Oral Session 3

Carbon-centric dynamics of Earth’s marine phytoplankton from underwater profiling robots

Marine phytoplankton are fundamental to Earth’s ecology and biogeochemistry. Our understanding of the large-scale dynamics of phytoplankton biomass has greatly benefited from, and is largely based on, satellite ocean color observations from which chlorophyll-a, a commonly-used proxy for carbon biomass, can be estimated. However, ocean color satellites only measure a small portion of the surface ocean, meaning that subsurface phytoplankton biomass is not directly monitored. And chlorophyll-a is an imperfect proxy for carbon biomass because cellular physiology drives large variations in their ratio. The global network of Biogeochemical (BGC)-Argo floats now makes it possible to complement satellite observations by addressing both these issues at once. In our study, we use about 100,000 water-column profiles from BGC-Argo to describe Earth’s phytoplankton carbon biomass and its spatiotemporal variability. We estimate the global stock of Earth’s phytoplankton biomass at about 342 Tg C, half of which is present at depths out-of-view from satellites. Nearly half of phytoplankton are present in the Southern Ocean and its subtropical boundaries. We also compare the seasonal cycles of carbon biomass stocks with those of surface chlorophyll-a visible from space and find that surface chlorophyll-a does not accurately identify the timing of the peak annual bloom in two-thirds of the ocean. Our study is a novel demonstration of global-scale, depth-resolved monitoring of Earth’s phytoplankton, which will be crucial for understanding future climate-related changes and the effects of geo-engineering interventions if implemented.

*Adam Stoer, Dalhousie University, Canada, https://orcid.org/0000-0002-4561-9347

Katja Fennel, Dalhousie University, Canada, https://orcid.org/0000-0003-3170-2331

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