Comparison of FlowCAM and microscope biovolume measurements for a diverse freshwater phytoplankton community

FlowCAM combines flow cytometry and imaging to rapidly enumerate, classify and measure particles. The instrument potentially increases processing speed of phytoplankton samples. FlowCAM, however, requires extensive comparison to microscopy before incorporation into monitoring and research. Past studies have compared FlowCAM and microscopy results for mostly marine rather than freshwater phytoplankton communities. We compared phytoplankton biovolume, density and taxonomic classifications between FlowCAM and microscopy for 113 samples from Lake Champlain, USA—a large freshwater system with diverse phytoplankton. Total biovolume estimates from FlowCAM were higher than microscope biovolumes due to higher individual particle biovolumes. Biovolume relationships, however, were closely correlated between the two methods. Shape-specific biovolumes from FlowCAM images slightly improved estimates compared to area-based biovolumes. Diatoms and filamentous cyanobacteria showed the strongest relationships between FlowCAM and microscope biovolumes. Microscope natural unit counts were generally higher than FlowCAM counts. Genus richness was weakly related between FlowCAM and microscopy, demonstrating a potential tradeoff between finer taxonomic resolutions with a microscope versus the higher number of particles processed with FlowCAM. Both methods produced reproducible biovolumes with replicate samples. We conclude that microscopy is more reliable when fine taxonomic resolution is needed and FlowCAM is suitable for rapid processing of major phytoplankton groups.