Through close interactions with biotic and abiotic environments, microbial communities in lakes mediate numerous biogeochemical processes that are essential in regional and global cycles of C, N and P. However, the relationship between bacterial community compositions and environmental conditions is still unclear. Lake Erie's natural gradient of nutrient supply and many other environmental parameters from the Sandusky Bay to the Central Basin provides an ideal experiment to examine how well bacterial community composition tracks environmental changes spatially and temporally. Surface water samples were collected along a transect that ran from the Sandusky Bay (hypereutrophic) via Sandusky Sub-basin (mesoeutrophic) to the Central Basin (oligotrophic) in June, July and August 2012. Zooplankton sample were also collected at each basin in June, July and August to see whether they respond to environmental conditions and to the changing bacterioplankton communities. Physico-chemical parameters were measured in situ. Bacterioplankton was collected on filters and filtrates were used for nutrient analyses, including ammonium, dissolved organic carbon, total dissolved nitrogen, nitrate, nitrite and soluble reactive phosphorus. Chlorophyll a concentration measurements confirmed the expected gradient of primary productivity among sites. Terminal restriction fragment length polymorphism (T-RFLP) analysis was conducted to compare of the microbial community structure and diversity along this natural gradient from the Sandusky Bay to the Central Basin. Additionally, zooplankton community structure and diversity was compared along the transect. Results showed that the free-living bacterioplankton structure differed significantly among sampling time, which was likely contributed by temporal variations in nutrient concentrations. As for the zooplankton community, Cyclopidae, Branchionidae and Synchaetidae were identified as major families (>78.4% of total zooplankton) in all samples. Zooplankton family structure had no clear separation based on site location on site location or sampling time. In addition, no significant correlation was identified between zooplankton community structure and environmental parameters or with zooplankton community structure and bacterioplankton community structure. Zooplankton diversity tests revealed significant differences in zooplankton diversity among sites and months. This research contributes a better understanding of the zooplankton and bacteria community structure found in Lake Erie.
Along with this natural nutrient gradient found in Lake Erie, harmful cyanobacterial blooms (cyanoHABS) is also a serious issue that affects wildlife, human health, recreation and local economics. CyanoHABs produce cyanotoxins, such as microcystins that lead to skin irritation, illness and liver tumors. Natural bacterial degradation of these microcystins play a key role in lakes by transforming these harmful toxins to less harmful metabolites that can be consumed by other organisms without a detrimental affect on their health and ecosystem health. Microcystin-LR is a toxin produced in harmful cyanobacteria blooms in Lake Erie and in Grand Lake St. Marys. This experiment specifically compared bacterial community structure and diversity from lakes with previous CyanoHAB exposure and their response to amended Microcystin-LR levels. Water samples were collected in June 2012 in the western basin, Sandusky sub-basin and central basin of Lake Erie and three recreational sites in Grand Lake St. Mary’s. Particulate-associated bacterioplankton was filtered out of the water samples, and the remaining filtrate was starved of all carbon and incubated in the dark for 1 week. After incubation, water samples were divided into triplicate microcosms. Microcystin-LR additions were added to the water samples as the sole carbon source for the naturally existing bacterioplankton community. After the microcystin-LR addition cell counts and microcystin concentrations were measure every 24 hours for two days. T-RFLP analysis was conducted to compare original bacterial community structure and diversity for each site to the Microcystin-LR amended bacterial community structure and diversity. Significant differences between start and end MC-LR concentrations (p < 0.05) measured in the incubation experiment indicated MC-LR degradation. Shannon diversity indices for bacterioplankton T-RF percent abundances were not significantly different between treatments for both lakes (ANOVA, p > 0.05). T-RFLP results showed that bacterioplankton community structures were significantly different between microcystin amended and original free-living bacterioplankton communities for Grand Lake St. Mary samples, but there was no significant difference between community structure for MC-amended treatments and non amended controls. In contrast, Lake Erie’s MC-amended communities experienced no shift in community structure. Non-amended controls had natural occurring MC-concentration, which suggests that there is a large subset of bacterioplankton that could degrade MC-LR before the treatments were administered. The extensive CyanoHAB history found in both lakes can explain these results. There are two dominating cyanobacterial species in Grand Lake St. Mary’s and four in Lake Erie, which suggests that these differences may affect the differences in MC degraders found in both lakes and the overall bacterioplankton community structure. Evidence of MC-degradation could be explained by bacterioplankton using MC-LR as an energy source.