Phytoplankton Functional Diversity and New Production during Spring and Summer Blooms in the Subarctic Atlantic Ocean

In the subarctic Atlantic Ocean, strong seasonal cycles in heat flux drive water column stratification, which governs the supply of nutrients to the euphotic zone that fuels the biological pump. The export efficiency of this pump is largely determined by the degree of phytoplankton nitrate (NO₃^-) assimilation and phytoplankton community size structure. We investigated nitrogen assimilation and phytoplankton community diversity and size structure on spring and summer cruises to 50-60°N, by using a combination of stable isotope tracer incubations, flow cytometry, microscopy, size-fractionated algal pigments, and nitrogen stable isotope measurements. As expected in springtime, the phytoplankton community was dominated by large (>20 µm) cells while in late summer these constituted only a minor fraction of the assemblage. The weaker density stratification of the water column in the spring compared to the summer allowed for surface nutrient concentrations that were not limiting phytoplankton growth (e.g., [NO₃^-] >5 µM). Despite stronger water column stratification in the summer, partial consumption of subsurface NO₃^-, which had recently been supplied to surface waters, allowed for total chlorophyll and particulate nitrogen (PN) to attain similar levels during both seasons. High ¹⁵N/¹⁴N of NO₃^- and PN in surface waters is consistent with NO₃^- utilization. In springtime, however, the phytoplankton community consumed NO₃^- at PN-normalized rates up to fivefold higher than in summer, despite having comparable uptake rates for ammonium and inorganic carbon. This observation implies that the large phytoplankton species that are abundant in spring, mostly diatoms, contribute disproportionally more to new production than summer phytoplankton communities that are devoid of these large species.