saxitoxin (sxtA) originated from Microcystis and multiple diazotrophic genera, respectively. There were highly significant correlations between levels of total nitrogen, microcystin, and microcystin synthesis gene abundance across all surveyed sites (p < 0.001), suggesting high levels of nitrogen supported the production of microcystin during this event. Consistent with this, experiments performed with low salinity water from the St. Lucie River during the event indicated that algal biomass was nitrogen-limited. In the fall, densities of Microcystis and concentrations of microcystin were significantly lower, green algae co-dominated with cyanobacteria, and multiple algal groups displayed nitrogen-limitation. These results indicate that monitoring and regulatory strategies in Lake Okeechobee and the St. Lucie River and Estuary should consider managing loads of nitrogen to control future algal and microcystin-producing cyanobacterial blooms.