Abstract
ABSTRACT Escherichia coli and enterococci spp. are used as fecal indicator bacteria (FIB), i.e. proxies for the presence of human waste and pathogens in surface waters. Their use as indicators is complicated by many biotic factors such as differential survivability of microbes under varying conditions, and many abiotic factors such as mixing and transport, along with the fact that both FIB can originate in the guts of many warm-blooded species. Management steps to identify and eliminate human waste in natural waters requires identifying source locations and times, which are deeply uncertain owing to those confounding factors. Many source-identification approaches apply multiple lines of evidence – DNA sequencing, chemical and biological indicators, etc. – but variation in the environment continues to confound. This research addresses FIB variability by using an “outbound-flow” sampling method to control for environmental factors in tidally influenced freshwater, a natural system widely present in coastal southern U.S. and elsewhere. It also applies an “inland-to-coast” sampling approach to capture spatial changes in FIB within the waterbody as the water moves past a variety of land uses, discharges, and inputs. The objective was to compare data from the outbound-flow method to data from traditional FIB sampling to determine if it can better characterize the presence, magnitude, and variability of FIB in a tidal system by applying it to one case study and analyzing ways in which that case study may represent other tidal waters. .
Samples were collected in the Estero River, a Southwest Florida tidal stream draining a densely developed suburban watershed identified as impaired for bacteria under the U.S. Clean Water Act. The outbound-flow sampling methodology collects samples in order from the furthest inland site to the site nearest the ocean discharge, and during times when each location was experiencing moderate to high outbound flows produced by ebb tide, so that sampling essentially followed a packet of water as it moved seaward to the waterbody’s mouth. That sampling structure is expected to capture conditions when tidal mixing and sediment resuspension are at a minimum, controlling for those two abiotic factors; and also to identify locations and times when FIB may be entering the channel or attenuating with die-off or dilution. Samples were collected at ten locations within tidally-affected reaches of the Estero River over the course of eight extraction events during the wet-weather season (July through October) 2022. Samples were tested in the laboratory for abundance of E. coli and enterococci and results were compared to data for previous time periods, collected under traditional monitoring methods (i.e., collected once monthly at a regular date and time and not controlled for tidal conditions nor ordered spatially to reflect flow in the system), provided as publicly-available data promulgated by Lee County, Florida Environmental Lab.
Findings:
1. Variability in abundance of E. coli was greatly reduced by the outbound-flow technique as compared with traditional sampling, and this clarity revealed the absence of a sustained E. coli source in the watershed. Sources of E. coli appear to be distributed throughout the watershed, such that E. coli exhibits little attenuation or buildup along the course of the waterbody during outbound-flow conditions. The data for enterococci were also less variable with the outbound-flow technique, but did suggest a sustained source in the inland portion of the watershed. These results indicate that tidal mixing and resuspension were a dominant source of variability in traditional FIB sampling but were controlled with the outbound-flow method.
2. Substantial sources of enterococci appear to exist in the inland portions of the watershed, with inland sample sites exhibiting elevated levels during the single wet weather season that was analyzed. The traditional sampling data from Lee County, which does not control for tidal mixing, suggests higher abundance in seaward parts of the waterbody, but this is believed to be an artifact of tidal mixing and/or resuspension that is controlled in the outbound-sampling method. Both traditional and outbound-flow methodologies identify and overall elevated enterococci abundance in the system compared to regulatory targets.
3. The different spatiotemporal abundance patterns of E. coli and enterococci indicate those two FIB are originating from different sources.