Rocky intertidal shores are quite susceptible to damage by oil spills depending on the amount and characteristics of the oil to which they are exposed. The 1985 Oil in the Sea report adequately characterized the damage of shorelines to spills and stressed the critical role of geomorphology in the recovery of these shorelines. We reiterate here the importance of the interactions of wave and tidal energy with shoreline geomorphology in determining recovery and punctuate this with lessons from more recent studies.
The persistence of the oil and the time to recovery are a function of the energetic fluxes where oil is deposited. If the initial oiling from a spill is an outer, exposed coast, and the rocky substrate is continuous without substantial low energy interstices, then oil will not persist long and recovery will be relative quick (e.g., see Chan, 1977 for an account of recovery on heavily oiled rocky coasts after the San Francisco Bay spill). If the shoreline is relatively sheltered or there are significant interstices where the oil can enter and be sheltered from the energetic fluxes of waves and tides, then oil will persist and recovery may take substantially longer.
The degree of impact and recovery from a spill on the rocky intertidal is very much a function of the circumstances of a spill. Not only is the aforementioned geological structure of the shoreline important, but the type of oil, the weather conditions following the spill, the thickness and lateral continuity of the slick, the time of year, and the recent history of disturbance of the biological communities are all important factors affecting severity of impact. One example of how low energy environments can retain oil and effects can persist is a southern ocean spill at Macquarie Island. In this spill, most intertidal components appear to have recovered within several years after the spill occurred, but in the holdfasts of kelp, which is an environment not unlike mussel beds, oil was retained for years and the fauna of this microhabitat has not recovered (Smith and Simpson, 1995).
By far the greatest acute injury to intertidal communities as a whole arises from direct contact with oil. Heavy deposits of oil essentially smother intertidal organisms. Toxicity also occurs from elevated concentrations of the soluble components of oil in small pools of water, in wetted surfaces and in the water of rising tides. The common organisms found on rocky intertidal shores of North America—Fucus, mussels, periwinkles, starfish, and barnacles—are all susceptible to the toxic effects of oil (Chan, 1977; Stekoll et al., 1993). Recovery of these components can be quite substantial within a year or two, or nearly complete. Subtle long-term effects are possible, however (Peterson, 2001). In the Exxon Valdez spill, the aggressive washing of the intertidal rock shores resulted in loss of a significant amount of silt from the rock interstices and the associated bivalve fauna has not been fully re-established and may not be until these sediments have been replenished by natural processes (Driskell et al., 1996).
The above caveats about the nature of the oil, the thickness and extent of the slick and the weather conditions determining impact also apply to softer substrates. Of particular note is the stranding of oil in protected, low-energy environments, such as bays and harbors. If oil arrives in one of these otherwise low-energy environments under storm conditions and gets worked into the substrate, it will likely be there for years and possibly decades. Two examples are the Florida spill in West Falmouth, Massachusetts in 1969 (Burns and Teal, 1979) and some areas affected by the Amoco Cadiz spill in France in 1978 (Dauvin and Gentil, 1990). It was clear at the time of the Oil in the Sea report (NRC, 1985), that the combinations of circumstances resulting in acute effects can also result in recovery times of years and even decades.
In the last 17 years there has been more focus on chronic contamination by PAH, the sensitivity of meiofauna, and indirect effects mediated by changes in predator-prey relationships, as well as by the direct toxic impacts. In particular, chronic exposure of fauna and potential effects have been studied over more realistic time scales and concentrations. Microcosm experiments where realistic doses of PAH are maintained in sediments to provide a chronic exposure regime have been particularly valuable. For example, in salt marsh sediments in Louisiana concentrations of high molecular PAH (up to 16 ppm) were found to decrease the biomass of epibenthic diatoms and cyanobacteria after 4-day exposures, with some indications that snails from high exposure treatments lost weight after initial gains (Bennett et al., 1999). Such experimental results point to the need to examine more closely estuarine food webs where concentrations of PAH in this range can be found.
The spatial scale of the affected sand or mud shoreline area will determine the rebound of the affected area. A practical example of this is the impact of the Amoco Cadiz oil spill on benthic crustaceans. Failure to recover in some subtidal habitats was due to the fractionated distribution pattern of favored habitat by some species of amphipods (Dauvin and Gentil, 1990). Nevertheless, the populations were able to recover; densities on the impacted site attained high values similar to those found before the spill within 15 years (Dauvin, 1998).
Oil can arrive in the subtidal by two mechanisms. Surface oil can weather, lose buoyancy and eventually sink, and it can associate with particulate matter suspended in the water and eventually sink, thereby affecting the benthic community (Elmgren et al., 1983). A second route of oil to the benthos is the transport of oil or contaminated particles from nearby oiled beaches.
As with the intertidal fauna, the most sensitive organisms in the subtidal benthos appear to be the crustaceans. Major effects on the crustacean fauna were documented in the