California Sea Grant College Program

Certain structurally complex species such as corals and trees can create habitat that provides the foundation upon which ecological communities are built. Thus, understanding the biotic and abiotic limits of these “foundation species” may provide a means for conserving biodiversity and key ecosystem functions. For my dissertation, I studied native oyster habitat (Ostrea lurida) in Tomales Bay, CA, which acts as a foundation species by increasing community species richness and densities. Much of this habitat, however, has been depleted in areas where native crabs and whelks have been replaced by invasive crabs and whelks (Chapter 1). This work shows that in many cases invaders lack a shared evolutionary history and do not recognize each other as predator and prey. Therefore, predator-prey interactions like trophic cascades that normally benefit oysters have been short-circuited. Underlying these potentially important “top- down controls” is a spatial gradient in phytoplankton abundance that is tied to coastal upwelling (Chapter 2). During the summer-upwelling months, the lack of fresh water flowing into the estuary allows the daily pumping of water with each high and low tide (i.e., tidal excursion) to lengthen water residence times as distance from the ocean increases. The tidal excursion’s exchange with coastal waters creates a gradient of coastal nutrients that interacts with water residence times to promote consistent phytoplankton blooms in the middle portion of the estuary. These food subsidies influence oyster habitat from the “bottom-up” by allowing juvenile oysters to grow faster in the middle-bay region when compared to oysters in the outer- and inner-bay regions. In addition to phytoplankton, recruitment of oyster larvae is another important bottom-up control (Chapter 3). But the 4-6 week planktonic stage of these larvae allows them to accumulate in the inner bay where residence times are higher. Because high abundances of recruitment and phytoplankton are decoupled in space and because recruitment is inconsistent, the top-down and bottom-up controls identified in this system interact differently over time. These interactions then create temporally varying gradients of oyster density and size that cannot be understood without simultaneously considering the effects and underlying mechanisms of mortality, growth, and recruitment.