The counterintuitive effect of summer-to-fall mixed layer deepening on eukaryotic new production in the Sargasso Sea

The Sargasso Sea is characterized by strong summertime stratification that is thought to drive oligotrophy, but export production is surprisingly similar to that of high-latitude regions with ample major nutrient supply. Here we use the summer-to-fall progression in the northwestern Sargasso Sea to investigate the relationship between upper ocean stratification and phytoplankton nitrogen (N) uptake. Euphotic zone particles collected in July, October, and December were sorted by flow cytometry, and the N-15/N-14 of separated prokaryotic and eukaryotic phytoplankton was analyzed. The N-15/N-14 of Prochlorococcus and Synechococcus was always low, indicating uniform reliance on recycled N. In July and in two fall profiles, the N-15/N-14 of eukaryotic phytoplankton was high, reflecting consumption of subsurface nitrate. In three other fall profiles, eukaryotic N-15/N-14 was similar to prokaryote N-15/N-14, suggesting a shift toward more complete reliance on recycled N. The progressive deepening of the mixed layer from summer to fall, although reducing the surface-to-deep density contrast, increases the density difference of the euphotic zone as a whole from underlying nutrient-rich waters, which may play a role in the observed decline in euphotic zone nitrate supply into the fall. The apparent summertime nitrate supply to the euphotic zone, when the mixed layer is shallowest, may help to explain the surprisingly high export production of the subtropical and tropical ocean.