Samples were collected at the selected depth using Niskin bottles and immediately fixed with Lugol's solution (1%). Quantitative analysis is carried out using an inverted microscope after the settling of a variable volume of sample (Utermhöl, 1958). Volumes vary in relation to the number of cells present in the sample, which is estimated on the basis of the chlorophyll a concentration or of the Secchi Disk depth. Counting is done over varying fractions of the sedimentation chamber (transects, random fields, or entire chamber), depending on the characteristics of the sample.