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.

Evaluating historic and modern optical techniques for monitoring phytoplankton biomass in the Atlantic Ocean

“…Phytoplankton may respond to climate change in different ways. For example, through changes in community composition, phenology, metabolic rates, geographical distribution, and vertical structure (Sathyendranath et al, 2017;Brewin et al, 2022) to name a few. Arguably one of the most important metrics to monitor is phytoplankton biomass, considering this metric is explicitly linked to many of these responses.…”

Distinct habitat and biogeochemical properties of low‐oxygen‐adapted tropical oceanic phytoplankton

“…The niche that the low‐oxygen, low‐light phytoplankton occupy makes it difficult to observe these phytoplankton. Satellite remote sensing of ocean color is recognized as an important tool to monitoring phytoplankton (Moisan et al 2017; Sathyendranath et al 2019; Brewin et al 2022). The capability to monitor phytoplankton abundance on global and synoptic scales allows for an assessment of the entire global surface phytoplankton distribution and abundance (Gordon et al 1980; Groom et al 2019).…”

“…The capability to monitor phytoplankton abundance on global and synoptic scales allows for an assessment of the entire global surface phytoplankton distribution and abundance (Gordon et al 1980; Groom et al 2019). Although effective at representing the ocean surface (< 30 m), satellites are unable to observe phytoplankton abundance within stratified regions at depths of 50–200 m (Brewin et al 2022). Monitoring phytoplankton communities below the surface layer in these seasonally and permanently stratified regions (that constitute ~ 70% of the global ocean) requires in‐situ measurements.…”

“…Alongside collecting environmental data, BGC‐Argo floats have optical sensors measuring chlorophyll a (Chl a ) pigment concentration and particulate backscattering (), both of which have been used as measures of phytoplanktonic biomass (Martiny et al 2022). Brewin et al (2022) used a conceptual model to partition vertical profiles of Chl a and , collected using a BGC‐Argo float in the northern Red Sea, into the contributions of two phytoplanktonic communities (C 1 in the surface mixed layer, and C 2 at the deep chlorophyll maximum), to examine their seasonal dynamics.…”

“…Here, the two‐community model of Brewin et al (2022) is extended by adding a third community that thrives in low‐oxygen, low‐light waters at the base of the euphotic zone. By applying the model to data collected by a BGC‐Argo float in the eastern tropical North Pacific (ETNP) over a 5‐year period (2016–2021), we explore the environmental niche and dynamics of this unique third phytoplanktonic community.…”