Biology Ph.D. Dissertations
Spatio-Temporal Distribution Of Microbial Communities In The Laurentian Great Lakes
Freshwater microbial communities have received comparatively little attention compared to their marine counterparts, despite the importance of these systems. Using next-generation sequencing (Illumina itags), this study examined the microbial communities of the Laurentian Great Lakes during both the summer-stratified period and during the winter. Additionally, the winter communities of the Laurentian Great Lakes were compared with the winter community of Lake Onega, one of the largest freshwater lakes in Europe. Winter communities were examined from 2010 through 2013. Lake Erie was examined during periods of high- (2010 and 2011) and low ice cover (2012). Lower ice-cover resulted in an 89% decrease in phytoplankton biovolume between years of expansive ice cover and nearly ice-free 2012. Principal coordinate analysis (PCoA) of UniFrac distance matrices revealed a strong separation between high-ice year 2010 and low-ice 2012, indicating a shift in microbial community structure. An examination of winter communities in 2013 from both Lake Erie and the upper Great Lakes revealed phylogenetically different communities for Lake Erie, Lake Michigan, and the waters of the St Mary’s River. Samples from Lake Michigan and the Straits of Mackinac clustered with Lake Erie samples, which were correlated with concentrations of chloride and sulfate. The communities of the Laurentian Great Lakes were then compared with the communities of Lake Onega, and revealed strong differences in community structure.
Summer communities were examined from 2011 and 2012. A cruise from oligotrophic Lake Superior to eutrophic Lake Erie revealed differences in community structure of the surface mixed layer. Concentrations of phosphorus and ammonium were correlated with the PCoA plots. A comparison of the surface waters of the upper Great Lakes with their oxygenated hypolimnions revealed a unique community in deep waters. This community had high abundances of Planctomycete and Chloroflexi reads, which were stable across spatially and temporally. Resampling in 2012 confirmed the stability of this community, and also examined cyanobacterial communities in both Lake Superior and Lake Erie. The community of Lake Erie’s hypoxic 'dead zone' was also examined.