| 12 Oct 2026
From Surface Warming to Vertical Phytoplankton Reorganization in the North Pacific Subtropical Gyre
Abstract. The North Pacific Subtropical Gyre (NPSG) has experienced sustained warming and biological change in recent decades, yet how these changes through the water column are less well resolved. Here, we combined satellite observations from 1998–2024 with physical and biogeochemical Argo data and a long-term record from Station ALOHA to examine changes across the upper 300 m. Surface chlorophyll declined as sea-surface temperature increased. A marked physical transition around 2013–2014 involved changes in mixed-layer depth, ocean heat content, density structure, and the separation between the mixed layer and euphotic-zone depth. Subsequent biological changes were strongly depth-dependent, with chlorophyll declining most in the 75–105 m range, and the deep chlorophyll maximum deepened despite a comparatively stable euphotic-zone depth. These chlorophyll changes did not translate directly into changes in phytoplankton carbon biomass. In the upper water column, recent carbon biomass increased even as chlorophyll declined, while increases in chlorophyll relative to carbon at depth suggested that the deepening chlorophyll maximum did not simply represent a deeper biomass maximum. Station ALOHA observations also revealed depth-dependent changes in phytoplankton community composition that may have added to these patterns. Together, these results show that long-term warming in the NPSG has occurred alongside a reorganization of upper-ocean physical structure and phytoplankton vertical distribution, rather than a uniform decline in phytoplankton biomass through the water column.
Review on BG egusphere-2026-5927 by Adroli et al
Under the accelerating effects of global warming, our concerns are growing on the impacts of rising sea surface temperature and strong stratification on the productivity and structure of marine ecosystems and biogeochemical cycles. In this study, the authors examined how physical oceanographic properties such as water temperature, salinity, and stratification are changing in the North Pacific and its impacts on phytoplankton in the subtropical gyre of the North Pacific.
They described changes in the vertical physical structure in response to global warming and major events such as the 2013-14 Blob and 2015-16 El Nino and found the different responses with time lag by 3 depth layers (surface – MLD, MLD to Zeu, and Zeu to 300 m). The authors described “These timing suggests that upper-ocean reorganization developed progressively across several years spanning the Blob and subsequent 2015-2016 El Nino, rather than through a single instantaneous transition.” This finding is very important to understand the ecosystem response to global warming and emergent events. The ecosystem responses to the event were also not instantaneous transitions, indeed, which are well described in this ms. The trends and responses to the events on DCM depth (Fig. 2e) and phytoplankton communities (Fig. S3) are important findings for understanding the impact of global warming on marine ecosystems.
The methods used in this study are well described, and careful evaluation of the data is conducted, such as Bio-Argo data from different locations, so creating useful datasets for the targeted analysis. Results were well presented, and scientifically logical conclusions are derived from the data. One of the strengths of this paper is that it clearly distinguishes between conclusions that can be drawn from the data and those that cannot, such as the influence of strong stratification on nutrient supply and the biological carbon pump because of missing vertical mixing parameters.
Although the ms is written well, I have a couple of major concerns.
The first is on Cphyto. The algorithm of Behrenfeld 20025 relies on a fixed scaling factor to convert the backscattering coefficient to phytoplankton carbon. So, phytoplankton community shift to Prochlorococcus (0.5-0.7 um diameter) dominated from one with Synechococcus (~0.8-1.5 um) and picoeukaryotes (>1.5 um) as observed in this study may induce overestimation in Cphyto or incompatibility between the early and later parts of the observation. Please evaluate the impact of the change in phytoplankton size and its influence on the trend obtained. This also influences the Chl:Cphyto ratio.
There are different types of Prochlorococcus with high light adapted (usually occurring in the upper layer) and low light adapted (deep eutphotic layer) (e.g., Biller et al. 2015, https://doi.org/10.1038/nrmicro3378). Their Chrlorphyll:carbon ration is totally different. I also expect some discussion on this issue.
The other is the use of AOU. Because AOU reflects the cumulative biological respiration since the water mass is separated from the air-sea interface, it is ideal for evaluating long-term (years~10 years or longer) oxygen consumption and carbon export rather than short-term fluctuations. In the sea surface, continuous oxygen exchange with the atmosphere quickly erases the biological oxygen signals. Also, rapid seasonal heating or cooling can outpace the rate of oxygen equilibration with the atmosphere, generating artifact AOU values unrelated to biology. Thus AOU is not suitable for capturing annual/interannual biological variability there.
In the subtropical gyre of N.Pacific, non-Redfield biological production, such as nitrogen fixation and the use of organic phosphorus, is common. Dust events or an increase in specific organisms with functional genes that use organic phosphorus destroy the assumption of AOU. I am also concerned about the accuracy of P sensors and the inter-sensor variation, especially such low P concentrations. I’m not sure AOU is essential to this study. If the authors use AOU data for this ms, need further quality control of the Bio Argo P data and to reply to my concerns above.
Compared to the detailed description and discussion of the change in the vertical physical properties in the upper water column, the discussion on the depth-resolved annual and monthly biological trend (Fig. S3) is brief. There are several studies investigating phytoplankton distribution and community around DCM (e.g., Sato et al. 2022 https://doi.org/10.1002/lno.12222; Cornec et al. 2021 https://doi.org/10.1029/2020GB006759 ). It is worth discussing the findings of this study in more detail.
Minor comments:
What is the definition of Zeu? 1% light depth to the surface?
Salinity: In Supplement, AS is used (Fig S5) as an abbreviation of “Absolute Salinity” but SA is used in the text (eg, page 16). Unify the term. However, the ARGO or ALOHA data is practical salinity, so just “Salinity” is fine.