The prevailing weather and sea conditions (wind, swell, tides, currents,
salinity, temperature) influence the viscosity,
spread rate, dispersion and destination of the oil slicks. Thus a drastic
change in the wind direction saved the French Charente-Maritime region from
a serious threat of pollution from the Erika, but instead impacted the Pays
de la Loire region.
The time of year is also a factor which should be taken into account. The influence of the seasons on the water temperature and biological cycles is by no means negligible. Many species are more sensitive to oil pollution during their reproduction, nesting and migratory periods. For annual vegetation, the most sensitive time is the beginning of the growth period.
Other factors which directly influence the impact are the duration of exposure, the form and concentration of hydrocarbons, the thickness of deposits, the degree of contamination of the substrate and the chemical evolution of the hydrocarbons. Many of these are related to the degree of energy and water exchange in the receiving environments.
Finally, the presence of other pollutants and/or previous exposure to oil can increase the sensitivity of certain organisms to pollution, reduce the survival capacity of a biological community or cause the population of resistant species to grow in relation to sensitive species.