Fecal coliform bacteria disappearance rates in a north-central Gulf of Mexico estuary

Fecal coliform levels in surface waters often peak after a major rain event. Thereafter, they decrease or disappear from water with time and can concentrate in sediments at high densities. Their dynamics in coastal waters is a function of bacterial loading from inflowing streams and rivers, mass transport, and losses due to death and sedimentation. We used water quality data collected from Mississippi Sound before, during and after 12 rainfall events to assess the effects of rainfall on fecal coliform levels, estimate fecal coliform disappearance rates, and evaluate the influence of season on fecal coliform loss rates. Fecal coliforms typically peaked within 48 h after a rain event, and then decreased precipitously, whereas the Pearl River (a major inflow into the western Mississippi Sound) stage often peaked within 96 h. Fecal coliform disappearance rates per day varied from −0.214 to −0.743 (mean = −0.428). Loss rates were higher in November/December (−0.64 ± 0.06SE) than in January (−0.45 ± 0.03SE) or February/March (−0.35 ± 0.03SE), perhaps due to seasonal differences in water temperature and/or intensities of solar radiation. It took an average of 6 days (range: 0.3-12.9 days) for the geometric mean FC levels to fall to 14 MPN per 100 ml. These data are useful for managing shellfish harvesting areas in Mississippi Sound and other river-influenced estuaries.