Understanding Historical Fecal Coliform Trends in North Carolinas Shellfish Growing Waters

Within coastal waters, designated shellfish growing and harvesting areas are regularly tested for fecal coliform bacteria to maintain public health and safety standards for shellfish consumers. In North Carolina, the North Carolina Division of Marine Fisheries (NCDMF) Section for Shellfish Sanitation and Recreational Water Quality, has regulated shellfish growing waters since the 1920s. While site specific knowledge aids in waterway management, a formal assessment of trends, and correlations between trends, has yet to be conducted. The vast spatiotemporal record of fecal coliform monitoring has provided a rich resource for short-term decision making, but long-term trends remain understudied due to the structure and scale of the data. In this study, we (1) integrated water quality data into a normal-form database structure, and (2) analyzed spatiotemporal trends in fecal coliform concentrations. We assessed trends in fecal coliform concentrations for a 20 year period from 1999 to 2021 using data collected from spatially fixed sampling sites (n = 466) specifically used for the management of shellfish growing waters dating back to 1979. The spatial and temporal correlations in land usage, precipitation over watershed, distance from shore, and salinity were found in conjunction with the fecal coliform trends. Mann-Kendall trend testing along with the analysis of Sens slope was utilized to determine the degree and significance of trends in fecal coliform levels. Trends in precipitation and land use were evaluated as a function of distance from shore to examine potential drivers of change. Stations located along the central part of North Carolinas coastline have shown the highest degree of downward trends in fecal coliform concentrations (mean tau = -0.085; mean sen slope = -0.03) over the past 20 years. Those same stations have also exhibited the highest degrees of upward change in salinity values (mean tau = 0.12; mean sen = 0.07) as opposed to their more northern and southern counterparts (mean tau = -0.16; mean sen = -0.09) that fall within similar distances to shore. Ultimately this study informs future management strategies, provides a programmatic and statistical workflow for future trend analysis within similar study systems, and reveals new insights into the complexities of estuarine and coastal systems.