Petroleum hydrocarbons and their effects in subtidal regions after major oil spills

Abstract

The question often arises after large oil spills as to the extent and effect of oil entering the subtidal zones adjacent to heavily oiled shorelines. Estimates for a number of large spills suggest that 1 to 13% of the spilled oil can enter subtidal regions. Hydrocarbon concentrations in these subtidal zones are generally orders of magnitude lower than shoreline sediments. For example, in the Exxon Valdez oil spill, subtidal sediment hydrocarbon concentrations attributable to the spill were very low in the first year after the spill and barely detectable in the second year. The conditions necessary to produce high concentrations of hydrocarbons in the subtidal region include large amounts of oil in a semienclosed estuary or bay and high concentrations of fine particulate matter to associate with hydrocarbons to allow them to disperse and sink. Such conditions do not often occur after spills, with some exceptions, such as low energy tidal estuaries in the Amoco Cadiz spill in Brittany, France. More commonly sea floor sediment hydrocarbon concentrations, where sediment-associated hydrocarbons have settled, are generally near to background levels, due to dilution and weathering. A number of methods have been used to evaluate the biological effects of oil spills on subtidal fauna. These include toxicity to amphipods, increases in the concentrations of fluorescent aromatic metabolites in the bile of fish, histopathology of fish, increases in opportunistic species and infaunal succession. Sediments collected from the subtidal zone below heavily oiled shorelines of the Exxon Valdez spill showed low toxicity using standard amphipod bioassays. Well documented effects on the subtidal biota adjacent to heavily oiled shorelines are the increases in the number of hydrocarbon degrading microbes which are fed on by opportunistic species of meiofauna which in turn are food for macrofauna. The documented biological effects of oil in the subtidal region are generally of short duration and recovery back to an equilibrium or ‘normal’ condition is typically quite rapid.