Variation in the diversity of bacterial communities and potential sources of fecal contamination of beaches in the Huron to Erie corridor.

Understanding the diversity of bacteria and E.coli levels at beaches is important for managing health risks. This study compared temporal changes of the bacterial communities of Belle Isle Beach (Detroit, MI) and Sand Point Beach (Windsor, ONT), both located near the Lake St. Clair origin of the Detroit River. Water samples collected 4 days/week for 12 weeks in summer, were subjected to 16S rRNA analysis of amplicon sequencing and E. coli enumeration. Bacterial communities changed over time, as determined by cluster dendrogram analysis, exhibiting different communities in July and August than in June and different communities at the two beaches. After June, alpha diversity decreased and relative abundance of Enterobacter (Gammaproteobacteria) increased at Sand Point; whereas, Belle Isle maintained its alpha diversity and dominance by Betaproteobacteria and Actinobacteria. Contamination at both beaches is dominated by birds (23% to 50% of samples), while only ∼10% had evidence of human-associated bacteria. High E. coli at both beaches was often associated with precipitation. Nearshore sampling counts were higher than waist-deep sampling counts. Despite the dynamic changes in bacterial communities between the two beaches, this analysis based on 16S rRNA amplicon sequencing is able to provide information about bacterial types associated with high E. coli levels and to use bacterial sequences to more precisely determine sources and health relevance of contaminants.