Spatial planning for shellfish aquaculture and seagrasses in US West Coast estuaries : considerations for adapting to an uncertain climate

Shellfish aquaculture has been an important contributor to the local economy of several communities along the US West Coast for almost 100 years and in most of the estuaries where it occurs it has coexisted with the native seagrass (eelgrass, Zostera marina). Eelgrass provides numerous ecosystem services including nursery habitat for juvenile fish and invertebrates, but is declining in many locations worldwide, and is therefore now protected by no net loss provisions in US laws and regulations. We have studied the interaction between oyster aquaculture and eelgrass at both the local process scale and at the estuarine landscape scale in Willapa Bay, Washington. While there are important differences, most US West coast estuaries like Willapa Bay are small relative to the nearby coast, experience less riverine influence during the summer growing season and have shorter residence times than estuaries where oysters are cultured on eastern edges of continents, and the majority of culture takes place in intertidal areas. The ecology of eelgrass and cultured Pacific oysters as well as their role as habitat is thus directly influenced by bathymetry and proximity to the coastal ocean. We summarize studies on the interaction between eelgrass and oyster culture at local scales, present data that suggest culture practices are important to consider, but the cumulative interaction is not necessarily negative for eelgrass at the estuarine landscape scale in Willapa Bay. We also briefly summarize studies on the influence of sea level rise (SLR) and sea water chemistry (ocean acidification, OA), two projected changes in climate that are expected to occur over a broader spatial and longer temporal scale. SLR is projected to enhance the presence and interaction of eelgrass with oyster aquaculture in Willapa Bay, though this effect was mostly driven by bathymetry in our simplistic model and landward eelgrass expansion could be restricted elsewhere. OA has already influenced oyster culture especially in commercial hatcheries where it changes aragonite saturation state and the ability of larval oysters to deposit shell. Evidence for direct effects of carbonate chemistry in estuaries like Willapa Bay is more equivocal due to complex interactions, yet still related to the proximity of cold upwelled ocean water which likely influences natural spawning on this coast. The presence of eelgrass may also buffer water chemistry at least over short time scales, but this is less likely to directly affect survival of juvenile than larval oysters and instead eelgrass may impact juvenile oyster growth via this and other mechanisms. These physical effects of structure also influence conspecific eelgrass plants and appear to be more important than water chemistry feedbacks. Recent initiatives to expand shellfish aquaculture in US west coast estuaries have received increased regulatory scrutiny due in large part to this interaction with eelgrass. Our review suggests that a permit process that simply evaluates and prevents direct negative effects of oyster culture on eelgrass at small spatial and immediate temporal scales would greatly and perhaps unnecessarily restrict expansion of culture operations. A broader adaptive approach could be employed that considers bathymetric and along estuary gradients that affect both of these resources and the services they provide, especially given projected future climate.