Amplification of Indicator Bacteria in Organic Debris on Southern California Beaches
Certain recreational beaches in southern California frequently exceed state water quality standards for indicator bacteria (total coliform, fecal coliform, and enterococcus). In San Diego County, two sites have been particularly problematic: Mission Bay, a large coastal embayment; and Dog Beach at the mouth of the San Diego River. Recent studies designed to investigate sources of indicator bacteria at these sites suggested that densities of indicator bacteria can be amplified through extended survival and reproduction in organic debris deposited on area beaches. This process was most prevalent in two common features of recreational beaches: organic debris deposited on the beach in the form of a wrack line and tidally influenced storm drains where organic debris frequently accumulates. Field investigations showed that the wrack line acts as a bacterial reservoir that can impact receiving waters. Indicator bacteria were concentrated in the organic debris deposited on the beach during spring tides, maintained in the wrack above the water line during neap tides, then released back to the receiving waters during subsequent spring tides. At some locations, this process was considered to be a significant cause of bacterial water quality standard exceedances. In laboratory experiments that simulated tidally influenced storm drains, bacterial amplification was even more dramatic. Fecal coliform and enterococcus bacteria were shown to reproduce rapidly under conditions typical of coastal storm drains, with densities increasing three to four logs in 48 hours. The results have potential implications for managing recreational beach water quality in southern California.
HIGH-TIDE WASHING SURVEYS
The appearance that bacterial levels increased following high tides resulted in a study that focused sampling efforts around the high tide. High tides tend to wash the wrack line collecting on the beach (the wrack line is composed of marine vegetation, typically kelp and eelgrass, and other debris found deposited at the extent of the last high tide). Observations on the beach showed that dog feces tend to be within the wrack line as well. Therefore, it seemed that the source of elevated bacteria levels may be the washing of dog feces in the wrack line. On Dog Beach, four surveys were conducted that collected samples approximately 3 hours prior to high tide, during high tide, and approximately 3 hours following high tide. Samples were also collected in three specific regions of the beach, west (surf influenced), north (strong tidal current flow, steep beach, narrow river channel), and east (variable tidal current flow, flat beach, mudflats). The west and north regions were further segmented into areas with observed higher and lower concentrations of dog feces in the wrack line.
The results showed a significant difference between pre- and during high-tide samples. Samples collected at high tide were significantly higher than those collected 3 hours prior, with concentrations of all three bacterial indicators typically greater by an order of magnitude or more (Figure 1). Interestingly, samples collected in areas with high concentrations of dog feces versus areas with low concentrations of dog feces were not significantly different from each other. Regrowth of bacteria on the wrack line with an unknown mechanism of transport (i.e., flies or tidal washing) was assumed to be the likely cause of elevated bacterial concentrations in areas without apparent dog feces. Consequently, tidal washing of the wrack line was a contributing factor to elevated bacterial levels.