Vertical distribution and migration of Cyanobacteria and cyanotoxins across diel to seasonal timescales in Lake Mendota

Cyanobacteria, commonly known as “blue-green algae,” are a phylum of bacteria that produce toxic or otherwise bioactive peptides that can affect the brain, liver, and skin of humans and animals. Rapid growth of Cyanobacteria in water bodies—known as “blooms”—affect the health and recreation of many nutrient-rich lakes, including Lake Mendota (Madison, WI). Freshwater Cyanobacteria are known to grow and migrate throughout the water column to access varying light and nutrient conditions, but perceptions of blooms are often limited to lake surface observations. Many previous studies lack investigation of potential vertical heterogeneity of both Cyanobacteria and the cyanotoxins they produce, posing a challenge to water quality managers and health officials assessing the extent of toxic threat from cyanotoxins in freshwater lakes. In this study, we assess the vertical distribution and migration of Cyanobacteria and cyanotoxins in Lake Mendota on both a diel and seasonal time scale throughout the cyanobacterial growth season (May-Nov) 2021. To assess diel vertical migrations of the cyanobacterial community, we designed and constructed an in situ automated phycocyanin fluorescence water column profiler. We observed consistent yet taxonomically distinct diel migrations of Cyanobacteria throughout the epilimnion, as well as unique patterns in vertical distribution of cyanobacterial taxa across the growth season, observed via 16S rRNA gene amplicon sequencing. We also analyzed the concentration of 19 cyanotoxins using high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) from weekly depth-discrete samples and across two 24-hour diel studies. We observed high seasonality of epilimnion cyanotoxins and high variability in concentration between layers of the stratified lake. This study provides insights into potential coupled strategies for light avoidance, nutrient access, and toxin production throughout the water column. The combination of vertical measurements across time scales of hours to seasons provides a detailed characterization of the distribution and migration of cyanobacterial taxa and cyanotoxins throughout the water column.

Primary Presenter: Riley Hale, University of California, San Diego (haleriley@gmail.com)

Authors:

Riley Hale, Scripps Institution of Oceanography  (haleriley@gmail.com)

Robin Rohwer, University of Texas at Austin  (rohwer@utexas.edu)

Todd Miller, University of Wisconsin-Milwaukee  (millertr@uwm.edu)

Katherine McMahon, University of Wisconsin-Madison  (trina.mcmahon@wisc.edu)