Persistence of oiling in mussel beds after the Exxon Valdez oil spill.

Persistence and weathering of Exxon Valdez oil in intertidal mussel (Mytilus trossulus) beds in Prince William Sound (PWS) and along the Gulf of Alaska was monitored from 1992 to 1995. Beds with significant contamination included most previously oiled areas in PWS, particularly within the Knight Island group and the Kenai Peninsula. In sediments, yearly mean concentrations of total petroleum hydrocarbons ranged from < 60 micrograms/g in reference beds to 62,258 micrograms/g wet wt., or approximately 0 to 523 micrograms/g dry wt. total polynuclear aromatic hydrocarbons (TPAHs). In mussels, mean TPAH concentrations ranged up to 8.1 micrograms/g dry wt. Hydrocarbon concentrations declined significantly with time in some, but not all mussels and sediments, and should reach background levels within three decades of the spill in most beds. In 1995, mean hydrocarbon concentration was greater than twice background concentration in sediments from 27 of 34 sites, and in mussels from 18 of 31 sites.

Lack of physiological responses to hydrocarbon accumulation by Mytilus trossulus after 3-4 years chronic exposure to spilled Exxon Valdez crude oil in Prince William Sound.

Mussels, Mytilus trossulus, were sampled in 1992 and 1993 from beaches in Prince William Sound that had been oiled by the Exxon Valdez spill of March, 1989. At some of the oiled beaches, mussels were collected from beds overlying oiled sediments, and from bedrock adjacent to these beds. Mussels were also collected from beaches within the Sound that had not been impacted by the spill. Polynuclear aromatic hydrocarbon (PAH) concentrations in mussel tissue, physiological responses (byssal thread production, condition index, clearance rate, and glycogen content), were determined for each group of mussels. Total PAH concentrations in mussel tissue ranged from 0 to 6 micrograms g-1, and were significantly greater in mussels from oiled beds than those from reference beds. No significant differences were noted in byssal thread production, condition index, clearance rate, or glycogen content between oiled sample sites and reference sites. The lack of physiological response was surprising because mussels in this study were chronically exposed to PAH for 3-4 years, and none of the physiological responses measured appeared to be affected by that exposure. The lack of a physiological response suggests that chronically exposed mussels may develop a physiological tolerance to PAH, but we recognize that these measures may not have been sensitive enough to discriminate response from background noise.