Carbon Biomass Budgets for the 2008 North Atlantic Spring Bloom Microplankton

The carbon flux dynamics of the North Atlantic spring bloom in 2008 was studied using robotic Lagrangian floats and Seagliders between early April and late June. The instruments on these platforms were ground truthed during four cruises. One goal of the project was to determine how well optical measurements such as backscattering and beam attenuation could be used as proxies for organic carbon biomass in planktonic systems. Critical to this question is whether, and by how much, the proportion of plankton-to-total organic carbon changes over the course of the bloom. Samples were taken in the vicinity of the floats and gliders for plankton biomass determination using flow cytometry and imaging-in-flow technology. In early May the surface plankton community was dominated by chains of the diatom, Chaetoceros. As the bloom progressed the community shifted to a mix of large mixotrophic ciliates (cf. Laboea), dinoflagellates (Ceratium) and different genera of chain diatoms. The Synechococcus and phototrophic nanoplankton abundances increased over this succession as well. This surface succession coincided with a flux of Chaetoceros resting stages to subsurface depths. We estimate the carbon biomass of phototrophic and heterotrophic pico-, nano- and microplankton groups using cell abundances, forward light scatter calibrated to size, and established cell carbon density (carbon mass per unit biovolume) conversions to create a carbon budget during different stages of community succession. The difference between total particulate organic carbon, measured chemically, and the total plankton biomass is inferred to be detrital carbon. Our budget is compared to previously determined carbon budgets for North Atlantic spring bloom communities.