An assessment of the ability of Cladophora mats to sequester E. coli and microcystin LR and RR was conducted in the near-shore waters of Grand Traverse Bay (7 sites), Little Traverse Bay (2 sites), and Saginaw Bay (8 sites). The sampling locations were at public beach access points where Cladophora mats were previously observed. The goals of this research were to determine the spatial and temporal variability of E. coli populations in Cladophora mats in these recreational waters and if cyanotoxins (microcystin LR and RR) are sequestered in the detached algae. The collection of Cladophora samples was coordinated with local beach monitoring programs to facilitate the comparison with ambient water bacteria concentrations. This project provided important data for the assessment of public health impacts and the development of beach management programs to address the problems associated with Cladophora accumulations.

Based on the results from this investigation, Saginaw Bay appears to be more heavily impacted by detached Cladophora than Grand Traverse/Little Traverse Bays. Mean E. coli concentrations in detached Cladophora were higher in Saginaw Bay (2,796 cfu/g dwt) than Grand Traverse Bay/Little Traverse Bay (1,775cfu/g dwt); however, the difference was not statistically significant (Mann-Whitney ρ=0.40). Cladophora deposits exhibited spatial and temporal variability in both systems. At most beaches in Grand Traverse Bay, Cladophora deposits were limited to small pockets at 1 location. Clinch Park had only one site with Cladophora on the last sampling event and two locations at the Traverse City State Park were free of detached algal accumulations. In contrast, Cladophora deposits in Saginaw Bay covered approximately 1 meter (m) of the shoreline at most beaches. Two locations in Saginaw Bay also had no accumulations of Cladophora during the study period (White’s Beach and Pinconning Park). Differences in Cladophora accumulation between Saginaw Bay and Grand Traverse Bay/Little Traverse Bay may be attributed to higher total phosphorus levels in Saginaw Bay. Levels of E. coli in detached Cladophora in both systems were similar to concentrations previously reported in the Great Lakes (1,000 cfu/g dwt – 60,000 cfu/g dwt). In Saginaw Bay, the highest levels of E. coli in detached Cladophora were consistently found at beaches near the Saginaw River. Even within individual sites, locations near tributaries and drains at Wenona Beach and South Linwood Beach were significantly higher than locations farther away from a point source. This relationship also was noted in Grand Traverse Bay, where the location near Mitchell Creek at the Traverse City State Park, had elevated E. coli concentrations in detached Cladophora compared to the other beach locations. These results suggest that Cladophora can trap bacteria from point sources and also be stimulated by nutrient discharges. Two locations, Pinconning Park and White’s Beach, had very limited Cladophora growth. Both locations had Chara growing on the lake bottom. Chara is known to exhibit allelopathic activity that can limit the growth of other aquatic plants. No correlation was found between E. coli levels in the open water (designated beach monitoring locations) and the near-shore zone, where the detached Cladophora samples were taken. As noted in previous studies, Cladophora appears to hold trapped E. coli and does not release the entrained bacteria into the offshore water.

This investigation was the first to document the accumulation of microcystins in the detached Cladophora of Saginaw Bay. Total microcystins in detached Cladophora had a grand mean of 57 μg/g dwt for the study period. Saginaw Bay has a history of Microcystis blooms in the late vi summer months that produce both microcystin LR and RR. Since Microcystis has a high requirement for sunlight, cyanobacteria may become stressed when they are trapped in the detached algae mats. While accidental ingestion by humans of microcystins trapped in Cladophora is unlikely, these compounds can act as skin irritants. Walking through Cladophora accumulations to get to deeper water may provide sufficient exposure to cause irritation in sensitive individuals if microcystins are present. Although the data suggest that swimming areas (1 m depth) are not impacted by the E. coli accumulations in detached Cladophora, entrained bacteria and cyanotoxins may pose a hazard to children playing in the nearshore water and beach sand. Current regulations discourage beach grooming and altering the nearshore zone. The presence of elevated bacteria and microcystin levels in the nearshore environment of Saginaw Bay suggests that the current policy should be reevaluated to balance potential impacts to public health with the ecosystem services provided by coastal wetlands.