Effect of preservation on dinoflagellate and diatom cell volume and consequences for carbon biomass predictions

Plankton biomass estimates are often made using cell size measurements from samples preserved with fixatives such as Lugol's or glutaraldehyde. However, preservation of plankton samples can introduce artifacts to species abundance and cell volume estimates. The goal of this study was to quantify the extent of fixation-induced cell volume change in 18 species of dinoflagellates and 8 species of diatoms in order to provide correction factors to improve biomass estimates. Highly variable and species-specific cell volume changes were found with respect to cell covering, fixative type and strength, time of storage and analysis method (light vs epifluorescence microscopy). Both swelling and shrinking were observed with both fixatives in dinoflagellates and diatoms. Dinoflagellate cell volume changes ranged from over 60% shrinkage to 30% swelling. Cell volume of some diatom species shrank over 30% and swelled almost 30%, These results contradict the common assumption that diatoms are not affected by preservation due to their silica frustule. The observed cell volume changes for both dinoflagellates and diatoms differed from those reported for ciliates and other flagellates. Therefore, application of previously reported correction factors would lead to inaccurate estimates of biovolume and thus carbon biomass. Fixation caused significant changes in biomass predictions of individual dinoflagellate and diatom species, However, biomass estimates for samples containing several species were not significantly different whether based on live or fixed cell volumes.