Influence of seagrass on juvenile Pacific oyster growth in two US west coast estuaries with different environmental gradients

Authors: Dumbauld, Brett; McIntyre, Brooke

Submitted to: Aquaculture Environment Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2023
Publication Date: 9/14/2023
Citation: Dumbauld, B.R., Mc Intyre, B.A. 2023. Influence of seagrass on juvenile Pacific oyster growth in two US west coast estuaries with different environmental gradients. Aquaculture Environment Interactions. 15:287-306. https://doi.org/10.3354/aei00466.
DOI: https://doi.org/10.3354/aei00466

Interpretive Summary: Rapid change in the concentration of carbon dioxide in the atmosphere is causing ocean acidification that threatens many marine organisms, but especially those that build shells like oysters. The shellfish industry already contends with the negative effects of ocean acidification on oyster larvae due to strong seasonal upwelling that brings acidic water into estuaries along the US west coast. Hatcheries can control conditions to mitigate these negative effects on larvae, but growers must then outplant juvenile oysters into estuaries where conditions are variable and acidic water is still present. Seagrass has been suggested as a potential refuge habitat for oysters within these estuaries because it could alter pH of surrounding water and reduce stressful conditions. However, the net effect of seagrass habitat on juvenile oyster growth has been less studied and difficult to quantify, given the other conditions, such as food supply, that vary in these estuaries and also affect oyster growth. In this study, juvenile Pacific oysters (Crassostrea gigas) were outplanted in seagrass and adjacent unvegetated habitat in two commonly used culture positions (on- and off-bottom) at intertidal sites in Willapa Bay, WA, and Netarts Bay, OR. Oyster growth and water conditions were monitored monthly over a summer season. Results show that seagrass habitat did not create more favorable growing conditions for juvenile oysters at any outplant site, as it did not alter carbonate chemistry conditions (pH) and actually reduced the amount of available food (chlorophyll-a concentration). Despite observing little to no effect of seagrass habitat on oyster growth, oysters grew larger when planted at sites closer to the estuary mouth and on-bottom in both estuaries. Only carbonate chemistry (here pH) and not the amount of food (relative chlorophyll-a concentration) correlated with oyster growth across all sites in both estuaries. Overall, results from this study suggest that planting juvenile oysters in seagrass habitat in these estuaries will not necessarily improve their growth.

Technical Abstract: A rapid increase in the concentration of CO2 in the atmosphere is intensifying ocean acidification that threatens many marine organisms including oysters. Shellfish hatcheries mitigate for negative effects of ocean acidification on oyster larvae due to strong seasonal upwelling that brings deep, nutrient-rich acidic water into estuaries along the US west coast, but juvenile oysters are then out-planted into these estuaries. Seagrass habitat has been suggested as a potential refuge for juvenile oysters because it has the potential to ameliorate stressful carbonate chemistry conditions and augment food availability, but the net effect of seagrass on oyster performance has been difficult to quantify in situ given other environmental conditions that vary spatiotemporally in these estuaries. In this study, juvenile Pacific oysters (Crassostrea gigas) were out-planted in seagrass and adjacent unvegetated habitat in two intertidal culture positions (on- and off-bottom) at sites in two US west coast estuaries. Oyster growth and water conditions were monitored monthly over a summer season. Results showed that seagrass habitat did not alter local carbonate chemistry conditions (pH), consistently reduced available food (relative chlorophyll-a concentration) at all sites, and only affected juvenile oyster growth at one site. Oysters grew larger when planted at sites closer to the estuary mouth and on-bottom. Integrated water conditions related to hydrogen ion concentration (a proxy for carbonate chemistry), and not relative chlorophyll-a concentration (a proxy for food availability), were correlated with oyster growth across all sites, suggesting that carbonate chemistry and/or some metric other than relative food availability affects juvenile oyster growth along unique gradients in these estuaries.