reefs, live-bottom habitats, mangrove swamps, salt marshes, oyster reefs, and seagrass and kelp beds. Here the concern is that even though oil may not persist following an oil spill, the time required for recovery of damaged populations of organisms that provide the physical structure of the habitat may be many years. In some biogenic habitats, such as mangroves and mussel beds, oil can sometimes penetrate into the lower-energy sediments associated with these habitats and have potentially long-lasting effects. Biological communities that are integrally dependent on physical structures, which are themselves formed by living organisms, may be inherently slow to recover from severe impacts. In some cases where the structure-forming species actually stabilize the habitat, it is conceivable that permanent modification of that habitat could result from an acute incident that kills the key structuring species. Recovery from the effects of an oil spill in a community in which organisms provide the physical structure of the habitat depends on structural damage incurred during cleanup operations, the persistence of contamination, and the inherent ability of the community to recover.
The 1985 Oil in the Sea report focused extensively on the effects of oil spills on tropical habitats including coral reef ecosystems and mangroves. At the time, there were multiple field studies documenting effects on corals including decreased feeding response, coral colonization and premature expulsion of coral planula. The end result was coral tissue death, coral bleaching, and the loss of an entire year’s larval recruitment class. One lament of the 1985 Oil in the Sea report was the lack of information on concentrations and composition of oil in the water that prevented comparison of spill effects between coral sites.
Since 1985, a wealth of field and laboratory studies have increased our knowledge of the effects of oil on coral reefs. The 1986 Galeta spill into Bahia las Minas, Panama is arguably the most studied oil spill in the tropics. Large amounts of medium weight crude oil (see Box 5-5) spilled into mangroves, seagrass beds, and coral reefs on the Caribbean coast of Panama (Burns and Knap, 1989; Jackson et al., 1989; Guzman and Holst, 1993; Guzman et al., 1994; Box 5-5). Another notable tropical oil spill was the consequence of the Persian Gulf War in 1991 where 1,770,000 tonnes of oil were spilled into the marine environment (Price and Robinson, 1993). Despite a 120-fold difference in total volume of oil spilled, the long-term effects (greater than five years) of oil in Panama were more pronounced and detrimental due likely to repeat inoculation of oil from the surrounding mangroves into the coral ecosystem. In contrast, no long-lasting effects to the coral reef ecosystem were reported from the Persian Gulf War spills (Price and Robinson, 1993).
Corals located in intertidal reef flats are exposed to oil slicks and are more susceptible to damage and death than corals in subtidal reefs. Coral located subtidally or in areas with high wave action are not directly exposed to the marine surface layer where oil slicks can coat them. Instead, only the water-soluble fraction of oil generally affects submerged coral. The water-soluble fraction is primarily composed of benzene, toluene, ethylbenzene, and xylene, which can rapidly evaporate to the atmosphere. One laboratory study found that 15 percent of the benzene and toluene and 80 percent of the xylene were lost after 24 hours of exposure to the atmosphere (Michel and Fitt, 1984).
Acute and chronic exposures of oil on coral have been studied in the laboratory and field (reviewed by Peters et al, 1997). The symbiotic algae associated with coral are affected after 24 hours of exposure to the water-soluble fraction of oil (benzene, toluene, ethylbenzene, and xylene; see Box 5-2). Photosynthetic capacity can recover fully if there is only short-term exposure to oil (less than 72 hours), and no adverse affects were measured for exposure of less than one hour (Michel and Fitt, 1984). Mixtures of dispersants and oil are more toxic to coral than just the oil (Peters et al, 1997). Branching coral (e.g., Acropora palmata) is considered more sensitive to oil exposure than massive coral (e.g. Montastre, Bak, 1987).
Mussels often occur in dense intertidal aggregations and their interlocking byssal threads provide a low-energy habitat with protection from the rigors of breaking waves above the bed. The interstices of mussel beds are micro-habitats rich in intertidal life (Ricketts and Calvin, 1948). As with other bivalves, mussels effectively accumulate high concentrations of a variety of contaminants including petroleum hydrocarbons from the water and their food.
Mussels can be affected by the accumulation of petroleum compounds. Low concentrations of petroleum hydrocarbons can interfere with cellular and physiological processes like cellular immunity (McCormick-Ray, 1987; Dyrynda et al., 1997), lysosome characteristics (Pelletier et al., 1991), byssus attachment (Linden et al., 1980), growth (Widdows et al., 1987, 1989), and ability to tolerate air exposure (Thomas et al., 1999). Thus, there is a basis for expecting population impact under some conditions. Oil exposure or vigorous cleanup of the intertidal zone results in damage to these beds, and it may take years for the beds to re-establish their former richness. At the same time mussel beds effectively trap oil and under some circumstances allow the oil to persist for years after a spill. For example, after a 7,000 tonnes spill into a tropical estuary with mangrove habitats, damage to mussels was apparent one year after the spill (Garrity and Levings, 1993). A spill of Bunker C fuel oil, spilled from a collision of two tankers in San Francisco Bay in 1971, resulted initially in smothering of intertidal invertebrates. Five years after the spill, there was no evidence of long-lasting effects of the oil spill on recruitment patterns of intertidal invertebrates in high energy environments (Chan, 1977).