Cladophora promotes escherichia coli growth and contamination of recreational waters in Lake Michigan

Abstract

In recent years, thick mats of the green alga, Cladophora, have washed onto beaches along Lake Michigan. These mats become stranded on the shoreline, begin to rot, and are malodorous. The rotting mats are a nuisance to recreational beach users and shoreline property owners. Cladophora mats have the potential to harbor Escherichia coli, which is used as an indicator of recent fecal contamination to determine beach closures. Cladophora mats may allow bacteria to survive and replicate by providing nutrients, elevated temperatures, and protection from UV light. Bacteria-laden mats potentially increase bacterial concentrations in surrounding recreational waters. To test this hypothesis, stranded sampled stranded Cladophora mats were sampled for E. coli at two beaches along Lake Michigan in Door County, Wisconsin. Random and gradient water samples were collected to determine E. coli concentrations in water underlying the mat and in surrounding water. Algal samples were collected and washed to determine E. coli concentrations attached to Cladophora. Since E. coli in recreational water should correlate with increased concentrations of pathogenic bacteria (also found in fecal material), pathogen concentrations in and attached to Cladophora mats were examined. Standard microbial culture methods were used to identify and enumerate pathogens such as Salmonella, Shigella, and Campylobacter.

Escherichia coli concentrations in water underlying mats were significantly greater than surrounding water (p < 0.001). Below mat E. coli increased as the stranded mats persisted at the beach swash zone. Water adjacent to Cladophora mats had lower E. coli concentrations but surpassed EPA swimming criteria the majority of sampling days. A significant positive association was found between E. coli concentrations attached to Cladophora and in underlying water (p < 0.001). The attached E. coli likely acted as a reservoir for populating water underlying the mat. Removal of Cladophora mats from beach areas may improve aesthetic and microbial water quality at affected beaches. Fecal bacterial pathogens, however, could not be detected by microbiological culture methods either attached to mat biomass or in underlying water. These associations call into question the efficacy of using E. coli as a recreational water quality indicator of fecal contaminations.