Photographic records of sea-surface microlayers as a survey of pollution daily rhythm in coastal waters

For one month photographic records of the sea surface were taken (every five minutes) in a coastal area and used with records of wind speed and direction to determine the frequency of formation and spatial extent of slicks. The analysis of photographic records showed that slick sizes and occurrence frequencies decrease as a direct function of the wind speed. When the weather is under the influence of local winds (heat flux exchanges between the sea and the continent), a periodic rhythm occurs for a 24-h period: in the morning and late afternoon slicks are generally large (coverage ˜35%) and frequent) (occurrence frequency ˜65%), but in the middle of the day, when the winds increase to maximum velocity, slicks are small (coverage ˜12%) and rare (occurrence frequency < 40%). During the night, a high slick coverage (39%) can be deduced from wind conditions. The comparison of the present data to the results of a previous experiment conducted in another coastal area, and the fact that meteorological conditions which have prevailed during this study were not particular, allow us to extend our conclusions to the larger coastal zones. In situ sampling in the surveyed area has shown higher concentrations of anionic detergents and hydrocarbons in slicks as compared to the surrounding rough surface waters (Garabetian et al., 1993). Thus, this daily rhythm of slick formation may have some important consequences for the dynamics of pollutant dispersion.