Amador, Jose [faculty advisor, Department of Natural Resource Science]




fecal indicator bacteria; subaqueous soils; eelgrass; Rhode Island


A beach closure during the height of the summer can cost a coastal community approximately $37,000. In 2006, 349 beach closures in Rhode Island were due to presumed human fecal contamination. Fecal contamination is determined based on enumeration of fecal indicator bacteria. Fecal enterococci (FE) are the commonly used indicator bacteria for human fecal contamination in fresh and coastal waters. The EPA has suggested that FE is the best indicator of human health risk in salt water used for recreation and shell fishing activities. Recently, molecular analysis of Bifidobacterium adolescentis has been introduced as a more reliable, specific indicator of human fecal contamination. To be useful, bacterial indicators must die soon after entering a water body, so that their presence can be related to a specific contamination event. Some studies have suggested that FE may survive for long periods after entering receiving waters. We assessed the potential role of subaqueous soils and eelgrass (Zostera marina) as refugia for B. adolescentis and FE in a coastal marine environment. Subaqueous soils are mineral soils that exist in shallow water and support rooted vegetation. Eelgrass is a submerged aquatic plant in the shallow water areas of Narragansett Bay. It provides critical habitat for fish, shellfish and crustaceans and conservation concerns have arisen in the past few years due to its marked decline. The study was conducted in Bluff Hill Cove, Point Judith, Rhode Island. Subaqueous soil samples were extracted using a vibracore and eelgrass sampled from the soil extraction sites. FE was analyzed using the Enterolert method and B. adolescentis was detected using nucleic acid analysis. Particle size distribution of each sample was determined to assess relationships between FE and B. adolescentisn concentration and soil texture. FE were present in both the eelgrass and subaqueous soil samples, with a higher concentration found in the latter. By contrast, B. adolescentis was not detected in either soil or eelgrass samples. These results suggest that FE bacteria may not be a reliable indicator of recent human fecal contamination, since soil and plants both appear to serve as refuge for these organisms in coastal environments, and thus a potential source of these bacteria to the water column. Re-evaluation of FE bacteria as an indicator of human fecal contamination in coastal wasters appears to be warranted.

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