PS 83-228
Quantifying tidal elevation effects on oyster growth and survival to assess climate change effects on restoration success in San Francisco Bay, CA
Both biotic and abiotic factors interactively determine the abundance and distribution of organisms. Understanding the relative importance of these factors is key to designing successful restoration strategies and increasing the resilience of organisms to projected changes in climate variables. Olympia oysters (Ostrea lurida) are an important foundation species in western estuaries, creating habitat for many native species, but since the mid-1800s populations have declined precipitously. Restoration efforts are underway from Washington to California, but little is known about factors limiting this species and how climate change may influence current limits. As part of a larger “Living Shoreline” project, where shoreline protection is an additional goal of oyster restoration, we investigated temperature effects on Olympia oyster performance at two San Francisco Bay sites, San Rafael and Hayward. In August 2012, using large experimental arrays established for restoration, we placed five substrate types, spanning three tidal elevations with bi-directional surface orientations. Substrate types selectively provided differing amounts of surface complexity, interior / exterior space, and vertical / horizontal surfaces. At 48 km apart, restoration sites differ in shoreline aspect, bathymetry, and species composition. A non-native oyster predator, Urosalpinx cinerea, is present only in Hayward. Data were collected November 2012 through April 2014.
Results/Conclusions
Oyster recruitment was greater at the San Rafael site and on substrate types with more interior and vertical surfaces. Oyster cover was greatest at lower tidal elevations and on north-facing, vertical surfaces, across all substrate types. We recorded higher surface temperatures and lower oyster survival at higher tidal elevations and south-facing surfaces, suggesting thermal stress as a likely factor driving distribution patterns. In contrast, Hayward had lower initial recruitment and a reverse distribution pattern, higher tidal elevations having higher oyster survival. A possible driver of this pattern is the presence of predatory whelks (Urosalpinx cinerea), which we observed exerting greater predation pressure at lower elevations. We suggest the optimal tidal elevation for oysters, at the Hayward site, occurs at higher elevations where heat stress limits predation by drills but is still tolerable to oysters. Increased air and water temperatures, a projected result of future climate change, could have complicated influences on the vertical distribution of Olympia oysters. Increased temperatures may lower oyster tidal distributions. However, predicting effects is more complicated with predator presence and depends on future changes in whelk tidal distribution. We conclude that predicting climate change on restoration success will require understanding both trophic interactions and physiological responses.