Can post-oil spill patterns of change be used to infer recovery?

Abstract

Assessing recovery from oil spills in areas not previously studied must be based on comparisons of changes in affected areas to changes in unaffected areas. The present study on impacts and recovery in intertidal biological communities affected by the Exxon Valdez oil spill in Prince William Sound, Alaska, began after the spill. The study was designed to contrast recovery processes of two types of oiled stations: stations that were treated with hot water delivered at high pressure to remove oil from the shorelines and untreated stations. Natural variation in species composition and abundance was concurrently monitored at unoiled stations. Correspondence analyses of data collected at rocky mid-intertidal stations from 1989 to 1997 show that unoiled algal assemblages remained relatively unchanged. Algal assemblages at treated and untreated oiled stations returned to patterns of species composition and abundance similar to those at unoiled stations. The convergence of patterns provides evidence of recovery. In contrast, changes in invertebrate abundances were characterized by high year-to-year variation in that exceeded within year differences among oiled and unoiled stations. Convergence of patterns was not clearly evident in the analysis, and any recovery in the invertebrates can only be based on the fact that variations observed at the oiled stations appeared as great as natural variation. These results indicate that recovery for some species can be assessed when post-spill monitoring occurs for a sufficient length of time, but is also dependent on characteristics of the study design.