Recovery Monitoring and Restoration of Intertidal Oiled Mussel Beds in Prince William Sound, Alaska

Study History:
After the 1989 Exxon Valdez oil spill, many researchers began measuring hydrocarbon concentrations in mussels, sediment, and other matrices in Prince William Sound to assess resource damage. The specific focus of this project was dense aggregations of intertidal mussels (Mytilus trossulus) within the slick trajectory. Oil accumulated in these filter-feeders; total polynuclear aromatic hydrocarbon concentrations were high in 1989 and remained relatively high for several years in mussel beds on unconsolidated sediment. The persistence of oil in these beds in Prince William Sound and along the Gulf of Alaska caused concern, and beginning in 1992 heavily oiled beds were systematically surveyed for oil content in mussel tissue and underlying sediment. Projections from earlier data (through 1995) were that oil would persist for up to three decades, thus funding for study was extended through 1999. This report synthesizes all available natural resource damage assessment data to develop a coherent picture of spatial and temporal oil distributions in mussels and sediment.

Abstract:
Exxon Valdez oil trapped in intertidal sediment in Prince William Sound degraded slowly, was biologically available for at least a decade, and was toxic for at least 9 years. Habitat condition controlled the biological availability of oil. Exposure duration was short where mussels were only exposed to oil in water (months) and long (6 to 10 years) at locations where oil remained in sediment. Limited evidence suggests some oiling extended outside previously reported slick boundaries; polynuclear aromatic hydrocarbon (PAH) sources were verified with three independent petrogenic composition models. After an initial peak, oil concentrations typically declined in sediment and mussels, although distribution was non-uniform and variability was often high. Oil may persist in some intertidal sediment for >50 years, but declines in mussel tissue suggest it became less available to surface organisms and this community is recovering. Attempts to manually accelerate hydrocarbon loss from mussel beds were equivocal, demonstrating that removal of oil from intertidal sediment is difficult. Exposure to residual oil may explain why some vertebrate populations (pigeon guillemots, Cepphus columba, and sea otters, Enhydra lutris) that forage in the most heavily impacted areas have not yet fully recovered from the spill.