Une procédure systématique pour le sous-échantillonnage des prélèvements de plancton

It is often necessary to sub-sample plankton samples even if the precision of the estimation is thereby decreased. A methodological optimization is looked for. The variance depends on the procedure of construction of the aliquot (strategy) and the number of individuals counted (effort). The sub-sampling effort is allocated in a general context of repartition of the total effort between sampling and sub-sampling. This effort depends on the time consumed by the sub-sampling procedure, the volume of the aliquot and the number of individuals counted. For the same effort, a systematic procedure for the construction of sub-samples is more precise than a random one because it travels through the entire sample and is thus less dependent on the spatial distribution of the organisms in the sub-sampling tank. The calculation of the subsampling variance in routine is suggested because deviations from the theoretical models are frequent, especially for the high levels of abundance.