Cyanobacterial community structure changes with seasonal mixing and dissolved oxygen dynamics in three urban eutrophic lakes

Background/Question/Methods ; Nutrients contribute to the development of toxic cyanobacterial blooms yet information about other drivers is needed to better predict when and where blooms will occur. We monitored cyanobacterial community dynamics in three Gulf Coast urban lakes. The SW and SE lakes were both sulfidogenic and ferrugenic whereas the NE lake was ferrugenic. The lakes were sampled during Sept.-Oct. 2016 and during early April, early July and late October 2017. Chemical analyses followed standard protocols. Dissolved oxygen (DO) and temperature were monitored continuously throughout 2017. The lakes were recovering from a weather-related mixing event in April 2017 when a defined oxycline was not observed. Similarly, in October 2017 the lakes had been impacted by a mixing event that led to a 50% reduction in DO in the upper portions of the lakes and lowered the oxycline by 1 to 2 m. A well-defined oxycline was observed in July 2017. Cyanobacterial communities were investigated using 16S rRNA gene sequences. Partial gene fragments (gfs) were amplified using the forward primer 515fB and the reverse primer 806rB targeting the V4 region of the 16S rRNA. Amplified gene fragments were parsed into operational taxonomic units (OTUs) at 97% sequence identity. Results/Conclusions ; Ten Cyanobium sp. OTUs accounted for 75 % of the total gfs among the 20 most abundant OTUs. Planktothrix sp., Limnothrix sp., and a Microcystis sp. accounted for 11, 1.7, and 1.3 % of the top 20 OTU gfs, respectively. Cyanobium sp. were found throughout the aerobic zone and in two instances occurred in the anaerobic zone. The deepest penetrations occurred in April for all three lakes where oxygen levels decreased from the surface to 0 mg/l at 7 m. However, the overall distribution of specific Cyanobium spp. varied with lake and time of sampling. A single Planktothrix sp. OTU was found only throughout the anaerobic zones in the SE and NE lakes in July, where Fe2+ concentrations were greater than 40 µM in the bottom waters. Anaerobic conditions and elevated Fe2+ combined have been reported for initiating Planktothrix blooms. A single Microcystis sp. bloom was observed in the SE lake in October 2016. These results indicate the distribution of Cyanobium spp. and the Planktothrix sp. varied with DO levels in the water column. Weather conditions had a definite impact on the depth and stability of the oxycline which limited the depths where certain Cyanobacteria could grow.