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

Abstract

We use data collected by Biogeochemical Argo (BGC‐Argo) float, over a 5‐year period (2016–2021), to study the dynamics of a unique low‐oxygen‐adapted phytoplanktonic community in the eastern tropical North Pacific. We isolate this community using a model that partitions vertical profiles of chlorophyll a (Chl a) and particulate backscattering into the contributions of three communities of phytoplankton: C1, the community in the mixed‐layer; C2, at the deep Chl a maximum; and C3, in low‐oxygen waters at the base of the euphotic zone. We find that C3 has a similar chl‐specific particulate backscattering to C2, both lower than C1. C2 and C3 contribute significantly to integrated stocks of Chl a, both at around 41%, and both around 30% of integrated particulate backscattering (after removing a background signal attributed to nonalgal particles). Found at depths of around 100 m, the peak biomass of C3 is lower than C2 (located at around 60 m), and yet, C3 makes similar contributions to integrated stocks, because it has a broader peak than C2. In relation to C1 and C2, C3 thrives in a lower temperature, higher density, lower light, lower oxygen, and higher saline habitat. This work illustrates how BGC‐Argo floats, in combination with simple conceptual models, can be used to observe the dynamics of unique communities of phytoplankton in extreme environments. The projected climate‐driven changes in oxygen minimum zones add urgency to understand the vulnerabilities of these communities both in terms of stocks and composition.

Climate variability shifts the vertical structure of phytoplankton in the Sargasso Sea

“…Below the surface mixed layer of the seasonally and permanently stratified regions, covering over 70% of the global ocean, resides an understudied subsurface community which satellites cannot see (Cullen, 2015;Cornec et al, 2021). This community can contribute a significant portion of the total water column phytoplankton biomass and production (Bouman et al, 2020;Zhuang et al, 2020;Arteaga et al, 2022;Cox et al, 2023) and can have contrasting phenology to surface communities (Brewin et al, 2022;Hu et al, 2022). Subsurface phytoplankton often forms a deep or subsurface chlorophyll maximum (DCM or SCM) (Cornec et al, 2021), and blooms often occur for longer periods compared to surface communities (Ross et al, 2017).…”

“…This tool has been combined with BGC-Argo floats to observe phytoplankton biomass dynamics below the mixed-layer (Brewin et al, 2022;Cox et al, 2023). However, as these platforms are relatively recent additions to the ocean observing system, they do not yet yield the long-term observations that ship-based observations provide (Bittig et al, 2019).…”