Microbial pollution characterization of water and sediment at two beaches in Saginaw Bay, Michigan

Abstract Singing Bridge Beach and Whites Beach in Saginaw Bay have been designated as impaired water bodies by the Michigan Department of Environmental Quality (MDEQ). To quantify microbial loads and to determine probable sources of elevated E. coli levels, water and sediment samples were collected from both beaches during swimming season 2011. Sources of fecal pollution were assessed using multiple molecular markers: human-associated Bacteroides thetaiotaomicron α 1–6 mannanase (B. theta), bovine-associated Bacteroides (BoBac), human adenoviruses (HAdVs), and bovine adenoviruses (BAdVs). Fecal indicator bacteria ( E. coli and enterococci) were also monitored. In Singing Bridge Beach, 50% of water samples ( n  = 78) exceeded the recreational water quality guidelines for E. coli , while in Whites Beach all water samples met the E. coli standard. B. theta, BoBac, HAdVs, and BAdVs were detected in both beaches indicating influence of multiple sources of contamination. Cell culture assays indicated six water samples from Singing Bridge Beach and seven water samples from Whites Beach contained infectious viruses. Both frequency and concentrations of B. theta and BoBac were higher in sediment samples than in water samples. Elevated BoBac as compared with B. theta in water and sediment samples in both beaches, which are located in a watershed characterized as intensely farmed, suggest that runoff from agricultural areas was one of the probable dominant pollution sources. This study demonstrates that Bacteroides markers used in combination with land use information may help to characterize impacted beaches in mixed watersheds and design management plans.