Natural recolonisation

By the time clean-up operations are complete, the wildlife populations will very likely have been altered, sometimes locally eradicated, by the effects of the oil and clean-up. The affected sites may be partially depopulated and become newly habitable areas for other species. First, colonising species settle in them, then over time populations similar to the original species return. However, this colonisation sequence may be threatened by the existence of abnormally high quantities of hydrocarbons in the water and sediments for months or even years after the spill. Scientific follow-up can be set up to determine whether the original balance has been regained and the timeframes involved.

Studies have shown that the areas seriously impacted by a major oil spill would typically regain a balance comparable to that of surrounding areas in the following timeframes:

• 2 to 6 years for low sensitivity areas (rocky points and other areas beaten by waves)

• 5 to 15 years for moderate sensitivity areas (beaches and other areas of moderate hydrodynamic activity)

• 10 to 25 years for high sensitivity areas (coastal marshes and other areas of very low hydrodynamic activity).

These timeframes cited here as typical for species/population recovery rates after a major oil spill are not much higher than those required in similar environments following a natural disaster, such as a major flood, a large mudslide or a volcanic eruption.

Impact of the Amoco Cadiz in France

An example of the problem of assessing impact

(Source: DAUVIN J.C. Surveillance du milieu marin : travaux du RNO de la qualité du milieu marin, édition 1996)

The graphs showing the evolution of the biomass of populations in the bay of Morlaix (Brittany, France) after the Amoco Cadiz spill (1978) seem to contradict each other if we only consider the overall quantities. We must look at individual species, or groups of species, to understand what really occurred.

In fine-grain sand, Ampelisca (type of amphipod), a sensitive species, practically disappeared for two years and only returned to its normal level some 14 to 15 years after the disaster. In muddy sand, Lanice (segmented worms) developed abnormally and became abundant the year of the disaster and for the following 7 years, before returning to the former modest numbers.

Recovery of the ecological environment

The areas of the Breton coastline which were successively hit by the pollution from the Torrey Canyon (1967), the Amoco Cadiz (1978) and the Tanio (1980) suffered two major disruptions to ecosystems in recovery phase. Today, the vast majority of their populations have regained a balance similar to that of surrounding areas, however traces of overabundances of opportunist species or abnormally low levels of sensitive species still remain measurable in certain environments. At temperate latitudes, a period of 6 to 7 years is generally enough to see all traces of a major oil spill disappear. However this is not always the case. Some highly protected environments, such as maritime marshes and mudflats, where arrivals of oil from the Amoco Cadiz were widespread, were still polluted 13 years after the vessels grounded.