Chronic impacts of oil pollution in the sea: risks to vertebrate predators

Abstract

Following the 1989 ‘Exxon Valdez’ oil spill in Alaska, several studies of the biological consequences were conducted (e.g. Wells et al. 1995, Rice et al. 1996), targeting especially the vertebrates, whose high diversity, abundance, and importance helps distinguish the coastal ecosystem of the northern Gulf of Alaska. Virtually unique among field programs studying impacts of oil spills has been the availability of support for years after the ‘Exxon Valdez’ spill for assessment of some of the potential processes of long-term impacts of oil in the marine environment. This Theme Section synthesizes results of the Nearshore Vertebrate Predator project, which was initiated in 1995 to test several hypotheses to explain delays in recovery of some sentinel vertebrates in the coastal ecosystem. The project involved 4 species: 2 marine mammals and 2 marine birds, each of which had demonstrated acute impacts of the spill at the population level and also showed some evidence of delayed recovery (Rice et al. 1996). One bird (harlequin duck: Histrionicus histrionicus) and one mammal (sea otter: Enhydra lutris) feed largely on benthic invertebrates, plus another bird (pigeon guillemot: Cepphus columba) and mammal (river otter: Lontra canadensis) consume a diet dominated by fish. Parallel studies of each species were conducted to examine: (1) the status of population recovery 6 to 9 yr after the oil spill; (2) whether food availability constrained the recovery; (3) whether ongoing exposure to oil toxicity delayed recovery; and (4) whether demographic limitations intrinsic to the species and unrelated to food availability or chronic oil toxicity limited the rate of recovery. The role of acute mortality to marine organisms from exposure to petroleum hydrocarbons can be inferred from well-designed observational sampling over short time intervals. Consequently, a substantial body of information on acute risk has been gathered in nature following oil spills and in the laboratory using protocols of acute toxicity testing (e.g. National Research Council [NRC] 1985). The role of longer-term mortality from oil exposure has proved a much more elusive research target for marine ecologists despite a longstanding recognition of its potential significance (Gray 1982, National Research Council [NRC] 1985, Boesch & Rabalais 1987). Such research is more costly to conduct because it involves time frames of years instead of days and requires simultaneous evaluation of multiple mechanisms of potential impact to biological systems. Unlike acute mortality, which mostly occurs as an immediate narcosis reaction to toxicity (Rice et al. 2001) or some other direct process like physical smothering of rocky-shore invertebrates, long-term consequences of exposure of marine ecosystems to petroleum hydrocarbons can occur via several direct and