Evolution saisonnière des populations de diatomées benthiques dans les lagunes aménagées de Certes

In lagoons and estuarine ecosystems, the benthic microflora contributes considerably to the primary production of the area and consists largely of diatoms Monthly samplings have been made from December 1976 to March 1978 in a shallow station characterized by the lack of macrophytes. Two sets of monthly sam­plings were made allowing in theone hand a qualitative analysis and on the other hand a quantitative study by cellular counts. The diatom populations show a cyclic evolution with two peaks : one in February 1977 and the other in September 1977. This cycle is confirmed by the di­versity and dominance analysis. Seasonal evolution is characterized by the domi­nance of one or more species with well marked succession of dominant species (Nitzschia Navicula et Amphora). Finally a comparison of the number of cells per cm2 and of the degree of coverage demonstrated the numerical prevalence of diatoms with a size of less than 500 um2. Accordingly large diatoms with long doubling time are frequently underestimated. Thus, Surirella stritula Turpin (9000 um2) which is always found in reduced number becomes dominant by transformation in coverage.