A 1990/1991 shoreline ecology program to assess the fate and effects of the Exxon Valdez oil spill in Prince William Sound was updated in 1998 and 1999. This update included a sediment sampling program for organisms at “worst case” sites and at randomly chosen reference sites. Correspondence analysis (CA), a statistical method that examines animal communities in terms of their similarity, was used to define community structure Statistical analysis of the degree of similarity between communities was used to assess effects of site-specific variables (sediment grain size, total organic carbon (TOC) and wave energy), interannual variation, and degree of oiling. Interannual variability had a significant effect on community structure, whereas site specific variables and degree of oiling did not. Differences in communities between 1998 and other years were particularly dramatic. The importance of interannual change demonstrates the importance of multi-year sampling and of appropriate study designs for separating impact effects from the natural occurring environmental factors which affect biological communities.

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