Ecological Significance of Residual Exposures and Effects from the Exxon Valdez Oil Spill

Abstract

An ecological significance framework is used to assess the ecological condition of Prince William Sound (PWS), Alaska, USA, in order to address the current management question: 17 y following the Exxon Valdez oil spill (EVOS), are there any remaining and continuing ecologically significant exposures or effects on the PWS ecosystem caused by EVOS? We examined the extensive scientific literature funded by the Exxon Valdez Trustees or by ExxonMobil to assess exposures and effects from EVOS. Criteria to assess ecological significance include whether a change in a valued ecosystem component (VEC) is sufficient to affect the structure, function, and/or health of the system and whether such a change exceeds natural variability. The EVOS occurred on 24 March 1989, releasing over 250,000 barrels of crude oil into PWS. Because PWS is highly dynamic, the residual oil was largely eliminated in the first few years, and now only widely dispersed, highly weathered, or isolated small pockets of residual contamination remain. Many other sources of polycyclic aromatic hydrocarbons (PAHs) exist in PWS from past or present human activities or natural oil seeps. Multiple-lines-of-evidence analyses indicate that residual PAHs from EVOS no longer represent an ecologically significant exposure risk to PWS. To assess the ecological significance of any residual effects from EVOS, we examined the literature on more than 20 VECs, including primary producers, filter feeders, fish and bird primary consumers, fish and bird top predators, a bird scavenger, mammalian primary consumers and top predators, biotic communities, ecosystem-level properties of trophodynamics and biogeochemical processes, and landscape-level properties of habitat mosaic and wilderness quality. None of these has any ecologically significant effects that are detectable at present, with the exception of 1 pod of orcas and possibly 1 subpopulation of sea otters; however, in both those cases, PWS-wide populations appear to have fully recovered. Many other stressors continue to affect PWS adversely, including climate and oceanographic variability, increased tourism and shipping, invasive species, the 1964 earthquake, and overexploitation of marine resources, with associated cascading effects on populations of PWS fish and predators. We conclude that the PWS ecosystem has now effectively recovered from EVOS.

Quantifying population‐level risks using an individual‐based model: Sea otters, Harlequin Ducks, and the Exxon Valdez oil spill

“…Using the Gentile and Harwell (1998) criteria for assessing ecological significance (see also Harwell et al 1994), Harwell and Gentile (2006) evaluated a suite of approximately 2 dozen VECs that characterize the PWS ecosystem, concluding that by that point in time recovery had essentially occurred for almost all of the VECs; similar conclusions were presented in Integral Consulting (2006), with a few exceptions. However, the Exxon Valdez Oil Spill Trustees (EVOSTC 2010) continued to report several endpoints as not recovered, including characterizing sea otters and Harlequin Ducks as “recovering” but not yet recovered species.…”

“…In Harwell and Gentile (2006), we applied these concepts to assessing the significance of any continuing effects on the Prince William Sound (PWS), Alaska, ecosystem more than 15 years after the Exxon Valdez oil spill (EVOS) (see also Harwell, Gentile, Cummins et al 2010). In the ecological risk context, the issue is 2-fold, 1 pertaining to exposure and the other to effects: 1) Have the stressors caused by the oil spill decreased to the point where no further risk exists to the PWS ecosystem and its biota, and 2) Have VECs in PWS that were initially impacted now recovered?…”

Selection of valued ecosystem components in cumulative effects assessment: lessons from Canadian road construction projects

“…Introduced by Beanlands and Duinker (1983) in Canada, VECs were recommended as a means to avoid the inefficient 'count everything' approach that characterized early IAs (Kennedy & Ross 1992;Treweek 1999). They were also meant to direct the focus of assessors to a more holistic view of ecological health and integrity (Harwell & Gentile 2006;Barnes et al 2010;Nunes 2010) rather than simply the effects of a particular development on 'just water' or 'just vegetation', for example (Beanlands & Duinker 1983). Similar processes exist outside the Canadian practice, e.g.…”

“…A scan of the literature reveals some attention to the values that influence VEC choices, characterization of stressors affecting VECs, and application of the VEC concept within different assessment frameworks (see, for example, Lohani et al 1997;Harwell & Gentile 2006). In a recent review of project-based IAs in southern Saskatchewan, Ball et al (2013) found that aquatic VECs were usually selected to address a proponent's exposure to liability and penalty under federal legislation (e.g.…”

Effects of Oil in the Environment

“…67 They applied a risk assessment paradigm to evaluate a range of organisms from invertebrates to seabirds and marine mammals, and concluded that those resources not in decline at the time of the spill recovered within six years, with others approaching recovery by 15 years post-spill. Notable exceptions were those ecosystem attributes related to orca pods.…”