Oyster aquaculture shapes intertidal communities: from foundation species to higher trophic levels

Abstract

The work presented in this dissertation sits at the intersection of two major global issues, rapidly increasing stressors on marine systems, and simultaneously, a need for sustainable food solutions. Thus, a wicked problem presents itself: how can we support ecosystem function in marine systems, while also leveraging these systems for food production? The first step to addressing this question is to better understand how aquaculture, in this case oyster aquaculture, shapes intertidal communities. Our work was conducted in Willapa Bay and Grays Harbor, Washington, a globally important region for production of the introduced Pacific oyster (Crassostrea gigas, now Magallana gigas), where we investigated the effects of oyster aquaculture across multiple trophic levels, considering different methods of culture as well as the impacts of associated disturbances. We evaluated the use of oyster culture as habitat to a wide variety of intertidal taxa, focusing first on waterbirds (Chapter 1) and then on nekton (Chapter 2), investigating how organisms respond to culture habitats versus seagrass and mudflat habitats (Chapters 1 & 2). Taxa responses to habitat type were species-specific for birds and nekton generally, with some taxa utilizing oyster culture while others preferred seagrass or mudflat habitats. Some nekton taxa were found to be structure-generalists, meaning they used seagrass and culture habitats similarly. There were very few instances of negative effects of oyster culture on birds or nekton habitat-use, and we concluded that maintaining a mosaic of habitat types best supports the diverse organisms in the estuary. While structures associated with oyster culture provide habitat to a variety of taxa, disturbance events from mechanical shellfish harvest can cause substantial damage to seagrass (Zostera marina, i.e. eelgrass) beds, another habitat-forming foundation species. We investigated how timing of large-scale dredge disturbance interacts with eelgrass phenology and determined that disturbance during the slow-growing overwintering phase or early spring provides the best outcome for eelgrass recovery, with a major contribution from seedlings if the seedbank is given the opportunity to regenerate. Estuaries serve as a resource oasis that link the land to the sea with many organisms spending part of, or the entirety of, their lives as inhabitants. While many estuaries worldwide have been heavily modified anthropogenically and restoration to a “pristine” pre-industrial condition may not be possible, research-based management decisions can conserve or restore function of the ecosystem to better serve native wildlife, commercial fisheries, and aquaculture. Overall, findings from this work can be applied to co-management of estuaries and shellfish aquaculture, with the goal of maintaining ecosystem functioning.