Biogeographical Classification of the Global Ocean From BGC‐Argo Floats. Issue 6 (12th June 2022) - Bock, Nicholas
Abstract: Biogeographical classifications of the global ocean generalize spatiotemporal trends in species or biomass distributions across discrete ocean biomes or provinces. These classifications are generally based on a combination of remote‐sensed proxies of phytoplankton biomass and global climatologies of biogeochemical or physical parameters. However, these approaches are limited in their capacity to account for subsurface variability in these parameters. The deployment of autonomous profiling floats in the Biogeochemical Argo network over the last decade has greatly increased global coverage of subsurface measurements of bio‐optical proxies for phytoplankton biomass and physiology. In this study, we used empirical orthogonal function analysis to identify the main components of variability in a global data set of 422 annual time series of Chlorophyll a fluorescence and optical backscatter profiles. Applying cluster analysis to these results, we identified six biomes within the global ocean: two high‐latitude biomes capturing summer bloom dynamics in the North Atlantic and Southern Ocean and four mid‐ and low‐latitude biomes characterized by variability in the depth and frequency of deep chlorophyll maximum formation. We report the distribution of these biomes along with associated trends in biogeochemical and physicochemical environmental parameters. Our results demonstrate light and nutrients to explain most variability in phytoplankton distributions for all biomes, while highlighting a global inverse relationship between particle stocks in the euphotic zone and transfer efficiency into the mesopelagic zone. In addition to partitioning seasonal variability in vertical phytoplankton distributions at the global scale, our results provide a potentially novel biogeographical classification of the global ocean. Plain Language Summary: Due to the enormous spatiotemporal variability in environmental dynamics within the global ocean, understanding and predicting biological processes at broad scales requires identifying ecologically similar marine provinces. However, because such provinces are typically defined using data from satellites or from climatologies averaging many decades of ship‐based measurements, they are limited in their ability to account for environmental variability beneath the sea surface. Over the last 10 years, the development of the global Biogeochemical Argo (BGC‐Argo) networks of autonomous profiling floats has significantly relieved this constraint, providing measurements of physical, bio‐optical, and biogeochemical parameters within the upper kilometer of the ocean. In this study, we explore the potential for using BGC‐Argo time series data to develop a novel biogeographical classification based on the direct measurement of subsurface bio‐optical parameters. We identify six phytoplankton biomes in the global BGC‐Argo data set: two high‐latitude biomes characterized by large phytoplankton blooms during summer months and four mid‐to‐low‐latitude biomes characterized by complex subsurface dynamics reflecting basin‐scale trends in environmental forcings. Our results capture the environmental drivers of phytoplankton distributions at the global scale, in addition to relationships between phytoplankton distributions in the upper sun‐lit layer of the ocean and particle accumulation within the mesopelagic zone. Key Points: Data from Biogeochemical‐Argo provides a novel basis for defining six biogeographical provinces of the global ocean Light and nutrient fields explain most of the variability in phytoplankton distributions for each province Small particle concentrations in the euphotic zone are inversely related to their transfer efficiency toward the mesopelagic zone