Greater Selenium Threat Accompanies Invasion Of Exotic Bivalve Species In San Francisco Bay, Says Study

Biological invasions of estuaries have become an increasing problem worldwide and are known to change community structure and function. For example, beginning in the mid-1980s, an invading species of bivalve Potomocorbula amurensis became the predominant benthic macroinvertebrate in San Francisco Bay, one consequence of which has been the essential elimination by this voracious feeder of the standing stock of phytoplankton from the water column of Suisun Bay, in northern San Francisco Bay. A paper in Aquatic Toxicology shows another apparent result of the invasion: an increase in selenium concentrations in the biota of the bay.

Selenium is an environmental toxicant that has been responsible for adverse reproductive effects and local extinctions of fish and birds in wetlands receiving agricultural drainage and river ecosystems draining selenium-rich agricultural lands. Although it is nutritionally essential, the window between essential concentrations in food and concentrations that can cause adverse effects is narrow.

Bivalves are frequently particularly prone to bioaccumulate selenium in their tissues. It is possible that P. amurensis may have a greater bioaccumulation ability than other bivalves, because it is an unusually voracious filter feeder and utilizes a variety of food sources; however, say the study's authors, direct comparison with other bivalve species in the Bay was difficult because P. amurensis has essentially displaced them all. However, they note that, whether or not it bioaccumulates selenium to a great extent than other bivalve species, the fact that P. amurensis has decimated the food web of the water column and thus shifted the community structure toward benthic organisms can in itself "enhance adverse effects of selenium in the system, by expanding the availability of a contaminated food supply."

The highest concentrations of selenium in P. amurensis, say the study's authors, "exceed values that other studies have shown reduce growth or cause reproductive damage when ingested in experiments by birds and fish." Indeed, they exceed values at which "a high frequency of adverse effects is found" by twofold. Earlier studies showed selenium concentrations in the livers of surf scoters (a species of diving duck) seven to fourteen times higher than those from ducks at a reference site. Similar high levels were found in other benthivores, such as sturgeon and Dungeness crab; predators that fed from the water column, such as striped bass, appeared to have lower concentrations than the benthivores.

The authors conclude "that the invasion of the non-native bivalve P. amurensis has resulted in increased bioavailability of a potent environmental toxin to certain benthivores in San Francisco Bay. Changes in contaminant cycling and potential effects are yet another reason to be concerned by the threat of invasive species in our estuarine ecosystems."

Source: Linville, R.G., et al. 2002. Increased selenium threat as a result of invasion of the exotic bivalve Potomocorbula amurensis into the San Francisco Bay-Delta. Aquatic Toxicology 57: 51-64. Contact: Samuel N. Luoma, U.S. Geological Survey, 345 Middlefield Rd., Mail Stop 465, Menlo Park, CA 94025. E-mail: snluoma@usgs.gov

 Accompanies Invasion Of Exotic Bivalve Species In San Francisco Bay, Says Study

Biological invasions of estuaries have become an increasing problem worldwide and are known to change community structure and function. For example, beginning in the mid-1980s, an invading species of bivalve Potomocorbula amurensis became the predominant benthic macroinvertebrate in San Francisco Bay, one consequence of which has been the essential elimination by this voracious feeder of the standing stock of phytoplankton from the water column of Suisun Bay, in northern San Francisco Bay. A paper in Aquatic Toxicology shows another apparent result of the invasion: an increase in selenium concentrations in the biota of the bay.

Selenium is an environmental toxicant that has been responsible for adverse reproductive effects and local extinctions of fish and birds in wetlands receiving agricultural drainage and river ecosystems draining selenium-rich agricultural lands. Although it is nutritionally essential, the window between essential concentrations in food and concentrations that can cause adverse effects is narrow.

Bivalves are frequently particularly prone to bioaccumulate selenium in their tissues. It is possible that P. amurensis may have a greater bioaccumulation ability than other bivalves, because it is an unusually voracious filter feeder and utilizes a variety of food sources; however, say the study's authors, direct comparison with other bivalve species in the Bay was difficult because P. amurensis has essentially displaced them all. However, they note that, whether or not it bioaccumulates selenium to a great extent than other bivalve species, the fact that P. amurensis has decimated the food web of the water column and thus shifted the community structure toward benthic organisms can in itself "enhance adverse effects of selenium in the system, by expanding the availability of a contaminated food supply."

The highest concentrations of selenium in P. amurensis, say the study's authors, "exceed values that other studies have shown reduce growth or cause reproductive damage when ingested in experiments by birds and fish." Indeed, they exceed values at which "a high frequency of adverse effects is found" by twofold. Earlier studies showed selenium concentrations in the livers of surf scoters (a species of diving duck) seven to fourteen times higher than those from ducks at a reference site. Similar high levels were found in other benthivores, such as sturgeon and Dungeness crab; predators that fed from the water column, such as striped bass, appeared to have lower concentrations than the benthivores.

The authors conclude "that the invasion of the non-native bivalve P. amurensis has resulted in increased bioavailability of a potent environmental toxin to certain benthivores in San Francisco Bay. Changes in contaminant cycling and potential effects are yet another reason to be concerned by the threat of invasive species in our estuarine ecosystems."

Source: Linville, R.G., et al. 2002. Increased selenium threat as a result of invasion of the exotic bivalve Potomocorbula amurensis into the San Francisco Bay-Delta. Aquatic Toxicology 57: 51-64. Contact: Samuel N. Luoma, U.S. Geological Survey, 345 Middlefield Rd., Mail Stop 465, Menlo Park, CA 94025. E-mail: snluoma@usgs.gov