Presenter: Jessica Lacy (US Geological Survey)

Description:
Modeling cohesive sediment transport requires specifying sediment properties, one of which is the erodibility parameter M. Erodibility is known to vary both spatially and temporally, but the factors governing its variation in cohesive sediments are not well understood. During summer 2019 and winter 2020 we collected field data to investigate how hydrodynamic forcing, physical properties of sediment, and benthic infauna influence erodibility in the shallows of San Pablo and Grizzly Bays in northern San Francisco Bay. We determined M from near-bed measurements of vertical turbulent sediment flux. During each two-month study period we measured bulk density and grain size distribution of surficial bed sediments 5 or 6 times, and the abundance of benthic infauna every two weeks.  Our results demonstrate both biotic and physical influences on M, as well as the dependence of biotic effects on species-specific manners of inhabiting the sediment. Amphipod abundance was much greater at both sites during winter 2020 than summer 2019. The dominant amphipod in the fresher Grizzly Bay was Corophium alienense, which burrows below the sediment surface, whereas in San Pablo Bay it was Ampelisca abdita, which dwells in tubes protruding above the sediment surface and can form dense mats. Ampelisca increased bed roughness (k_b) and decreased erodibility: 5-day average k_b was significantly correlated with Ampelisca abundance, and the mean and standard deviation of M were lower in winter 2020 than summer 2019. Five-day median wave-current bed shear stress was significantly, negatively correlated with 5-day average M lagged by 2.5 days in summer 2019 in San Pablo Bay, when Ampelisca abundance was low and M variance was high. This relationship did not hold in winter 2020 in San Pablo Bay, or in either season in Grizzly Bay. In Grizzly Bay, sediment bulk density was inversely correlated with the abundance of Corophium, but there was no associated change in either M or k_b.