1 Living with the oceans. – A report on the state of the world's oceans



4.15 > A specially equipped ship deploys a boom, consisting of inflatable floats, in an attempt to contain the crude oil spilled at sea by the oil tanker Sea Empress after it went aground off the coast of Wales in 1996. In rough seas, however, the use of these skimmers has little effect. © David Woodfall/Getty Images 4.15 > A specially equipped ship deploys a boom, consisting of inflatable floats, in an attempt to contain the crude oil spilled at sea by the oil tanker Sea Empress after it went aground off the coast of Wales in 1996. In rough seas, however, the use of these skimmers has little effect.

How oil damages habitats

It is generally not possible to protect an entire coastline from the effects of a major oil spill, so the authorities have to set priorities for their oil spill response. It goes without saying that designated conservation areas, such as national parks, or sensitive marine areas are particularly worth protecting and are given high priority in clean-up efforts. As a rule, however, these areas are too large to be protected in their entirety. Here, sensitivity rankings can facilitate the oil spill response: these describe the general sensitivity of the various shoreline types to oil pollution. In exceptional cases, it may even be possible to define “sacrificial areas” which are less important from a nature conservation perspective and where no protective measures are taken. When defining these sensitivity rankings, one factor which is taken into account is whether the section of coastline is a “high-energy” area, e.g. with rocky or sandy shores that are subjected to direct wave action, or whether they are relatively calm, “low-energy” areas such as the Wadden Sea, which are protected by sandbanks or offshore islands. Of course, within the major habitats described here, other more detailed sensitivity rankings can be defined for a targeted oil spill response.

EXPOSED ROCKY AND SANDY SHORES: Exposed rocky and sandy shores are classed as areas of relatively low sensitivity because the oil deposited by the sea is cleared very swiftly by wave action. Nonetheless, major oil spills can change the composition of biological communities in these habitats over the longer term. In such cases, populations of former dominant species such as crustaceans and molluscs may decline. In rocky crevices, rough gravel and on mussel beds, the oil pollution may persist for many years.

SANDY BEACHES: Here, a different situation applies. The extent to which the oil penetrates the ground and how long it remains there depend primarily on the structure of the beach. An extensive beach with little surf and with branching channels, for example, is far more vulnerable than a steep beach with a less diverse structure. Coarse-grained sediment facilitates oil penetration, makes the clean-up process more difficult, and increases the risk of follow-up damage from re-surfacing oil. Beach areas used as habitats or breeding sites by endangered species, such as turtles, are classed as particularly sensitive.

CORAL REEFS: Corals are also highly sensitive to oil pollution. Various studies show that damaged coral reefs are very slow to regenerate. Oil pollution can also affect entire communities. For example, less sensitive species of algae can colonize oil-contaminated areas which were previously coral habitats. Very little research has so far been undertaken to investigate how oil spills affect the relationship between corals and the many species asso- ciated with them. The linkage between numerous specialized species and the great significance of symbioses within these ecosystems indicate that far-reaching and long-term impacts can be anticipated after major oil spills.

MANGROVES: Mangrove habitats react with particular sensitivity to oil pollution. Here, an oil spill can inflict severe damage on trees and the sensitive organisms living in them and in sediment. This damage is caused by toxic hydrocarbons, but can also occur as a result of oil cover, which shuts off the oxygen and freshwater supply. The regeneration of damaged populations of flora and fauna is a lengthy process. As the harmful hydrocarbons are removed from sediment very slowly in mangroves, habitat recovery is further delayed.

SOFT SUBSTRATES AND SANDBANKS: Sections of coastline with soft substrates and sandbanks, such as the Wadden Sea on the North Sea coast, are classed as particularly or highly sensitive. The organisms living at great density in and on the sediment provide the basic food supply for fish and birds. Although in most cases, very little oil penetrates the often water-saturated fine pores of muddy sediment, these areas are generally densely populated by burrowing organisms whose activities cause the oil to sink deeper into the ground. On the other hand, the stirring and mixing of sediment by these organisms – known as bioturbation – also help to break down the oil by churning up the sediment, exposing deeper layers to the air and bringing oily sediment to the surface. As this activity promotes a healthy oxygen supply, the oil is then broken down more quickly by bacteria. If the organisms in the sediment have been killed by the oil, however, bioturbation ceases and the oil remains in the ground for longer, causing long-term habitat damage.

SALT MARSHES: Very few studies have been carried out to investigate how oil affects invertebrate organisms found in salt marshes, such as insects and worms. The ve­­­getation, however, can suffer long-term damage from oil pollution, with major implications for breeding and resting birds in the salt marshes, whose plumage may be co­­­vered in oil or which could lose their basic food supply. To sum up, the following regeneration periods can be assumed:
  • Exposed rocky and sandy shores: between a few months and 5 years;
  • Protected rocky shores and coral reefs: between 2 and more than 10 years;
  • Protected soft substrates, salt marshes, mangroves: between 2 and more than 20 years. >