The gravitational settling of organic particles from the surface to the deep ocean is an important export pathway and one of the largest components of the marine biological carbon pump (BCP). The strength and efficiency of the gravitational pump is often measured using metrics reliant on reference depths and empirical formulations that parameterize the relationship between depth and flux or concentration. Here, BGC-Argo profiles were used to identify the isolume where POC concentration starts to decline, revealing attenuation trends below this isolume that are remarkably consistent across the global ocean. We developed a semi-mechanistic approach that uses observations from the first optical depth to predict POC concentration from the surface ocean to the base of the mesopelagic (1000 m), allowing assessments of spatial and temporal variability in BCP efficiencies. We find that rates of POC attenuation are high in areas of high biomass and low in areas of low biomass, supporting the view that bloom events sometimes result in a relatively weak deep biological pump characterized by low transfer efficiency to the base of the mesopelagic. Our isolume-based attenuation model was applied to satellite data to yield the first remote sensing-based estimate of integrated global POC stock of 3.02 Pg C for the upper 1000 m, with 1.27 Pg C of this global carbon stock located above the reference isolume where POC begins to attenuate.

Phytoplankton photoacclimation is a well-known response to changes in light and nutrients, with the intracellular Chlorophyll to Carbon ratio (Θ) increasing at low light and decreasing under high light to optimize growth rate. Accurate quantification of phytoplankton growth rates, Net Primary Production (NPP), and carbon export from space requires careful consideration of Θ. The scientific community needs gap-free estimates of θ for precise global NPP quantification. Current NPP models interpolate Chlorophyll concentration (and other inputs) from clear-sky pixels under clouds, assuming no photoacclimation—an assumption we challenge.   Using data from ≈14,000 BGC-Argo floats and MODIS-derived cloud cover, we compared the response of Θ to various irradiances under both cloudy and clear skies. We found that phytoplankton photoacclimate similarly regardless of sky conditions, at the global, local, and seasonal scales. This study highlights an incorrect assumption in current NPP estimates and suggests ways to improve global assessments.