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.