Differential occurrence of Escherichia coli and human Bacteroidales at two Great Lakes beaches

This study compared the occurrence of Escherichia coli and the human-associated Bacteroidales marker (HF183) across the beach sand–water interface at two distinctively different beaches on Lake Ontario. E. coli concentrations were measured in both water and sand/sediment samples by culture methods and E. coli and HF183 concentrations were measured by qPCR methods. Both beaches had the same general profile of culturable E. coli concentrations across the beach sand–water interface, with the highest concentrations associated with foreshore sand and with concentrations diminishing with depth offshore. E. coli concentrations were also highest at both beaches in foreshore unsaturated sands relative to upshore sand and sediment samples at ankle- or knee-depth. E. coli concentrations were below our qPCR limit of detection for 90% of all water samples, rendering this particular E. coli qPCR protocol ineffective for monitoring these relatively clean beaches. The general profile of the HF183 marker across the beach sand–water interface was different from E. coli at both beaches. The frequency of occurrence and concentrations of HF183 did not exhibit the same gradient as E. coli with diminishing concentrations offshore. Instead, HF183 concentrations were similar across pore, ankle and chest water at Marie Curtis Beach, while they were significantly lower in foreshore pore water at Burlington Beach. It is possible that differential occurrence of culturable E. coli and the HF183 marker across the beach sand–water interface may lead to different indications of health risks associated with sand and water exposure at beaches.