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The Tiny Clams That Ate the Bay Delta

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Asian clams, Corbicula fluminea  | Photo: Center for Lakes and Reservoirs/Flickr/Creative Commons License

An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with for all the project's stories.

Spend enough time reading about the plight of fish and other aquatic animals in the Bay Delta and you'll come across a whole lot of mentions of an ongoing collapse of the estuary's ecosystem. That collapse has been in progress for a long time, but biologists started getting really worried in the 1980s. That collapse rivals diversions of water for our cities and farms as the major threat to the Delta's suffering wildlife.

The problem is pretty basic. Aquatic ecosystems are like almost any other: they wouldn't exist without the plants in them turning sunlight into biomass. In the Bay Delta, most of those plants are what's called "phytoplankton": the tiny floating algae and other photosynthesizers that form the base of the food chain. Phytoplankton feed small animals such as zooplankton, which feed larger animals, which feed even larger animals. The amount of phytoplankton in an ecosystem is thus a rough measure of an ecosystem's biological productivity. And for centuries, the Bay Delta was one of the most productive ecosystems in the world.

Until recently. In the last few decades, the amount of phytoplankton in the Bay Delta has dropped catastrophically. That collapse in productivity has echoed out along each link in the food web: animals from zooplankton to gigantic fish are starving as a result. And the likely culprits are tiny, have hard shells, and live in unimaginable numbers in the Bay Delta.

The Amur River clam (Corbula amurensis) and Asian clam (Corbicula fluminea) have essentially split the Bay Delta between them, and together they have cut off the Bay Delta's aquatic ecosystem at its base.

The Amur River clam originated in China and the Siberian region in Russia, though the clams in the Bay Delta may be descended from a lineage that first invaded estuaries in Japan. Corbula amurensis probably arrived in the Bay Delta when its free-floating larvae hitched a ride in ballast water in cargo ships, an important vector by which invasive marine organisms are moved around the world. It was first noticed in the Bay Delta in 1986, when a dredge in San Francisco Bay found three individuals.

In the three decades since, the Amur River clam has become near-ubiquitous throughout the saltier portions of the Bay and Delta, forming solid-packed colonies on many continually submerged surfaces, including mudflats. Those colonies can easily hold more than 2,000 half-inch clams per square meter of surface.

The Asian clam has been around probably four decades longer, and it's three or four times the size of the Amur River clam. It has spread similarly, forming massive colonies. Unlike the Amur River clam, the Asian clam prefers fresher water. And so the two species have neatly divided the Bay Delta estuary between them, with the Amur River clam downstream of the saltwater-freshwater mixing zone, and the Asian clam upstream.

That rough boundary between the two species' invaded turf shifts as that mixing zone moves upstream and down depending on how much fresh water flows out of the Sacramento and San Joaquin rivers and out to sea. The two species' ranges generally overlap a little right where the two rivers finally flow into Suisun Bay around the city of Antioch, but a few months of drought will encourage Corbula to move eastward along with the intruding salt water into the Delta proper.

Conversely, higher than average freshwater flows through the Delta -- remember those? We used to have those sometimes -- they will encourage Corbicula to colonize a little farther downstream and into the Bay.

Regardless of whether the clams in the neighborhood are Corbula in salty or brackish water, or Corbicula in fresh, their main effects on the local environment are roughly the same. Both clam species are take up any contaminants in the water and incorporate it into their tissue, whether they're pesticides or oil refinery effluent or improperly disposed motor oil. Any organism that eats the clams ingests every substance the clams have accumulated.

That's especially significant in the case of selenium, a mineral that occurs naturally in soils of the San Joaquin Valley, and gets washed into the Bay Delta with irrigation runoff -- though it's also a component of pollution from the Bay Delta's oil refineries.

Selenium made national headlines back in the 1980s when irrigation water that ran off in the Kesterson National Wildlife Refuge ended up causing reproductive toxicity to waterfowl. Pictures of deformed baby ducks graced the evening news, and we suddenly learned a whole lot about the effects of too much selenium on living things. Though Kesterson National Wildlife Refuge was closed shortly after and work has been done to reduce selenium concentrations in agricultural drain water, all the selenium in the San Joaquin river basin does eventually end up in the Bay Delta, and Corbula and Corbicula take it in.

When the handful of fish that have learned to eat the invasive clams -- including the green and white sturgeons and the Sacramento splittail -- eat the clams, they get a full dose of each clam's selenium uptake. Selenium has been shown to interfere with fish reproduction, meaning that fish like the sturgeons and splittail become less fertile. And they're already in enough trouble: the Bay Delta's green sturgeon is listed as a Threatened species under the federal Endangered Species Act, and the white sturgeon and Sacramento splittail ought to be.

But if there's nothing to eat, it doesn't matter whether you can reproduce.

Clams and many other mollusks eat by sucking in water, then expelling it through their gills, which filter out edible plankton suspended in the water. One clam by itself can make a surprisingly large impact on the amount of plankton in the water. Finding an Amur River clam or an Asian clam by itself in the Bay Delta would be unusual. With thousands of clams in some square meters of the Bay and Delta, the Amur River clam and the Asian clam become very efficient black holes for both phytoplankton and the zooplankton that eats it. The arrival of these two species of clams in the Bay and Delta, and their astonishingly fast domination of the submerged environment, meant a drastic and persistent decline in the amount of plankton in the water.

And that means less food for larger animals, including the Threatened Delta smelt, juvenile Chinook salmon, and steelhead that the state has spent countless millions of dollars and thousands of person-hours in unpleasant meetings attempting to restore.

The decades-long general decline in the Bay Delta's ecological productivity stems directly from a lack of phytoplankton, and that decline in phytoplankton can mainly be laid at the clams' feet.

An aquatic ecosystem like the Delta without its phytoplankton is like a grassland with no grass, or a forest with no trees. Though there are other contributors to the overall system decline in the Delta, including herbicides in agricultural runoff that have their own impacts on phytoplankton, the Amur River and Asian clams -- just in trying to survive in a new place -- have essentially clearcut the Delta's aquatic ecosystem. The destruction ripples outward along the food web -- which has a whole lot less food than it used to.

There's probably no way to change the fact that the Delta is now home to possibly billions of these invasive clams. Some researchers suggest that restoring some of the Delta's historic floodplains might help, by providing a "nursery" for phytoplankton during the spring floods that the clams wouldn't be able to colonize, seeing as such floodplains generally dry out completely for at least part of the year.

And as those flooded lands can also provide a nursery for some of the same native fish now struggling to make their way in a Delta with less phytoplankton, restoring floodplains might prove an elegant solution to a few different problems in the Delta.

Until that happens, attempts to restore fish in a Delta without phytoplankton might be about as likely as restoring bison herds on a grassless plain. Without a sustainable food source, the restored wildlife faces little hope of survival.

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