Biogeochemically-contrasting phytoplankton populations of the Eastern and Western Subarctic Atlantic

Recent findings from an intensive multi-year field program (NAAMES, the North Atlantic Aerosol and Marine Ecosystem Study) in the western North Atlantic have uncovered a unique picture of the springtime phytoplankton bloom climax community. Specifically, maximum pigment concentrations and phytoplankton biomass during the bloom climax were dominated by a taxonomically diverse, yet small (<20μm) pico- and nano-eukaryotic population. The small sizes of these cells convey a different biogeochemical function than the anticipated large (often chain forming) phytoplankton species and requires reassessment of extrapolations between pigment, biomass, size, and ecosystem function.

Here, we will highlight differences in field measurements made during the bloom climax during NAAMES and previous studies in the subarctic Atlantic Ocean. We use satellite ocean color products and comparable ecosystem model output to examine these differences across the broader subarctic Atlantic basin from west to east. Similarities in many bulk observable properties (i.e., chlorophyll concentration) exist from east to west despite the observed differences in community composition found in field measurements. In particular, commonly used ocean color satellite algorithms and ecosystem models predict significant contributions to pigment and biomass by diatoms (or other microphytoplankton) on both sides of the basin. One important consequence of this contradiction is that biogeochemical calculations based upon chlorophyll concentration, cell size or functional type may be inappropriate in these cases. We perform sensitivity analyses to show the effects of differences in phytoplankton community structure on estimates of basin-scale export ratios and export production rates. Last, we describe mechanisms that might support these observed east-west differences in climax community composition and their potential significance globally.