Altered Phytoplankton Dynamics Associated with the North Pacific Blob Provides a Glimpse of Future Warming Oceans

The Northeast Pacific is recognized as a critical region for carbon sequestration despite chronic iron limitation of phytoplankton. Although chlorophyll concentrations are relatively constant in this region, contributions of large cells (≥ 5 μm) such as diatoms versus small cells (< 5 µm) such as cyanobacteria and small flagellates can vary substantially, significantly influencing carbon export potential. Much of the export is believed to be associated with ephemeral iron inputs resulting in diatom blooms. Through a synthesis of microcosm experiments conducted over the last decade and a half, we show that the dynamics of the phytoplankton response to iron enrichment in the Northeast Pacific is primarily a function of the ambient community composition. At times when large cells initially dominate, the response to iron enrichment is large and rapid. Conversely, when small cells initially dominate, there is a delayed and curtailed response to iron enrichment. This phenomenon was most pronounced in association with the North Pacific Blob, a large feature of unusually warm water first appearing in the Fall of 2013. As perceived through RNA sequencing, the diatom community response to iron enrichment was also very different during the Blob event compared to previous years. Our findings suggest that in future warmer oceans, proportions of large cells will likely decline, substantially influencing food web dynamics and the iron-induced carbon export potential in this region.