This Saturday is Coastal Cleanup Day, but for an Olympia oyster, every day is coastal cleanup day. Oysters, like other bivalves, are filter feeders. They suck in water over their gills hoovering up anything floating around. As an oyster species, Olympias are tiny but mighty. A full grown Olympia is about the size of a quarter and filters around three gallons of water a day, that is about 30 gallons of water filtration power you can fit in the palm of your hand.
One of the most common questions we receive is - if we successfully restore Olympia oyster populations to the bay, would it be safe to eat them? With our history of water quality that is a valid concern. A century of degradation and pollution have led us to look dubiously at anything harvested directly from our bays waters. But oysters are discerning eaters, looking specifically for phytoplankton aka algae to eat. Everything else sucked up but rejected as ‘not food’ is expelled by the oyster as ‘pseduofeces’. This is an important distinction, any inorganic particles and grit the oyster is not digesting are still removed from the water column and become part of the benthic environment, reducing the turbidity of the water. Turbidity is a measure of how cloudy the water is. High turbidity generally means low water quality. The clearer the water, the more sunlight can penetrate, allowing the growth of more beneficial aquatic plants, like sea grass to grow.
When oysters filter feed, they are also extracting nutrients like nitrogen and phosphorus from the water. This is very important in places like San Francisco Bay where high concentrations of available nitrogen and phosphorus in the water from urban, industrial, and agricultural runoff can cause harmful and potentially toxic algal blooms in a process known as eutrophication. Toxic algal blooms as well as bacteria from untreated sewage can bioaccumulate into oyster tissues which is what makes them potentially unsafe to eat. We can do our part by preventing as much untreated runoff from flowing into the bay as possible, allowing oysters to do their job, and the more oysters we have in the water the better.
There are many factors that go into what constitutes good water quality and it can mean different things to different species. Improved water quality is just one benefit of a restored oyster reef that cascades into other benefits and improves conditions for other species that live in the bay to thrive. So while you may not eat an Olympia oyster harvested today, the work we do now to leads to a swimmable edible bay in our future.