Salish Sea snapshots: Mussel memory

Among the most widely used organisms for monitoring toxics in marine life are mussels, which feed on microscopic plants and animals that they strain out of seawater. In the process of this filter feeding mussels also pick up all sorts of contaminants, so at any given time their body tissues record data about water quality over the previous two to four months.

“They’re basically tasty sampling devices,” says Jim Gawel, an aquatic chemist at the University of Washington in Tacoma.

PAHs, or hydrocarbons from fossil fuels, are the most abundant contaminants found in mussels in Puget Sound, reaching levels up to an order of magnitude greater than other toxins. PAHs are also widespread: in a 2013 study from the Mussel Watch program sponsored by the Washington Department of Fish and Wildlife, researchers found them in mussels from every single one of 108 monitoring sites.

In general, PAHs are higher in mussels from more urbanized areas. Those from the Elliott Bay waterfront and Eagle Harbor ferry terminal on Bainbridge Island have the highest levels of these contaminants.

The ratio of different PAHs found in the mussels suggests that at nearly 90 percent of monitoring sites, these molecules come partially or primarily from combustion – that is, burning of fossil fuels, especially in vehicle exhaust – rather than from oil spills or other unburned petroleum sources. Combustion PAHs are especially prominent at ferry terminals, suggesting that vehicle exhaust may be the major source of petroleum toxicants in developed waterfront areas.

In the past, scientists had to rely on wild mussels for their studies, but in these cases they were able to transplant mussels to the study sites with the help of citizen volunteers.

Native mussels (Mytilus trossulus) were spawned and reared in an aquaculture facility were transplanted and collected about two months later. There were some differences between wild and transplanted mussels, but scientists hope that transplanted mussels will be an increasingly valuable monitoring source in the future.