PHY-GO · Evaluating linkages between phytoplankton community structure and carbon export using BGC-Argo floats

Marine phytoplankton are a primary vector in the transport of carbon dioxide from the atmosphere to the ocean interior via the biological carbon pump. The magnitude and efficiency of this flux hinges on phytoplankton community structurethe relative abundance of different phytoplankton groups present within a given water mass. Therefore, accurate estimates of phytoplankton community structure are crucial for understanding global carbon cycles and for anticipating the impact of global climate change. Phytoplankton distributions within the global ocean are typically inferred from bio-optical proxies reflecting phytoplankton physiology (e.g., chlorophyll fluorescence) or concentrations of particulate carbon (e.g., attenuation coefficients or optical backscatter coefficients). The deployment of biogeochemical Argo (BGC-Argo) autonomous profiling floats is transforming the global coverage of these measurements, providing new insight into connections between phytoplankton distributions and export processes. However, few studies have evaluated relationships between bio-optical proxies as measured by BGC-Argo floats and direct measurements of phytoplankton community structure, and none have performed an intercomparison of these relationships across different ocean basins. Therefore, I propose to 1) utilize a combination of novel and existing flow cytometry methods to estimate group-specific phytoplankton biomass across four disparate ocean provinces, 2) evaluate the bio-optical proxies that best predict group-specific phytoplankton biomass within each province, 3) leverage relationships identified in objective 2 to develop a novel transfer function for estimating phytoplankton biomass based on bio-optical proxies alone, and 4) apply this function to existing BGC-Argo data to quantify the importance of variability in phytoplankton community structure to carbon export to the mesopelagic zone, as compared to other mechanisms.