# Low-latitude mesopelagic nutrient recycling controls productivity and export

Keith Rodgers, Olivier Aumont, Katsuya Toyama, Laure Resplandy, Masao Ishii, Toshiya Nakano, Daisuke Sasano, Daniele Bianchi, Ryohei Yamaguchi

► To cite this version:

Keith Rodgers, Olivier Aumont, Katsuya Toyama, Laure Resplandy, Masao Ishii, et al. Low-latitude mesopelagic nutrient recycling controls productivity and export. Nature, 2024, 632 (8026), pp.802-807.
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# Low-latitude mesopelagic nutrient recycling controls on productivity and export

**Keith Rodgers**

keithbrodgers@gmail.com

Center for Climate Physics, Institute for Basic Science https://orcid.org/0000-0002-6465-8923

**Olivier Aumont**

IRD

**Katsuya Toyama**

Climate and Geochemistry Research Department, Meteorological Research Institute, Japan
Meteorological Agency

**Laure Resplandy**

Princeton University https://orcid.org/0000-0002-1212-3943

**Masao Ishii**

Meteorological Research Institute https://orcid.org/0000-0002-7328-4599

**Toshiya Nakano**

Nagasaki Ocean Academy, NPO Nagasaki Marine Industry Cluster Promotion Association
https://orcid.org/0000-0003-1846-907X

**Daisuke Sasano**

Atmosphere and Ocean Department, Japan Meteorological Agency https://orcid.org/0000-0002-7228-823X

**Daniele Bianchi**

University of California Los Angeles https://orcid.org/0000-0002-6621-0858

**Ryohei Yamaguchi**

Japan Agency of Marine-Earth Science and Technology https://orcid.org/0000-0002-7800-5798

## Physical Sciences - Article

**Keywords:**

**Posted Date:** September 28th, 2023

**DOI:** https://doi.org/10.21203/rs.3.rs-3341728/v1
**License:** © Ⓟ This work is licensed under a Creative Commons Attribution 4.0 International License.

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**Additional Declarations:** There is NO Competing Interest.

**Version of Record:** A version of this preprint was published at Nature on August 21st, 2024. See the published version at https://doi.org/10.1038/s41586-024-07779-1.
**Abstract**

Low-latitude oceans (30°S-30°N) account for approximately half of the global net primary production and export. A long-standing paradigm holds that the Southern Ocean dominates low-latitude primary production and export, with implications for the response of global primary production and export to climate change. Here we apply observational and model analyses to argue instead that 66% of low-latitude primary production and 50% of export is controlled by local mesopelagic macronutrient recycling. An additional supply of nutrients locally upwelled from the deep ocean balances the loss of nutrients to the deep ocean via the soft tissue pump, with only a 5% supply through shallow (considered over the upper 870m) transfer from the Southern Ocean. Further analyses of a set of five CMIP6 models run to 2300 under high emissions reveal significant disparities in their projections of not only the amplitude but also the sign of low latitude primary production, with temperature-dependent remineralization promoting enhanced low-latitude mesopelagic nutrient retention and associated increases in primary production through low-latitude overturning and upwelling. This underscores the importance of mechanistic understanding of mesopelagic remineralization and its sensitivity to ocean warming for predicting future ecosystem changes.

**Main text**

The low-latitude ocean regions, considered as the expanse between the subtropical convergence zones (30°S-30°N), are important for both their rich biodiversity as well as for fisheries and food security¹⁻³. Both marine primary productivity (PP) and export over this region account for approximately half of their respective global totals⁴. Large PP and export occur in low-latitude oceans despite thermocline macronutrient concentrations being less than half of what is found in subpolar/circumpolar regions, indicating vigorous decadal renewal timescales for thermocline waters through the shallow overturning circulation⁵⁻⁷. An Earth system model has previously been considered under strong anthropogenic forcing (historical/RCP85) to argue that low-latitude (30°S-30°N) export and PP could decrease by 41% and 24% respectively by 2300 relative to the present climate state⁸. This collapse was attributed to a cascade of processes, initially triggered by sea ice retreat over the Southern Ocean, which in turn induced Southern Ocean subsurface nutrient trapping. Reduced surface nutrient concentrations in the Southern Ocean in turn lead to reduced nutrients exported northward to the low-latitude thermocline in mode and intermediate waters. This work is important in emphasizing a first-order role for a low-latitude response in PP and export under multi-century climate perturbations. The interpretation, however, invoked the mechanisms presented in an earlier analysis⁹, which found that 75% of low-latitude new production is contingent on a sustained supply of nutrients from the Southern Ocean using an ocean biogeochemistry model. A subsequent study with an inverse ocean biogeochemical model identified a contribution of 44% to PP to the north of 40°S as being due to Southern Ocean nutrient trapping¹⁰, thereby also arguing for a first-order role for the Southern Ocean. However, independent observational analyses indicate that at least for the case of the Pacific basin, the Subantarctic Mode Water mass cannot readily access regions north of 20°S due to strong potential vorticity barriers¹¹.
We are thereby motivated to propose that low-latitude PP and export are sustained primarily by
processes local to the tropics, and that the sensitivity to non-local perturbations over the Southern Ocean
is more modest than that previously proposed ⁹. In particular, we propose that remineralization within the
low-latitude mesopelagic domain (LLMD) (defined here to span 30°S-30°N and 150m-870m) and rapid
subsequent re-emergence of nutrients through the shallow overturning structures within the low-
latitudes are the dominant driver of low-latitude PP and export. This is investigated through the
combined use of in-situ observations and a new suite of model sensitivity studies with a fully prognostic
global ocean biogeochemistry model.

