Freshwater salinization increases survival of Escherichia coli and risk of bacterial impairment

Elevated levels of Escherichia coli (E. coli) are responsible for more designated freshwater stream impairments than any other contaminant in the United States. E. coli are intentionally used as a sentinel of fecal contamination for freshwaters because previous research indicates that salt concentrations in brackish or marine waters reduce E. coli survival, rendering it a less effective indicator of public health risks. Given increasing evidence of freshwater salinization associated with upland anthropogenic land-use, understanding the effects on fecal indicators is critical; however, changes in E. coli survival along the freshwater salinity range (<= 1500 mu S cm(-1)) have not been previously examined. Through a series of controlled mesocosm experiments, we provide direct evidence that salinization causes E. coli survival rates in freshwater to increase at conductivities as low as 350 mu S cm(-1) and peak at 1500 mu S cm(-1), revealing a subsidy stress response across the freshwater-marine continuum. Furthermore, specific base cations affect E. coli survival differently, with Mg2+ increasing E. coli survival rates relative to other chloride salts. Further investigation of the mechanisms by which freshwater salinization increases susceptibility to or exacerbates bacterial water quality impairments is recommended. Addressing salinization with nuanced approaches that consider salt sources and chemistry could assist in prioritizing and addressing bacterial water quality management. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Research shows that freshwater salinization increases the survival rates of E. coli at conductivities as low as 350 μS cm(-1) and peaks at 1500 μS cm(-1). Specific base cations, such as Mg2+, have different effects on E. coli survival rates. Further investigation into the mechanisms by which freshwater salinization affects bacterial water quality is recommended.