Potential Impacts of Ocean Acidification on Harvest and Other Ecosystems Services in Puget Sound

Over a third of the carbon dioxide released by anthropogenic activities since the industrial revolution has been absorbed by the ocean, where it has caused a decrease in pH of 0.1 units. Small changes in pH can affect a diverse suite of organisms, especially those that produce calcium carbonate structures. Changes in species’ growth and survival rates can alter their total biomass within an ecosystem, creating a new balance among primary producers, prey, and predators. Using an Ecopath with Ecosim model developed to approximate the food web of Puget Sound’s Central Basin, we explore the potential consequences of ocean acidification (OA) on an estuarine food web. We surveyed the literature on species responses to OA to determine which species groups would be susceptible and whether their response would be positive or negative. From this survey, we developed four test scenarios, decreasing the productivity of 1) non-cephalopod molluscs, 2) selected crustacea, 3) echinoderms, and 4) these three groups together. Given the high uncertainty in forecasting OA impacts at the ecosystem level, we consider our results more from a conceptual level than predictive of specific effects on certain functional groups. We found that our OA scenarios had both direct and indirect effects on the food web. Also, the magnitude of our simulations was often less than the forcing function we applied, indicating that direct and indirect effects of OA sometimes countered each other due to trophic dynamics. For example, forced declines in production of urchins and sea cucumbers were counteracted by forced declines in production of sea stars, predators on both groups. Simulating OA impacts on one or a limited number of species groups often yielded results counter to simulations on other or all directly affected species groups, emphasizing the importance of correctly characterizing those components of the ecosystem directly susceptible to OA. For instance, scenarios on euphausiid production increased the biomass of harvested groups that scenarios on copepod production decreased and vice versa. Our scenarios had non-additive effects on food web organization, indicating that trophic cascades maintain the system fairly close to its current state. Declines in crustacean and mollusc production, especially infaunal bivalves, had the biggest effect on food web organization.