Lethal effects

Aside from deaths caused by the effects of oiling, certain constituents of oil are incorporated in the water column, taken up by the biota, and can be toxic for marine plants and animals. This toxicity can be acute, causing the rapid death of organisms exposed to oil by contact or ingestion, or causing serious disturbance to their basic functions. Delayed toxicity occurs when the organism’s survival capacity is diminished, i.e. when there is a reduction in its growth rate, rate of reproduction, resistance to stress or to biological attack (illness, parasites, predators).

Acute toxicity is measured by the Lethal Dose 50 (LD50), the dose likely to kill 50% of the organisms to which it is administered over a given period. This expression of toxicity can be explained by the fact that all the members of a population do not have exactly the same sensitivity to any given toxic product. The LD50 measures the average sensitivity of a test population for a certain species.

Many laboratory-based studies have been carried out to determine the most toxic components of different oils. These are mainly aromatic compounds (polycyclic aromatic hydrocarbons or PAHs), which are fairly soluble and therefore quickly available for marine organisms.

Agitation in the surrounding environment is a significant factor in the generation of concentrations of aromatic compounds. In conditions of mild turbulence, only the most soluble aromatic compounds enter the water mass. The others quickly evaporate before they have the chance to dissolve in the water. In strong turbulence, the least soluble volatile compounds also enter the water column.

Polycyclic aromatic hydrocarbons are the contaminants present throughout the marine environment; on the one hand, due to their high stability and, on the other hand, because of the multiplicity of their sources (burning fuel, various industries, incinerating waste, etc.). The higher molecular weight PAHs are considered the primary contaminants of both marine and terrestrial ecosystems for their potentially carcinogenic and mutagenic effects. Their toxicity results from the formation of metabolites (epoxides, dihydrics)
by organisms, which bind with the DNA and result in malfunctioning.