Ostrea lurida is a foundation species found along much of San Francisco Bay’s rocky intertidal shoreline but has lost a significant proportion of its historic population. As a primary target for restoration, it is important to understand which sites will be most successful in the long-term when considering extreme climatic events. This field study measured O. lurida population density and recruitment, along with coinciding weather and water quality data from approximately 2014 to 2021. The aim of this research is to determine how extreme precipitation and temperature have affected O. lurida and identify potential refuges for O. lurida in San Francisco Bay as extreme events increase with climate change.
For sessile invertebrates such as oysters, the location where they settle is critical to prolonged survival. Benthic organisms typically experience less environmental variability toward the mouth of an estuary due to increased marine influence which modulates temperature and salinity fluctuations. In Central California, some downstream sites are more frequently protected by marine fog, reducing heat stress by scattering solar radiation. Thus, variation in conditions throughout the estuary can affect consistency of recruitment and long-term population persistence for O. lurida and other estuarine organisms.
While upstream sites tend to support high densities of oysters during drought years, upstream populations are also the most volatile in terms of adult density and juvenile recruitment (Figure 1). Our data indicate more stable but lower density and recruitment at downstream sites closer to the ocean. We found the greatest number of extreme heat events (in-situ temperatures >30C) at China Camp State Park, the most upstream site, which at times contains the greatest oyster density in the species’ range. O. lurida has also experienced 97% to 100% mass mortality at this same location in response to low salinity during flood years. As climate change increases the frequency and severity of flooding and heat waves, it may become increasingly difficult for O. lurida populations to recover before being wiped out by another extreme event. Understanding the spatial dynamics of extreme events can help anticipate population-level impacts of climate change and inform future restoration to promote long-term resilience.