Using FlowCAM data to estimate microplankton biomass: Is biovolume a good predictor of particulate organic carbon?

King County manages a comprehensive, long-term water quality monitoring program in Puget Sound’s Central Basin. Year-round observations are collected for a suite of physical, chemical, and biological parameters. In 2014, use of a FlowCAM (Fluid Imaging Technologies) particle imaging system was incorporated to add quantitative microplankton observations to the dataset. Plankton particles of two size fractions (5-100 and 100-300 µm) are imaged, classified and quantified with endpoints of abundance (particles/mL) and biovolume (mm3/L). Although phytoplankton biovolume is often used as a proxy for biomass, the suitability of using particle biovolume values generated by the FlowCAM software as a surrogate for biomass needed further validation. Conversion of plankton biovolume into carbon biomass is essential if these data are to be used in studies addressing carbon flow and trophic structure. The purpose of this study was to determine the relationship between microplankton biovolume, as measured by FlowCAM, and particulate organic carbon (POC) in the Central Basin. Surface samples for POC analysis were collected from Niskin bottles along with routine monitoring FlowCAM samples at eight Puget Sound Central Basin stations from June to November, 2015 and March to September, 2016. A total of 200 samples were separated into two size fractions and further analyzed by combustion for total POC. Preliminary analysis shows that the relationship between particle biovolume and POC was non-linear, and that particle biovolume to POC ratios varied for the two size fractions analyzed, reflecting both taxonomic and seasonal dependence on these ratios. The data will be compared with published values for different algal taxonomic groups, and the suitability of using biovolume as a proxy for microplankton biomass in Central Puget Sound will be discussed