Every summer, recreational oyster and shellfish harvesters brace themselves for the period from May through October when red tides prohibit recreational shellfish harvesting in coastal waters along the California coast.
“Red tides” (or “harmful algal blooms,” as scientists prefer to call them) occur when colonies of phytoplankton, a form of algae, begin to rapidly reproduce. The result: millions of cells per gallon of water. As the name implies, the bloom turns the water red because of a pigment present in each cell.
Phytoplankton is present in ocean waters throughout the year, but blooms occur only when ocean temperatures and salinity are favorable, usually May through October on the northern California coast.
But that may be changing.
The Pacific Ocean is a vast body of water whose relatively constant temperature moderates the climate and provides a stable habitat for marine life. But the coastal ocean in Marin and Sonoma counties is actually getting warmer.
The Bodega Ocean Observing Node at UC Davis’ Bodega Marine Laboratory in Sonoma County has been tracking ocean temperatures along our coast for decades. Ocean temperature at the lab in August 1988, as reported on its website, was 58.1 degrees. At the same location on Aug. 30 of this year, the temperature was 61.8 degrees.
A 3.7 degree gain is significant and troubling. In warmer waters, algal blooms will begin sooner and last longer, and with this longer growing season, they may begin to produce biochemicals not previously seen.
Dinoflagellates, a type of single-celled phytoplankton, produce a biotoxin called saxotoxin, which has been the primary concern during red-tide season. Saxotoxin accumulates in filter feeders such as mussels and oysters. Paralytic shellfish poisoning (PSP), which can be fatal to humans and marine life, is caused by the ingestion of saxotoxin.
According to the Alaska Division of Public Health, saxotoxin is 1,000 times more potent than cyanide. And PSP toxins are not destroyed by heating or freezing, so cooking contaminated shellfish does not make it safe to eat.
The California Department of Public Health (CDPH) ensures that, with thorough and regular testing, all commercially available oysters and other shellfish are safe for human consumption year-round. But recreationally harvested oysters and shellfish can be very high-risk.
Historically, PSP was the problem during red tide season. But in 1998, the Marine Mammal Center in Marin County diagnosed the first case of domoic acid poisoning in marine mammals. Domoic acid is a biotoxin produced by diatoms, another type of single-celled algae which blooms as seawater warms. Domoic acid causes amnesic shellfish poisoning as it accumulates in shellfish, sardines and anchovies, which may then be eaten by larger marine mammals.
On Aug. 26, the CDPH issued a warning to consumers in Humboldt and Del Norte counties to avoid eating bivalve shellfish because of dangerous levels of domoic acid. That warning was already in place in Santa Cruz, Monterey and Santa Barbara counties. According to John Largier, professor of coastal oceanography at the Bodega Marine Lab, the same warning may be issued for Marin and Sonoma counties in the next month.
“The Mendocino, Sonoma and Marin county coastal waters will become warmer in the next few weeks to a month as our cool ocean upwelling system weakens” says Largier, “and it is very likely that the diatoms will then begin to bloom and the same warning will be issued here as well.”
There may also be a third biotoxin, not previously present in significant concentrations on the north coast. “Okadaic acid, which causes diarrhetic shellfish poisoning, may become a problem as our coastal waters continue warming and we begin to see more temperature gradient stratification,” says Raphael Kudela, professor of ocean studies at UC Santa Cruz. Produced by several species of dinoflagellates, okadaic acid also accumulates in shellfish.
It’s possible red tides could last all year on the North Coast, and that could mean trouble for local shellfish lovers. Will there be a recreational shellfish harvest in Marin and Sonoma in 10 or 20 years?
The commercial oyster industry in Tomales Bay is already grappling with problems caused by ocean acidification. How will the industry cope with higher temperatures and the presence of multiple biotoxins in the coastal ocean environment?
As climatologist Nick Bond put it, “This may be a dress rehearsal for climate change.”