| 21 Jul 2026
Chlorophyll–particle backscattering displacement reveals multiple deep chlorophyll maximum structures in the North Pacific
Abstract. Subsurface chlorophyll-a (Chl) maxima are common features of stratified oceans, but a Chl maximum does not necessarily indicate a maximum in particle-associated optical structure. Here we use Biogeochemical-Argo (BGC-Argo) observations from the North Pacific during 2022–2025 to examine how the previously recognized separation between Chl maxima and particle-associated optical structure is expressed across a large set of vertically resolved in situ profiles. For 10,801 usable paired Chl and particulate backscattering at 700 nm (BBP700) profiles, we diagnosed the depths of the Chl DCM (deep chlorophyll maximum) and BBP700 maximum within the upper 300 dbar and classified profiles by their vertical displacement. Coupled profiles, in which the two maxima occurred within 25 dbar of each other, were the most frequent class and had median Chl DCM and BBP700 maximum depths of 52.5 and 47.5 dbar, respectively. In Chl deeper profiles, the Chl DCM was displaced downward relative to a shallower BBP700 maximum, consistent with a stronger contribution of photoacclimation or pigment adjustment to the Chl maximum. In Chl shallower profiles, a shallow Chl maximum coexisted with a deeper BBP700 maximum, demonstrating that Chl–BBP700 decoupling is not unidirectional. The three displacement classes showed coherent basin-scale spatial organization and distinct nitracline contexts, with the nitracline generally deeper than both optical maxima in Coupled and Chl deeper profiles, whereas Chl shallower profiles tended to show a deeper BBP700 maximum near the nitracline depth range. Together, these findings indicate that North Pacific Chl DCMs do not represent a single uniform vertical bio-optical structure. Instead, the previously recognized separation between Chl maxima and particle-associated optical structure is expressed in mature BGC-Argo observations as coherent vertical, spatial, and nitracline-related structures across the North Pacific.
Dear Editor and Referees,
While re-checking our archived figure source tables after posting of the preprint, we identified an inconsistency in the manuscript text, which we wish to report proactively.
The text states that the analysis used 10,801 usable paired Chl-BBP700 profiles (Abstract; Sect. 2.1; Sect. 3.2; Sect. 3.4). The correct number is 10,740. This is the population underlying all figures and all other reported statistics: the three displacement classes comprise 6,973 Coupled, 3,037 Chl deeper and 730 Chl shallower profiles (Fig. 3), which sum to 10,740.
The value 10,801 predates the final Pacific basin-mask correction, in which profiles lying outside the connected Pacific domain (Gulf of Mexico and Caribbean side) but inside the initial rectangular longitude-latitude window were removed. All figures, source tables and derived statistics were recomputed after that correction, but the total profile count was not updated in the four places listed above.
We have re-verified every other quantitative statement in the manuscript against the archived source tables in the Zenodo package (DOI: 10.5281/zenodo.20873450), and all of them are consistent with the corrected population: median Chl DCM / BBP700 maximum / displacement values of 52.5 / 47.5 / +5.0 dbar (Coupled), 107.5 / 47.5 / +50.0 dbar (Chl deeper) and 17.5 / 97.5 / -55.0 dbar (Chl shallower); class sample sizes of 6,973 / 3,037 / 730; 663 grid cells containing 10,630 profiles in the gridded class-fraction summary; and 9,402 profiles with a successful nitracline diagnosis, comprising 6,150 Coupled, 2,651 Chl deeper and 601 Chl shallower profiles.
No figure, no source table and no conclusion is affected. The correction concerns the reported total profile count only, and we will implement it throughout the revised manuscript.
We apologise for the oversight and thank the Editor and Referees for their time.
Taketo Hashioka and Maki N. Aita