Indirect effects

Marine plants and animals do not live in isolation from each other. They develop in an environment teeming with a wide variety of biological interaction. This interaction goes from predator/prey relationships to situations of dependence, symbiosis, parasitism and other situations in which an organism or group of organisms provides a shelter or another necessity for one or several species in the community.

Members of all the groups of organisms - algae, bacteria, invertebrates, fish, birds, mammals – can be affected by an oil spill. All damages caused by oil to individuals of a group may lead to changes in the structure and functioning of the biological community. Four broad types of indirect effects can be distinguished:
• death by starvation of organisms whose usual source of food is polluted, and where alternative food supplies are not available
• disturbance of certain interactions between species, due to the elimination or reduction of certain species
• large-scale proliferation of organisms which prey on the food usually consumed by other species present in the environment
• modification of habitats due to the loss of key species, such as seagrasses or mangroves, or the impacts of clean-up operations, such as the alteration or disappearance of the substrate.

A reference condition of vegetation on land (France)

In 2000-2002, the French National Botanical Conservatory in Brest coordinated the development of a reference condition for shoreline flora and fauna in Brittany and the Pays de la Loire (France). This work came as a response to the need, which had become apparent in the aftermath of the Erika oil spill, for a specialised tool to provide information on the issues affecting vegetation, which could act as a decision-making aid in the use of contingency
plans for marine pollution. This reference condition made use of previously acquired knowledge and provided a diagnosis of the flora (species) and the patrimonial shoreline vegetation (vegetation communities or habitats), with a particular focus on formatting the information necessary for those responsible for the fate of the natural heritage. To the information which was already available was added an interpretation of aerial digital photos georeferenced by the work of botanists and phytosociologists who listed, mapped and described in detail the sites of different species of vegetation and their habitats of high patrimonial value. The map below shows the Roscanvel peninsula in Finistère, France, on a scale of 1:25.

More information

The website of the French National Botanical Conservatory in Brest (French only).


All these effects are temporary. After the imbalance caused by the pollution, then the proliferation of resistant, recolonising organisms, the former balance is gradually restored, except if new pollution arrives. However, this return to normal can take years.

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Reference condition

The contamination of the environment and the ecological impact of pollution can only be fully appreciated when compared to reference values providing an accurate representation of the situation before the accident. The creation of reference conditions is the only way of demonstrating the causal relationship between the spill of pollutant and the effects subsequently observed. This implies that the information provided by these reference conditions must be coherent with the particular needs of the ecological follow-up, in order to provide an objective impact report.

To fulfil this need, and due to a growing demand for information on the state and evolution of benthic flora and fauna, Ifremer (the French Research Institute for Exploitation of the Sea), for example, launched and coordinated in 2001 and 2002 a strategy for setting up a new surveillance and reference data network for coastal benthic populations, known as the Rebent network. This network has been up and running since 2003.
For more information, see

Follow-up of botanical restoration