Tsesis spill (Elmgren et al., 1980), the Florida barge spill (Sanders et al., 1980), the Amoco Cadiz spill (Dauvin and Gentil, 1990), the Exxon Valdez spill (Jewett et al., 1999), and the 1996 North Cape oil spill where 8 to 9 million American lobsters were killed subtidally from a fuel oil spill (McCay, 2001). In addition, the rhepoxiniid amphipods, which appear to be particularly susceptible, are one of the few severely depressed faunal components in the benthic communities in areas of moderate petroleum seepage in the Santa Barbara Channel (Davis and Spies, 1980).
Not all spills demonstrated adverse effects in subtidal habitats. A study of the possible effects of tar residues from the Haven oil spill in Italy revealed no discernable differences between tar-affected and non-affected benthic communities (Guidetti et al., 2000). Exxon Valdez oil was generally not discernable below 40 meters in most portions of Prince William Sound and was never found in measurable quantities below 100 m depth. It is not surprising then that a study of deep benthic communities found no differences between various areas that could be attributed to oil from the spill (Feder and Feder, 1998).
During the Braer spill off the Shetland Islands, 84,700 tonnes of a light Gullfaks crude oil were released from the grounded vessel during hurricane-force winds, and an estimated 35 percent of the oil was deposited on the seabed in water depths from 2-100 m in an area of 4,000 km2 (Kingston, 1999). The sedimented oil provided a long-term pathway for exposure to benthic fisheries. For example, burrowing Norway lobster (Nephrops) remained contaminated for over five years, whereas epibenthic lobsters (Homarus) eliminated petroleum contaminants to background levels of PAH in one month (Kingston, 1999).
As discussed in Chapter 3, petroleum enters the marine environment from a variety of sources, at different rates, and in diverse settings. Understanding how the environment responds to releases associated with specific sources is an important aspect of understanding the overall impact of widespread extraction, handling, and use of petroleum hydrocarbons.
Natural petroleum seeps occur in many parts of the ocean, and can be utilized to understand the effects of oil contamination (Spies et al., 1980). As petroleum enters the ocean from the seabed, it is relatively unweathered in comparison to many other sources of oil that reaches the bottom (Reed and Kaplan, 1977; Steurmer et al., 1982). There are some significant consequences to this difference that limit the usefulness of oil seeps as effects models for other sources of oil in which weathering occurs before the oil is deposited in bottom sediments. Also, the possibility must be kept in mind that, with a history of thousands of years, animals living near seeps might have unique adaptations. Biological studies of seeps have concentrated on the extensively contaminated benthos (Spies and Davis, 1979; Spies et al., 1990).
There are two aspects to the effects of fresh seeping petroleum on benthic ecosystems. First, fresh petroleum, being a highly reduced source of energy, is readily oxidized by microbes (Bauer et al., 1988), which, in turn, can serve as a supplementary food source for benthic food webs in shallow water (Spies and DesMarais, 1983; Bauer et al, 1990). In the case of seeps in deep water, it can be a nearly exclusive carbon source. Second, at sufficiently high concentrations, the aromatic components of seep petroleum are toxic to marine organisms (Davis et al., 1981). There is also an interaction between toxicity of oil and microbial metabolism of petroleum. The decrease in oxygen in the surface layers of the sediments that results from microbial metabolism of petroleum is a limiting factor to benthic organisms. The oil, while supporting microbial growth that acts as a food source, may also be toxic to other organisms or indirectly decrease habitat quality through oxygen deficiency (Spies et al., 1989; Steichen et al., 1996). Microbial transformations of aromatic hydrocarbons may alter hydrocarbon composition and various oxidized products may be formed (Bartha and Atlas, 1987). Natural biogeochemical tracers indicate that both the petroleum carbon, particularly the lighter fractions, and sulfur from sulfide is incorporated into benthic meiofauna and macrofauna (Spies and DesMarais, 1983; Bauer et al., 1990). Circumstantial evidence for damage to gill tissues in bottom-feeding surf perches are linked to oil exposure through cytochrome P450 1A induction and aromatic petroleum metabolites in bile (Spies et al., 1996).
The most detailed investigations of petroleum seepages have been carried out in the Santa Barbara Channel off the coast of southern California. The following summarizes the findings of studies conducted at a depth of 20m in one of these oil seep areas, the Isla Vista seep. Starting with the fresh oil and gas in the sediments of a petroleum seep, several related phenomena occur. Bacterial populations, as measured by ATP content or by direct microscopic counts, are elevated several fold over surrounding sediments (Spies et al., 1980; Bauer et al., 1988). The sediments are highly reducing, oxygen is undetectable in sediments below a very thin surface layer, sulfate oxidizing activity is markedly elevated, hydrogen sulfide is abundant, and sulfide-oxidizing bacteria (Beggiatoa) are abundant at the surface of sediments, often forming prominent white mats.
The heavy seepage areas where the Beggiatoa mats form support a low-diversity benthic community consisting of large numbers of nematodes, a few polychaete worms (e.g., Capitella capitata), some oligochaete worms, and a limited number of harpacticoid copepod species (Spies et al., 1980; Montagna et al., 1987, 1989, 1995). Porewater concentrations of aromatic hydrocarbons within a few centimeters of an active seep were approximately 1 ppm. The nematodes