California Sea Grant College Program

Nearshore concentrations of fecal indicator bacteria (FIB) are controlled by a complex array of physical and biological processes. The studies herein evaluate the contribution of several such process to coastal FIB dynamics using laboratory studies, mathematical modeling, and fieldwork. Two field sites are discussed; Border Fields State Beach (BFSB) and Huntington Beach, both sites of chronic FIB contamination in Southern California. At Huntington Beach field measurements of alongshore currents and horizontal diffusion were used to parameterize a suite of individual based particle tracking models. These models were used diagnostically to evaluate the contribution of physics and mortality to FIB dynamics. Advection and diffusion were found to explain a significant fraction of surfzone FIB decay, but played a lesser role offshore, suggesting that offshore FIB loss may be dominated by mortality rather than physics. No single mechanism was identified that best-explained FIB mortality at this beach, although cross-shore variable mortality mechanisms had higher skill than spatially constant ones. At BFSB the relative contribution of physics and mortality to nearshore FIB dynamics varied temporally rather than in the cross-shore. Generalized linear model analyses at this beach linked FIB contamination to an anthropogenic beach nourishment. Field-based microcosm experiments (using FIB from this source), incombination with surfzone bacterial monitoring, showed that FIB concentrations were controlled by rapid mortality of a “sensitive” FIB fraction and physical dilution mixing, which became steadily more important as sensitive FIB died off, leaving behind a resistant community. Co-existing populations of sensitive and resistant FIB were also observed in laboratory experiments evaluating the effects of phytoplankton concentration on FIB mortality. Mortality in these experiments was biphasic, with loss of sensitive FIB occurring > 10x faster than resistant FIB. Notably, bloom concentrations of phytoplankton halved mortality rates of both FIB groups, enhancing survivorship. This signal was not obscured by physical transport/mixing (Huntington Beach particle tracking model), suggesting that phytoplankton have the potential to be an important factor controlling FIB concentrations. This dissertation highlights the importance of both physics and biology as factors controlling nearshore FIB and points to phytoplankton communities as an under-evaluated control on FIB growth/mortality in marine systems.