A Conceptual Approach to Partitioning a Vertical Profile of Phytoplankton Biomass Into Contributions from Two Communities

Abstract
We describe an approach to partition a vertical profile of chlorophyll-a concentration into contributions from two communities of phytoplankton: one (community 1) that resides principally in the turbulent mixed-layer of the upper ocean and is observable through satellite visible radiometry; the other (community 2) residing below the mixed-layer, in a stably stratified environment, hidden from the eyes of the satellite. The approach is tuned to a time-series of profiles from a Biogeochemical-Argo float in the northern Red Sea, selected as its location transitions from a deep mixed layer in winter (characteristic of vertically well-mixed systems) to a shallow mixed layer in the summer with a deep chlorophyll-a maximum (characteristic of vertically stratified systems). The approach is extended to reproduce profiles of particle backscattering, by deriving the chlorophyll-specific backscattering coefficients of the two communities and a background coefficient assumed to be dominated by non-algal particles in the region. Analysis of the float data reveals contrasting phenology of the two communities, with community 1 blooming in winter and 2 in summer, community 1 negatively correlated with epipelagic stratification, and 2 positively correlated. We observe a dynamic chlorophyll-specific backscattering coefficient for community 1 (stable for community 2), positively correlated with light in the mixed-layer, suggesting seasonal changes in photoacclimation and/or taxonomic composition within community 1. The approach has the potential for monitoring vertical changes in epipelagic biogeography and for combining satellite and ocean robotic data to yield a three-dimensional view of phytoplankton distribution

Citation
Brewin, R. J. W., Dall’Olmo, G., Gittings, J., Sun, X., Lange, P. K., Raitsos, D. E., Bouman, H. A., Hoteit, I., Aiken, J., & Sathyendranath, S. (2022). A conceptual approach to partitioning a vertical profile of phytoplankton biomass into contributions from two communities. Journal of Geophysical Research: Oceans. Portico. https://doi.org/10.1029/2021jc018195

Acknowledgements
We acknowledge the Remotely Sensed Biogeochemical Cycles in the Ocean (remOcean) project, funded by the European Research Council (GA 246777), and thank Hervé Claustre and all others involved in the deployment of the BGC-Argo float (WMO number 6901573) used in the study. This work was supported by: a UKRI Future Leader Fellowship (MR/V022792/1); the European Space Agency (ESA) project Biological Pump and Carbon Exchange Processes (BICEP); by the Simons Foundation Project Collaboration on Computational Biogeochemical Modeling of Marine Ecosystems (CBIOMES; 549947, SS); and by the King Abdullah University for Science and Technology (KAUST) Office of Sponsored Research (OSR) under the Virtual Red Sea Initiative (Grant # REP/1/3268-01-01). Additional support from the UK National Centre for Earth Observation is acknowledged. J.A. Gittings is funded by ESA under the Living Planet Fellowship program (POSEIDON). G. Dall'Olmo was supported by funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 862923. This output reflects only the author's view, and the European Union cannot be held responsible for any use that may be made of the information contained therein. We thank four anonymous reviewers for their constructive comments on our paper. This work is dedicated to the memory of Trevor Platt for his support at the outset

Publisher
American Geophysical Union (AGU)

Journal
Journal of Geophysical Research: Oceans

DOI
10.1029/2021jc018195

Additional Links
https://onlinelibrary.wiley.com/doi/10.1029/2021JC018195

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