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: C 1 , the community in the mixed‐layer; C 2 , at the deep Chl a maximum; and C 3 , in low‐oxygen waters at the base of the euphotic zone. We find that C 3 has a similar chl‐specific particulate backscattering to C 2, both lower than C 1 . C 2 and C 3 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 C 3 is lower than C 2 (located at around 60 m), and yet, C 3 makes similar contributions to integrated stocks, because it has a broader peak than C 2 . In relation to C 1 and C 2 , C 3 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.