Fecal bacteria variability in a Florida tidal river: implications for source identification and water quality restoration.

Fecal indicator bacteria (FIB) are monitored in natural waters to detect potential presence of pathogens originating in fecal pollutants, but confounding factors and natural variability in FIB survival and abiotic factors such as temperature, precipitation, and mixing limit the validity of monitoring results to detect and mitigate FIB sources. This research controlled for one particular confounder, tidal mixing, in a coastal subtropical low-relief watershed, the Imperial River in Lee County, Florida. Each sampling event followed stream flow from inland toward the coast at a time when tides were outbound from their maximum height at each sampling site, measuring FIB abundance under conditions when tidal mixing would be at a minimum and sources from upstream in the watershed would have their maximum influence on in-stream conditions. Results showed less mixing-induced FIB variation in the form of fewer "high-abundance observations," with zero occurrences of >1000 MPN/100 mL for either Escherichia coli or enterococci over the six four-site sampling events. In comparison, sampling in the same reach without controlling for tidal mixing measured 3.6% > 2420 MPN/100 mL and 17.9% > 0 MPN/1000 mL for E. coli and 10.7% > 2420 MPN/100 mL and 32.1% > 1000 MPN/1000 mL for enterococci. Those proportions were similar those in 5 to 10 years of once-per-month sampling in three similar tidal waterbodies by local agencies. The tide-controlling sampling method effectively reducing sampling variability, improving representativeness of the sample for the targeted conditions, which is valuable to directing management actions toward the watershed locations where FIB originate.