Response of phytoplankton biomass and primary production to changes in community size structure during the 2008 North Atlantic Spring Bloom

Oceanic primary production plays a critical role in the global carbon cycle. The flux of carbon material from the upper ocean, however, is dependent on the quality and quantity of the carbon biomass produced. In the North Atlantic, large diatom chains dominate the phytoplankton community structure early in the spring bloom and can promote a significant sinking particulate organic carbon flux. As the spring bloom progresses and stratification sets in, changing phytoplankton community structure results in more recycled and regenerated forms of organic carbon prohibiting efficient carbon flux from the upper ocean. During the May 2-21, 2008 North Atlantic Bloom Experiment southwest of Iceland, size-fractionated (total, <20um) fluorometric chl a concentration and photosynthetic parameters from short-term 14C-uptake incubations were measured between 10am - 2pm at the surface (10m) and at depth (usually 30m) to determine the evolution of phytoplankton population size structure and primary productivity during the spring bloom. During the first two weeks of the cruise, large phytoplankton (>20um, mostly large diatom chains) contributed over ~80% to the total chl a signal at both depths. Over the final week, however, the surface phytoplankton community transitioned to relatively smaller cells (<20um) that contributed at least 60% to total Chl a indicating a shift away from a diatom chain based community structure. A similar phytoplankton community structure shift appeared to begin at depth as the cruise ended with similar implications for total Chl a contribution. Quantification of photosynthetic parameters demonstrated a high degree of spatial and temporal variability at both 10 and 30m which appeared to be correlated with variability in chl a concentration over the course of the cruise. Pmax varied from approximately 20 to 90 mg C m-3 h-1 at the surface and 10 to 60 mg C m-3 h-1 at depth. When maximal values were observed, large cells dominated the production (>80%); when low values were observed small cells made a significant (30-80%) contribution. The pattern suggests that the smaller size fraction provides a relatively constant background level of biomass and production while the dynamic bloom is driven by the biomass and production of the large cells, in this case, chain-forming diatoms.