Sunday, August 30, 2015

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Ecosystem-based models are the tools used to try to predict how ecosystems will respond to shifts in various factors, such as ocean acidification.  Often these models are used to inform fisheries management, and management of other important natural resources.  Analyzing how this change in ocean chemistry will impact an ecosystem can be challenging because many other aspects of the ecosystem are effected  by and intertwined with one another.  Because these ecosystem feedbacks are complex, understanding the uncertainty associated with these models is critical to effective management.  Many people think of ocean acidification impacting just those organisms that are calcifiers. But scientists are exploring how OA may affect fish behavior (i.e predator-prey interactions) and the nutritional content of non-calcifying phytoplankton (and those impacts further up the food web).  These models are a tool that can be used to guage the complex effects of ocean acidification on entire ecosystems.

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NOAA Scientists Look at Potential Ocean Acidification Impacts on the Puget Sound Food Web

NOAA scientists at the Northwest Fisheries Science Center are beginning to understand future impacts of ocean acidification on Puget Sound’s food web. Drs. Shallin Busch, Chris Harvey, and Paul McElhany applied ocean acidification scenarios to a food web model to explore how the estuary’s food web and its ecosystem services (i.e., fisheries yield and ecotourism) may change over the next fifty years. The scenarios focused on species that calcify (i.e., shrimp, copepods, sea urchins), as they are most likely to respond to changes in carbon chemistry. Furthermore, the scenarios were designed to identify which acidification-sensitive species and groups the food web responds to most, information which can inform experiments and field investigations on the impacts of acidification.  Scenarios of ocean acidification on crustaceans affected the food web more than scenarios on echinoderms and molluscs, and results indicate that the food web is most sensitive to ocean acidification scenarios on copepods, an abundant group of zooplankton. This study demonstrates that the direct effects of ocean acidification can ripple through the food web to cause both negative and positive effects on other species groups, including those that are harvested and provide ecosystem services, and can restructure the food web, redirecting the flow of energy to species like polycheates and small gelatinous zooplankton.

Food web model of Puget Sound's central basin, with 65 functional groups (Harvey et al., 2010).  Box size is proportional to biomass, and the width of connecting lines is proportional to energy flow from prey to predator. Functional groups included in ocean acidification scenarios are circled. Single line = crustaceans, double line = molluscs, dashed line = echinoderms. 

This work highlights the complexity of interactions among and between species in an ecosystem and the difficulties inherent in understanding the impacts of ocean acidification on entire food webs. More accurate predictions of impacts will require better information on the relative sensitivity of local species to ocean acidification and how the changes in chemistry could alter species interactions. Important lessons emphasized by the study are that the effects of ocean acidification will be mediated by predator-prey interactions, the impacts of ocean acidification on a community cannot necessarily be predicted by a community’s or group of species’ average direct responses to ocean acidification, and, in some systems, the groups that seem most susceptible to ocean acidification (e.g., some molluscs) may influence the food web less than the groups that are less sensitive to acidification. Results from modeling exercises such as this can help inform resource managers about the types of changes in marine ecosystems that could occur under ocean acidification and can guide scientists developing monitoring activities and experimental work to explore the impacts of acidification.

This work was funded by NOAA's Northwest Fisheries Science Center and Ocean Acidification Program.