On bright hot days, standing in the shade can feel a lot better than standing in the sun. The same goes for oysters living in the inter-tidal shallows of San Francisco Bay. When the tide is low, the oysters bake in the sun. During extreme heat events they can even die. But in this coastal region there is one factor that could help mediate the heat: fog. Indeed, over the past year, San Francisco State graduate student Alexandra (Allie) Margulies has been examining fog data and monitoring oyster density at five sites around the Bay, and recruitment at 10 sites, building on a long-term monitoring dataset collected by the San Francisco Bay National Estuarine Research Reserve.
“Fog can help scatter solar radiation,” Margulies says. “So the prevalence of fog at some sites versus others may be something we should consider in siting new oyster reefs.”
For the last decade, SF State’s Estuary and Ocean Science Center, the Coastal Conservancy and other partners have been steadily experimenting with how best to build new oyster reefs in San Francisco Bay. The idea is that oysters are ecosystem engineers, attracting other marine species, and improving habitats and food webs. Early experiments off the Marin shore have now multiplied to include a large, multi-habitat project off the Contra Costa County shore, including not just oyster reefs but also eelgrass beds, tidal marshes, and upland transition zones. This fall, scientists went out to monitor progress at Giant Marsh, including Margulies who volunteered to help with the hard muddy work in the oozes.
“Going out there it was interesting to see how many oysters were living on the north side versus the south side of the oyster castles,” she says, referring to one of several reef element designs being tested. “Sunlight from the south is often stronger.” Shading from canopies of sea lettuce growing on the new reefs may also help with heat, scientists think.
Margulies’ early findings, comparing East Bay, South Bay, and Marin sites, suggest that conditions may be slightly less foggy the farther sites are from the Golden Gate. But she didn’t find much difference between Marin sites such as in Sausalito versus San Rafael. Nor has she yet found a higher density of oysters at sites with higher average fog cover that is statistically significant.
“One interesting thing I found is that even though the amount of fog at China Camp in San Rafael isn’t very different, this site had by far the most timepoints that recorded extreme heat,” she says. These temperature sensors, referred to as “data loggers,” are kept around the elevation where oysters are the densest.
Sites in the northeast Bay near Giant Marsh can also be subject to other extreme conditions — such as the flush of freshwater that poured over the saltwater-adapted oyster species during the atmospheric-river storms of 2017. A lesser 2011 storm caused 97-100% mortality of oysters at some sites. Margulies also found a big die-off in 2016 after a multi-year drought, but she isn’t sure of the cause. Oysters do come back, but the ups and downs can be problematic for long-term health.
Fog is certainly a factor for future consideration, as are dramatic salinity and air-temperature fluctuations. The recent heat dome in the Pacific Northwest killed thousands of mussels, for example. United States Geological Survey fog specialist Alicia Torregrosa told Margulies that fog can reduce temperatures on the ground by half a degree per hour of fog cover.
“If certain sites around the Bay provide refuge from extreme weather or conditions, then we can target restoration in those areas to create more stable populations,” says Margulies.
Top photo: Margulies collecting living samples from the reef balls. Photo: Jak Wonderly
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