Climate variability shifts the vertical structure of phytoplankton in the Sargasso Sea

Marine phytoplankton are essential to ocean biogeochemical cycles. However, our understanding of changes in phytoplankton rely largely on satellite data, which can only assess changes in surface phytoplankton. How climate variability is impacting their vertical structure remains unclear. Here we use 33 years’ worth of data from the Sargasso Sea to show distinct seasonal and long-term phytoplankton climate responses in the surface mixed layer compared with the subsurface. Seasonally, the surface community alters their carbon-to-chlorophyll ratio without changing their carbon biomass, whereas the chlorophyll a and carbon of the subsurface community covaries with no change in their carbon-to-chlorophyll ratio. Over the last decade, the subsurface phytoplankton biomass has increased in response to warming, whereas the surface phytoplankton have altered their carbon-to-chlorophyll ratio with minimal change in their carbon biomass. Given that satellites can only view the surface ocean, sustained subsurface monitoring is required to provide a full understanding of how phytoplankton are responding to climate change.