Direct Measurement of Group-Specific Phytoplankton Chlorophyll

Bulk chlorophyll a concentration is commonly used as a proxy for phytoplankton biomass in estimating ocean productivity. However, the heterogeneous nature of phytoplankton communities makes it difficult to interpret sources of variability in bulk chlorophyll concentrations at broad scales. We describe a method utilizing fluorescence activated cell sorting for the direct measurement of group-specific chlorophyll concentration for the globally distributed cyanobacterial groups Prochlorococcus and Synechococcus, in addition to that for picophytoeukaryotes, collectively called picophytoplankton. We present an application of this method to field samples collected from the Western Tropical North Atlantic and the Gulf of Mexico, investigating variability in group-specific and cell-specific chlorophyll in response to biogeochemical parameters. Preliminary results indicate that picophytoplankton account for 42 (±10.3) percent of total phytoplankton chlorophyll, with most variability in this proportion being attributable to changes in cyanobacteria-specific chlorophyll concentrations. These results highlight the significant contributions to total chlorophyll by picophytoplankton in both open ocean and coastal waters, and suggest the usefulness of the method for constraining model estimates of phytoplankton distribution.