Phylogenies of microcystin-producing cyanobacteria in the lower Laurentian Great Lakes suggest extensive genetic connectivity

Abstract

Lake St. Clair is the smallest lake in the Laurentian Great Lakes system. MODIS satellite imagery suggests that high algal biomass events have occurred annually along the southern shore during late summer. In this study, we evaluated these events and tested the hypothesis that summer bloom material derived from Lake St. Clair may enter Lake Erie via the Detroit River and represent an overlooked source of potentially toxic Microcystis biomass to the western basin of Lake Erie. We conducted a seasonally and spatially resolved study carried out in the summer of 2013. Our goals were to: 1) track the development of the 2013 summer south-east shore bloom 2) conduct a spatial survey to characterize the extent of toxicity, taxonomic diversity of the total phytoplankton population and the phylogenetic diversity of potential MC-producing cyanobacteria (Microcystis, Planktothrix and Anabaena) during a high biomass event, and 3) compare the strains of potential MC-producers in Lake St. Clair with strains from Lake Erie and Lake Ontario. Our results demonstrated a clear predominance of cyanobacteria during a late August bloom event, primarily dominated by Microcystis, which we traced along the Lake St. Clair coastline downstream to the Detroit River's outflow at Lake Erie. Microcystin levels exceeded the Province of Ontario Drinking Water Quality Standard (1.5 µg L(-1)) for safe drinking water at most sites, reaching up to five times this level in some areas. Microcystis was the predominant microcystin producer, and all toxic Microcystis strains found in Lake St. Clair were genetically similar to toxic Microcystis strains found in lakes Erie and Ontario. These findings suggest extensive genetic connectivity among the three systems.

Figure 1. Map of Lake St. Clair indicating sampling sites.

The underlined sites were the seasonal monitoring sites; starred sites indicate where DNA was extracted and sequenced for genetic diversity.

Figure 2. Percent biomass composition of the total phytoplankton community of the seven major phyla found in Lake St. Clair during the 23 August survey.

Figure 3. Total phytoplankton (black bars), cyanobacteria (grey bars) and Microcystis biomass (white bars) and total microcystins (MCs; solid black line) values at each site for the 23 August 2013 survey.

Figure 4. Maximum-likelihood tree of mcyA sequences sequenced from our study (LGL-1-6; bolded) and mcyA sequences from previous studies in Lake Erie (orange) and Lake Ontario (purple).

Numbers in parentheses indicate the number of identical sequences represented by the named sequence or LGL group. St.C  =  Lake St. Clair; Ont.  =  Lake Ontario; Erie  =  Lake Erie. Bootstrap values of >50% (for 1,000 replicates) are displayed at the branch nodes. The scale bar represents substitutions per site.