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

Abstract

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.

Fig 1

Sampling sites around Lake Okeechobee (A) and on the St. Lucie River Estuary (B) in Florida, USA. Sites sampled in both July and September 2016 (●) and September 2016 only (×) are represented. Inserts denote the general region sites were located on the peninsula.

Fig 2. July 2016 transect data of sites that represented a strong salinity gradient.

Included are microcystin and salinity values (A), chlorophyll a concentrations (B), densities of the five most abundant phytoplankton (C), and total (D) and dissolved (E) inorganic nitrogen and phosphorus concentrations and ratios. Algal densities are represented on the log scale in C. Error bars denote standard deviations.

Fig 3. Cyanobacterial taxa abundant in the 2016 summer transect.

Microcystis aeruginosa, bar is 20 μm (A) and 100 μm (B). Aphanocapsa grevillei, bar is 10 μm (C). Dolichospermum circinale, bar is 10 μm and asterisk (*) denotes heterocyst (D). Pseudanabaena spp., bar is 10 μm (E).

Fig 4. mcyE (white) and sxtA (shaded) gene abundances (copies mL^-1) at sites along the July 2016 transect.

Values are log-transformed and represented in boxplots.

Fig 5

Chlorophyll a (bar) and specific growth rate (black circle) data from July 2016 24 h nutrient amendment experiments from samples collected from sites LO 1 (A), SLE 1 (B), SLE 6.5 (C), and SLE 10 (D). Error bars denote standard deviations. Statistical significance (p < 0.05) among sites is denoted by different letter combinations. Top and bottom letters above bar graphs correspond to statistical differences in chlorophyll a and specific growth rate, respectively.

Fig 6. September 2016 transect data of sites that represented a strong salinity gradient.

Included are microcystin and salinity values (A), chlorophyll a concentrations (B), densities of the five most abundant phytoplankton (C), and total (D) and dissolved (E) inorganic nitrogen and phosphorus concentrations and ratios. Algal densities are represented on the log scale in C. Error bars denote standard deviations.

Fig 7. September 2016 diatom, cyanobacteria, and green algal pigment concentrations from all transect sites.

Error bars represent standard deviations.

Fig 8. Chlorophyll a (bar) and specific growth rate (black circle) data at 24 and 72 h from the September 2016 nutrient amendment experiment.

Samples were collected from sites LO 1 (A), SLE 1 (B), and SLE 6.5 (C). Error bars denote standard deviations. Statistical significance (p < 0.05) among sites is denoted by different letter combinations. Top and bottom letters above bar graphs correspond to statistical differences in chlorophyll a and specific growth rate. For B, statistical differences in chlorophyll a and growth rate were only present at 24 and 72 h, respectively.

Fig 9. Diatom, cyanobacteria, and green algal pigment concentrations at 48 h from the September 2016 nutrient amendment experiment.

Samples were collected from sites LO 1 (A), SLE 1 (B), and SLE 6.5 (C). Error bars represent standard deviations. Statistical significance (p < 0.05) of specific pigments among sites is denoted by different letter combinations. Letters are ordered relative to the order of parameters listed in the legend. For A, only cyanobacterial and green algal pigment concentrations exhibited statistical differences among treatments.

Fig 10. Regression of total nitrogen concentrations, total microcystin concentrations (black circles, dashed regression) and mcyE copies (white circles, dotted regression) during the July transect.