nutrients from anthropogenic pollution can degrade water quality and alter the balance of marine food webs. lying at the base of the trophic pyramid, plankton quickly respond to nutrient changes in the water, which can have repercussions throughout both pelagic and benthic food webs, and thus they serve as a good bioindicator of water quality. In early november 2009, we evaluated sediment pollution, water pollution, and plankton abundance at four shoreline sites in the San Francisco Bay. We tested the sediment for nitrogen, phosphorus, potassium, and pH levels, all factors that can affect growth of primary producers. In the water, we tested nitrate, phosphate, and pH levels. lastly, we sampled shoreline plankton abundance both morning and evening. Sediment phosphorus and water phosphates were strongly correlated with one another, but water nitrates remained relatively constant, at low levels, across sites. daytime plankton abundance showed a positive trend with water phosphate. these trends suggest nitrogen is quickly taken up by plankton, making nitrogen the limiting factor for them. the relationship between plankton and phosphorus is influenced by more complex factors.

Disease outbreaks are becoming more frequent as anthropogenic changes to ecosystem function stress species and create conditions favorable for pathogen infection. Marine outbreaks that were once locally constrained are re-emerging as large-scale epidemics with heightened mortality, decimating populations across multiple ocean basins. Effective prediction of future disease impacts requires a better understanding of their causes and consequences. In this dissertation, I explore the connections between environmental conditions, recovery of decimated host populations, and multispecies interactions to examine how a marine epidemic affects marine communities. I focus on a recent Sea Star Wasting Syndrome outbreak in rocky intertidal habitats along the Pacific coast of North America, with emphasis in Central California. Sea stars are an iconic intertidal species to many coastal visitors. Their absence has generated heightened public awareness of current challenges to ocean health, providing many opportunities for engagement in the process and outcomes of ecological research.

In my first chapter, I use data from long-term ecological monitoring to assess the recovery of sea star populations and predation pressure in the years immediately following the outbreak. I show that while sea star numbers are rebounding, these populations are made of smaller individuals that do consume the same amount of food. In my second chapter, I explore potential conditions associated with outbreak timing using data from citizen scientists and researchers combined. I identified associations between Sea Star Wasting Syndrome appearance, low tide exposure duration, proximity to other infected sites, and chlorophyll a concentration, though the relative importance of these factors varied across geographic regions of the coast. In my third chapter, I take advantage of a natural experiment created by the loss of predatory sea stars to measure the consequences of the outbreak on the intertidal community. I found that while mussels, the sea stars’ primary prey, had increased their coverage, they had not displaced the species living below the mussel bed. At local field sites, mussel increases were positively correlated with mussel recruitment and not pre-outbreak sea star levels. However, a coast-wide scale, pre-outbreak sea star density was positively correlated with pre-outbreak sea star density. Finally, I conclude with key insights from this work. My dissertation provides evidence that sea star wasting does not have uniform causes or population-level and community-level consequences across coastal regions. I discuss the value of long-term ecological monitoring and citizen science as critical information sources in the process of evaluating disease impacts.