| dcterms.abstract | Freshwater harmful algal blooms caused by cyanobacteria (CyanoHABs) are increasing in frequency, intensity, and duration. There remains a limited understanding of how bloom communities respond to multiple environmental drivers in ecosystem settings, in part because many prior studies have focused on individual genera or have considered CyanoHABs as a uniform community. To gain further insight into environmental controls on CyanoHABs this study explored shifts in total cyanobacteria biomass, cyanobacteria genera, and microcystin-producing genera within naturally occurring blooms. Specifically, spatial and temporal monitoring was performed in Lake Erie and Mill Pond (Watermill, NY, USA) and incubation experiments were conducted to assess CyanoHAB responses to elevated temperature, nitrogen (N) and phosphorus (P). Bar-coded metagenomic analysis of cyanobacterial-specific 16S rRNA and the microcystin synthetase gene (mcyE) was conducted using Illumina MiSeq next generation sequencing (NGS) technology. Two separate cyanobacteria blooms occurred in Mill Pond from May-October 2015, shifting from Microcystis (non-diazotrophic) dominated in the spring to Anabaena (diazotrophic) dominated in the summer, coinciding with a shift from high nutrients-low temperature to low-nutrients-high temperature. The Microcystis bloom was associated with a high proportion of uncultured cyanobacteria detected by the 16S rRNA sequencing. In Lake Erie, spatial analysis found cyanobacterial blooms occurred near the Maumee and Sandusky River outflows in areas of higher nutrient abundance than the surrounding bays and were dominated by Microcystis and Planktothrix, respectively. Sequencing analyses also revealed cryptic cyanobacteria diversity specifically detecting high abundances of two distinct clades of Synechococcus in each basin. Low taxonomic but high genetic diversity of potential microcystin-producing taxa was detected by the mcyE sequencing, with Microcystis being the sole toxigenic genus in Mill Pond and toxigenic Planktothrix and Microcystis present in Lake Erie. Microcystin concentrations paralleled with the 16S-based relative abundance of the genera identified in mcyE sequencing and were significantly increased by N enrichment. Cyanobacterial biomass significantly increased in response to elevated nutrients and temperature while the abundance of green and brown algae decreased. N, P and temperature were all a significant source of variation in abundance within the cyanobacteria community, with N being highly influential on non-diazotrophic genera, as Microcystis dominated under high N and P, and lower temperature, while diazotrophic genera were most abundant under low nutrient, high temperature conditions and promoted by elevated P. Across all sites and experiments, three types of cyanobacterial communities were characterized: Microcystis-dominated communities associated with high levels of microcystin found in high N and high P conditions, a mixed Synechococcus-Planktothrix community associated with low nutrient levels, and Anabaena-dominated community associated with high temperature and low nutrients. While nutrients and elevated temperature are known to promote harmful CyanoHABs, this study revealed the ability of N, P, and temperature to selectively promote individual genera of cyanobacteria. | |
