How to effectively sample the plankton size spectrum? A case study using FlowCAM

Any technique developed to enumerate plankton must take into account the size structure of the plankton community. Automatic sampling devices must be capable of analysing a minimum number of cells of the largest size to cover the whole size range intended to be sampled effectively. The Flow Cytometer And Microscope (FlowCAM (R)) has been used in the last decade to estimate the size structure of the plankton community. Few attempts, however, have been made to compare FlowCAM measurements with the results provided by traditional microscopy methods for size-structure estimations. FlowCAM can operate in three working modes: autoimage, fluorescence triggered and side-scatter triggered. Autoimage and fluorescence triggered cannot only count accurately a mono-specific suspension of cells, but they are also useful to estimate the size structure of natural samples. The side-scatter-triggered mode is not effective to estimate the size structure of natural samples, although it can count a sparse mono-specific solution accurately. The analysis of natural samples with FlowCAM requires a planned pre-processing of the samples to adjust the density of triggering particles (concentrating or diluting the sample) and to pre-filtrate the sample to avoid cell clumping or obstruction of the flow chamber. The size structure obtained with FlowCAM and with microscopy counts on preserved samples are comparable. Sample preservation, however, alters the size structure of the sample, which suggests that results based on preserved samples must be taken with caution. Automatic sampling devices like FlowCAM could provide a more precise analysis of plankton communities, increasing the resolution of surveys and avoiding the effects of preservation and sample storage.