'Exxon Valdez' oil spill: impacts and recovery in the soft-bottom benthic community in and adjacent to eelgrass beds

We assessed impacts of the 'Exxon Valdez' oil spill on benthic communities within and adjacent to eelgrass beds in Prince William Sound, Alaska, USA. The concentration of total polynuclear aromatic hydrocarbons (TPAHs),'benth ic community composition, diversity, biomass, and abundance were compared b etween matched pairs of oiled and reference sites in 1990 (approx. 16 mo af ter the spill), and in 1991, 1993, and 1995. TPAHs in sediments were high ( up to 15 300 ng g(-1)) at sites adjacent to oiled shorelines in 1990, but d eclined sharply thereafter. Some reference sites in 1990-91 also had elevat ed TPAHs in sediments and signatures matching Exxon Valdez oil, but concent rations were significantly lower than at oiled sites. Based on classificati on and ordination analyses, communities of infauna and small epifauna at so me oiled sites in 1990 differed from communities at reference sites, and fr om the same sites in subsequent years. Percent sand and mud and concentrati on of total chrysenes (PAH analytes indicative of crude oil) explained sign ificant proportions of the temporal and spatial variation in benthic commun ity structure. Total abundance and biomass of epifauna were generally highe r at oiled sites, primarily because of higher densities of epifaunal bivalv es. Otherwise, there were few consistent community-wide responses to oiling in diversity, richness, total abundance, total biomass, or the abundances of major taxonomic groups (e.g. polychaetes or bivalves). We attribute the lack of a stronger community-wide response to the varying sensitivities of constituent taxa to oil and organic enrichment. Over half of the dominant f amilies differed with respect to abundance at oiled versus reference sites. Most, including 9 families of polychaetes, were more abundant at oiled sit es. Most of these were stress-tolerant or opportunistic, and their increase was likely due to organic enrichment. Negative impacts were most evident i n oil-sensitive amphipods, especially the families Isaeidae and Phoxocephal idae. There were consistently more of these amphipods at reference sites, a nd abundances at oiled sites were likely reduced as a result of oil toxicit y. Most of these differences between oiled and reference sites persisted th rough 1995, 6 yr after the spill. We suspect that these differences are a r esult of the spill, but we rely on post-spill comparisons to infer impacts, and our conclusions rely on the untestable assumption of equality between oiled and reference sites in the absence of a spill. Future assessments of the impacts of oil spills or other accidental environmental disturbances co uld benefit from pre-impact studies that provide objective criteria for sel ection of matched pairs of sites, thereby supporting the assumption of equa lity in the absence of the disturbance.