Sediments as a Reservoir of Indicator Bacteria in a Coastal Embayment: Mission Bay, California,


Mission Bay is a large, heavily-used coastal embayment within the City of San Diego that includes over 27 miles of recreational shoreline. Historically, exceedances of state water quality standards for indicator bacteria (total coliform, fecal coliform, and enterococcus) have been a persistent problem at some beaches in Mission Bay. A 2-year comprehensive study was conducted to investigate and identify the numerous potential sources of bacterial contamination in the Bay receiving waters and surrounding watershed. As part of the investigation, intertidal sediments were assessed at some sites to determine the extent to which the beach sands act as a reservoir for indicator bacteria. The results suggested that bacterial densities in upper intertidal beach sands were significantly greater than those in lower intertidal beach sands. In addition, when the sediments in the upper intertidal zone were resuspended during simulated swimming activity, bacterial densities in the water column were an order of magnitude greater than those in samples collected when sediments were not disturbed. This pattern was not observed when the experiment was conducted in the lower intertidal zone. This phenomenon suggests that swimming activity may lead to greater bacterial densities in the water column and helps explain the pattern of bacterial contamination observed at some sites in Mission Bay. The study also has potentially-important implications for other recreational beaches in southern California.

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