High fecal indicator bacteria in temperate headwater streams at baseflow: implications for management and public health

Abstract

Fecal indicator bacteria (FIB) in rivers and streams serve as key markers of public health risks, but factors influencing spatiotemporal FIB variation in headwater streams at baseflow have received insufficient attention. We analyzed a 12-year dataset of FIB concentrations collected from 118 watersheds in the Delaware River Basin of the eastern United States across seasons to establish baseline conditions and investigate how landscape (watershed size and land cover) and in-stream (e.g., temperature and particulates) environmental variables relate to spatiotemporal FIB variations. We found that most probable number (MPN)-derived FIB levels in baseflow at 86 % to 96 % of sites (for Escherichia coli and Enterococcus, respectively) were above human health regulatory limits for primary contact during the recreational season. Variables affecting FIB dynamics included watershed size, seasonal variations in stream temperature, total particulate carbon and nitrogen in the water, and land cover types – specifically, the relative extent of forested, developed, and agricultural areas. Based on watershed size, smaller headwaters contained higher recreational period FIB concentrations than larger rivers. Headwater FIB concentrations were related to land cover, with lower concentrations observed in forested watersheds, and higher concentrations in developed and agricultural watersheds. Microbial source tracking suggested that FIB originated from human and bovine sources in headwaters with developed or agricultural land cover. FIB levels tended to be lower in non-recreational seasons (winter, spring, and fall) in small headwater streams. In addition to human, cow and other animals, we speculate that FIB survival and turnover in local environments might also cause FIB occurrence. These findings help guide choices of indicators to address fecal contamination of rivers and streams and prioritize restorative actions at the landscape scale.