FlowCAM: Sizing cells and understanding the impact of size distributions on biovolume of planktonic community structure

Instruments for in vivo identification and quantification of marine organisms are becoming more common, and the interpretation of data from these instruments is still evolving. In the present study, we compare the sizing performance of 3 instruments: (1) a black and white (B/W) FlowCAM II; (2) a BeckmanCoulter Multisizer III (MIII); and (3) an inverted microscope. We applied 3 different particle sizing algorithms available from the FlowCAM to suspensions of 5 different particle morphotypes (4 different phytoplankton species and a spherical NIST calibration bead). The FlowCAM generated size distributions similar to those reported by the MIII for the spherical calibration beads. However, differences in reported sizes emerged among FlowCAM algorithms as well as among instruments when applied to morphologically more complex particles, such as diatom chains. There was an immediate and substantial loss of cell counts when live cells were fixed in Lugol's solution, but only minor differences in cell size distributions among the different FlowCAM algorithms. The difference in sizing performance of the FlowCAM algorithms affects biovolume estimates of the natural plankton samples analysed. Species diversity was apparently higher in samples analysed by microscopy than with the FlowCAM, but the cell size distribution from the microscope was extremely narrow compared to FlowCAM and MIII. The present study demonstrates that the particle sizing algorithm has severe impact on the characteristics of the particle size distribution and on the total community biomass estimate.