Effects of Lugol's fixation on the size structure of natural nano-microplankton samples, analyzed by means of an automatic counting method

Accurate abundance and biomass measurements are essential steps for determining the role of nano-microplankton in the microbial food web. Owing to practical constraints, traditional microscope analysis of nano-microplankton requires preservation; but preservatives alter plankton samples and bias the measurements. The majority of studies on the effects of preservation have been based on cell cultures. However, new automatic counting systems offer the possibility to investigate the effect of fixatives on large numbers of natural samples. In the present study, cell counts of live and 1% Lugol's preserved samples were compared at 115 stations located in the Bay of Biscay. Additionally, the effect of different Lugol's concentration (1 and 5%) was studied. Analyses were performed with the FlowCAM (see Sieracki et al. in An imaging-in-flow system for automated analysis of marine microplankton. Mar. Ecol. Progress Ser., 168, 285-296, 1998), using plankton samples directly collected from the field. The results show that the analysis of natural samples preserved with a single fixative biases the abundance and biomass estimates of different size ranges of the nano- and microplankton, not only in the large sizes. This is due to changes in cell abundance, especially in the nanoplankton size range, and to the formation of aggregates.