A changing estuary: Understanding historical patterns in salinity and fecal coliform levels in the May River, SC

Highlights

• From 1999 to 2017, salinity levels decreased and variability increased in the headwaters of the May River, South Carolina.

• From 1999 to 2017, fecal coliform levels increased dramatically in the headwaters of the estuary.

• Fecal coliform correlated positively with rainfall, population, and El Niño but negatively with salinity.

• Increasing development and more frequent, intense El Niño episodes may lead to further degradation of the May River.

Abstract

The May River, South Carolina watershed has undergone rapid increases in population and development from 1999 to 2017. This study aimed to understand the factors that influence salinity and fecal coliform levels in this estuary and how these levels changed from 1999 to 2017. This analysis revealed that salinity levels decreased in the headwaters, while variability increased. Additionally, fecal coliform increased from 1999 to 2017 throughout the hydrological network, with drastic changes occurring in the headwaters. Salinity and fecal coliform were influenced by spatial (distance from the mouth of the river), temporal (year, season, and tidal cycles), environmental (El Niño Southern Oscillation and rainfall), and anthropogenic parameters (population). This analysis suggests that the synergistic nature of climate change, resulting in more intense and frequent El Niño events, and watershed development may lead to further decreases in salinity and increases in fecal coliform levels in the May River estuary.

Introduction

Coastal development threatens many estuarine environments in the southeast United States. Populations in coastal communities are increasing at a rate double the global population growth with greater than 50% of the population located along the coastline (Henrickson et al., 2001). This development has major implications for the health of estuaries, which support local economies and provide integral habitats for a diverse range of species (e.g. Lehnert and Allen, 2002; Cain and Dean, 1976; Weinstein, 1979; Shenker and Dean, 1979; Hackney et al., 1976). By monitoring salinity and fecal coliform levels, researchers can gain insight into the extent of habitat degradation.

The southeast United States is home to more than 320 high salinity estuaries and tidal rivers stretching from Cape Fear, North Carolina to Cape Canaveral, Florida, with nearly 50% comprising much of South Carolina's 4628-kilometer coastline (Vernberg et al., 1992). These watersheds play a vital role in providing nursery habitats for many ecologically and economically important invertebrate and fish species (e.g. oysters Crassostrea virginica, blue crabs Callinectes sapidus, red drum Sciaenops ocellatus, spotted seatrout Cynoscion nebulosus, and silver perch Bairdiella chrysoura; Able et al., 2001; Boesch and Turner, 1984). Salinity levels vary widely throughout these estuaries with lower salinities typically found in the headwaters and higher levels towards the mouth. In addition, temporal (e.g. tidal, lunar, and seasonal patterns) and environmental variables (e.g. rainfall and temperature) dramatically affect salinity levels (Ramos et al., 2011; Kawanisi et al., 2010; Able et al., 2001). Organismal response to salinity fluctuations tends to be species specific with salinity acting as an integral factor in their distribution (Upchurch and Wenner, 2008; Bulger et al., 1993; Gunter, 1961). Salinity influences osmoregulation and metabolic rates of estuarine species, which can influence reproduction, growth, and survival (Peterson, 2003; Buckel et al., 1995; Lankford Jr. and Targett, 1994). Many small, sessile organisms have an optimal salinity range, with large anomalies potentially stunting growth (Gunter, 1961; Bulger et al., 1993). Smaller, motile organisms tend to occupy lower salinities, and as these organisms grow, they generally migrate from habitats of lower salinity to areas of higher salinity (Labonne et al., 2009; Gunter, 1961). Greater species diversity generally occurs in higher salinity habitats, as many of these organisms are more sensitive to lower salinities (Labonne et al., 2009; Gunter, 1961).

In addition to monitoring salinity, fecal coliform provides another means to gauge estuarine health. Fecal coliform are a group of Fecal Indicator Bacteria (FIB) utilized to measure the presence and magnitude of fecal contamination within bodies of water (Arnone and Perdek Walling, 2007). Fecal Indicator Bacteria are often used as a proxy indicator for other pathogens (e.g. organisms in the genus Giardia, Cryptosporidium, and Vibrio) that might be fecal in origin but not as easily measured (Byappanahalli et al., 2012; Arnone and Perdek Walling, 2007). Since FIB correlate with other toxic fecal borne pathogens (i.e. bacteria, protozoa, and viruses), this measure is used to regulate shellfish harvesting activity in the United States (NSSP, 2017; Arnone and Perdek Walling, 2007). These fecal coliform bacteria generally occur naturally within the gastrointestinal tract of warm-blooded animals and comprise three main genera, Citrobacter, Klebsiella, and Escherichia (Jin et al., 2004). The exception is Klebsiella, which can originate from non-fecal sources, such as effluent discharge from pulp and paper mills (Jin et al., 2004; Caplenas and Kanarek, 1984).

