Evaluating long-term changes in phytoplankton community composition using an ocean reflectance inversion model: A case study in the northern Arabian Sea to explore the emerging frontier of hyperspectral ocean color

Ocean reflectance inversion models (ORMs) provide a mechanism for inverting the color of the water observed by a satellite into marine inherent optical properties, which can then be used to study phytoplankton community structure. Most ORMs effectively separate the total signal of the collective phytoplankton community from other water column constituents; however, few have been shown to effectively identify individual contributions by multiple phytoplankton groups over a large range of environmental conditions. Here, we evaluate the ability of an ORM to discriminate between two phytoplankton communities – Noctiluca miliaris and diatoms – under conditions typical of the northern Arabian Sea. The recent emergence of N. miliaris relative to diatoms in this region has begun to alter predator-prey relationships of higher trophic levels, which in turn has the potential to alter carbon export to the deep ocean. In this presentation, we show a series of simulation experiments and time-series analyses to demonstrate the existing capabilities and limitations of heritage ocean color satellites to monitor phytoplankton community structure using an ORM and to explore what will be possible with advanced hyperspectral space-borne instruments. Using lessons learned from our study region, we also describe the challenges of using an ORM when environmental conditions are highly variable, with a nod to emergent technologies to overcome this, and outline the difficulties of producing time-series of phytoplankton dynamics on temporal and spatial scales that accommodate seeing ephemeral features.