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

Abstract Marine phytoplankton are essential to ocean biogeochemical cycles. Yet, how climate variability is impacting phytoplankton vertical structure remains unclear. Here, we apply a conceptual model that partitions a profile of phytoplankton into two communities – one in the surface mixed layer and one in the subsurface below – to 33 years of data in the Sargasso Sea. 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. At the multidecadal scale, climate variability influences these two communities differently. Over the last decade, subsurface phytoplankton biomass has increased in response to warming. In contrast, the surface phytoplankton has altered their carbon-to-chlorophyll without modifying their carbon biomass. Given that satellites can only view the surface ocean, sustained subsurface monitoring is required to understand fully how phytoplankton are responding to climate change.