In estuarine environments, numerous factors influence fecal coliform levels including rainfall, which can affect salinity (Freeman et al., 2019). While higher salinity levels in estuaries negatively influence fecal coliform levels, the mechanism driving this relationship remains unclear. Numerous studies have investigated whether this relationship is caused by salinity's negative impact on survivability of the bacteria, or whether decreasing salinity levels due to freshwater input correlate with increasing fecal coliform levels associated with rainfall runoff (Korajkic et al., 2019; Cahoon et al., 2016; Schulz and Childers, 2011; Chigbu et al., 2004; Lipp et al., 2001; Šolić and Krstulović, 1992). Additionally, population growth and watershed development have been shown to increase fecal coliform levels (Alford et al., 2016; Viau et al., 2011; Schiff and Benoit, 2007; Corbett et al., 1997). In Anchorage, Alaska and New Hanover and Pender Counties, North Carolina, population growth and subsequent watershed development led to an increase in fecal coliform levels (Frenzel and Couvillion, 2002; Mallin et al., 2000). In addition to increased impervious surfaces associated with development, other factors can increase fecal coliform levels including transformation of forested land to agricultural use (Mallin et al., 2001), increased pet and livestock waste, and septic system failures (Sowah et al., 2017; Kelsey et al., 2004).

The May River is a recreationally, economically, and ecologically important estuary located within Bluffton, SC. It provides a wonderful setting and natural resources for residents and tourists alike. From oyster and blue crab harvesting to fishing for spotted seatrout and red drum to observing bottlenose dolphins (Tursiops truncatus), these Bluffton traditions and the natural resources on which they depend impart a sense of place. The local community relies heavily upon the river as a means of recreation, income, and sustenance. Within this region of South Carolina, the local estuaries are utilized for subsistence fishing by Gullah Geechee African American communities, who rely on these local waters as a food source (Ellis et al., 2014). Additionally, local eco-tourism companies rely on the health of the May River estuary and its natural resources. Within the Town of Bluffton, the Bluffton Oyster Company relies on local shellfish beds to provide oysters to numerous local and statewide restaurants.

As in most coastal towns and cities in South Carolina, the population of Bluffton has increased dramatically from approximately 794 residents in 1990 to 21,085 residents in 2017 in large part to increased coastal development and annexation of surrounding areas. This change equated to a 2696% population growth rate in just 27 years (US Census Bureau, 2019). Over the past two decades, land-use within the watershed has changed drastically, with an increase in developed land and a decrease in forested land (Homer et al., 2012). The associated expansion of housing, roads, commercial infrastructure, and increased recreational use of the May River have resulted in an increased risk to the health of the estuary and its natural resources.

In 2009, the South Carolina Department of Health and Environmental Control (SCDHEC) restricted harvesting oysters in the upper portions of the May River estuary, SC due to elevated levels of fecal coliform. In response to these restrictions, in 2011, the Town of Bluffton, in conjunction with a consultant team and local stakeholders, developed the May River Watershed Action Plan Advisory Committee (WAPAC). This organization detailed a plan of action to help reduce fecal coliform loadings in the river and help conserve the watershed (AMEC et al., 2011). One such initiative was the microbial source tracking program, which has revealed fecal coliform sources stemming from pet waste, wildlife, and humans (i.e. septic systems). Currently, agricultural input is not of concern as there are no large-scale farming or concentrated animal feeding operations occurring within the watershed. As a result of this degradation, the first objective of our research was to investigate factors that influenced salinity and fecal coliform levels from 1999 to 2017. These factors included temporal (year, season, lunar phase, tidal phase), geographical (distance from the mouth of the river), and environmental parameters. Our second objective was to explore how changes in salinity and fecal coliform levels along the May River related to changes in human population and land-use.