Long‐term variability of phytoplankton carbon biomass in the Sargasso Sea

Time series of phytoplankton carbon biomass are scarce yet may provide important insights into ocean productivity and carbon export to depth via the oceanic biological pump. We combine recent flow‐cytometric measurements with pigment concentrations and other standard measurements to reconstruct taxon‐specific phytoplankton carbon biomass in the Sargasso Sea over 22 years, using a multiple regression approach. The reconstructed series reveal an increasing trend (~3% per year) in total phytoplankton carbon, apparently driven by increasing nutrient supply by vertical mixing associated with a shift to a negative phase in the winter North Atlantic Oscillation index. Also, the reconstructed eukaryote biomass fraction shows a multiannual shift from ~45% in the early 1990s/late 2000s to ~70% in the late 1990s/early 2000s. We hypothesize that a multiannual shift in the seasonal pattern of mixing may have stimulated and restructured the eukaryote community while suppressing prokaryote populations by increasing photodamage and grazing mortality.