In estuaries, we often see predictable patterns of spatial and temporal variation in both physical and biological factors. This results in an excellent system in which to study variation in population and community dynamics. Longitudinal gradients are heavily influenced by mixing of inputs from the nearshore ocean or rivers. Vertically, tidal elevation on shore is a proxy for a gradient of stress due to factors like aerial exposure or food availability. Consequently, both longitudinal position along the estuary and vertical position on shore can affect demographic patterns of benthic estuarine invertebrates and interactions between species. To this end, I documented demographic factors of the Olympia oyster, Ostrea lurida, along both a longitudinal estuarine gradient that is heavily influenced by the nearshore ocean and a vertical elevation gradient in Tomales Bay, California (Chapter 1). Secondly, I investigated the composition and competitive effects of the sessile community on stages throughout oyster life history along these same gradients. I also investigated competitive effects on earlier life stages in San Francisco Bay, California, which has a different estuarine structure and sessile community than Tomales Bay (Chapter 2).
Position along both the longitudinal and vertical gradient in Tomales Bay influenced recruitment and growth but not survival. Increasing water residence time toward the head of the estuary best predicted recruitment patterns, while a longitudinal food gradient peaking mid-bay best predicted intertidal oyster growth. Subtidal oyster growth did not show significant longitudinal variation. Thus, while benthic and pelagic processes can be coupled in an estuarine system, this link can vary with tidal elevation.
Competition directly affected earlier life history stages but not later stages. In Tomales Bay, size of recruits by the end of their first growing season was significantly reduced but total recruitment was not affected. In San Francisco Bay, we found the opposite pattern such that total recruitment was reduced by presence of competitors but growth was not affected. There was variation in both percent cover and composition of the sessile communities in each bay. These results better our understanding of how variation in habitat characteristics and life history stage together alter demography and species interactions.