We examine the extent to which local nutrient recycling in the tropics contributes to sustaining low-
latitude PP and export by quantifying its contribution to the total low-latitude mesopelagic nutrient
inventory (here we use phosphate, or PO₄³⁻) using an observational product based on hydrographic
measurements, GLODAPv2¹²,¹³. We invoke a well-known diagnostic that decomposes total PO₄³⁻
concentrations¹⁴,¹⁵ into two components: a preformed component that represents the PO₄³⁻
concentrations at the surface when a new water mass is transferred from the surface into the interior by
subduction, and a regenerated component that reflects the cumulative impact of respiration of organic
matter that increases PO₄³⁻ concentrations as the water mass is transported in the ocean interior.

The distribution of PO₄³⁻ on the mid-thermocline density horizon s₀=25.5 kg m⁻³ in the ocean interior (Fig. 1a) reveals concentrations that are maximum in the equatorial band. Over the same density horizon the regenerated fraction of PO₄³⁻ (Fig. 1b) is larger than 50 percent over much of the region between 20°S and 20°N, with exceptions (minimum values) neighboring subduction regions. On the density layer associated with the upper boundary of Subantarctic Mode Water (SAMW) formed in the Southern Ocean and entering the Pacific Ocean just below the thermocline (s₀=26.8 kg m⁻³)¹⁶, the PO₄³⁻ concentrations are higher than in the thermocline (Fig. 1c). Additionally, the regenerated fraction of PO₄³⁻ is modestly higher than 50% throughout most of the tropics (Fig. 1d). Viewed north-south along 150°W in the Pacific, despite the large vertical gradients in PO₄³⁻ concentrations across the equatorial thermocline (Fig. 1e), the regenerated fraction exhibits remarkable homogeneity with values just above 50% over the region 10°S-10°N and 150m-700m. Further sensitivity analysis with varying stochiometric ratios of O₂: PO₄³⁻ indicates that the substantial regenerated pool of PO₄³⁻ identified here is relatively insensitive to our choice of a ratio (Extended Data Fig. 1),

Here we hypothesize that the large regenerated nutrient pool in the low-latitudes provides a dominant
control on the elevated low-latitude PP and export through local processes. To test this hypothesis, we
apply a set of perturbation simulations with the PISCES-v2 biogeochemistry model¹⁷ (see methods) that
all use identical circulation fields but that vary by perturbing nutrient remineralization in distinct latitude
bands. In PISCES-v2, the export of organic material from the euphotic zone to the interior of the ocean is
represented by the gravitational sedimentation of two particulate pools that differ by their mean size, and
thus by their sinking speed. These particulate pools are continuously solubilized to dissolved organic
matter according to a temperature-dependent rate. This dissolved pool is then ultimately remineralized
to inorganic nutrients. In addition to a standard experiment (STD)¹⁷, we performed a series of
perturbation experiments in which remineralization of nutrients and solubilization of organic carbon is
disallowed within specific latitude-bounded regions (see Methods for model perturbation details). For
PERT_TROP, solubilization was disallowed within the tropics over the depths comprising mode and
intermediate waters (30°S-30°N, 150m-1500m). For PERT_SOUTH and PERT_NORTH, solubilization was
disallowed between 150m and the next-to-bottom grid-box of the model domain, over 90°S-30°S and
30°N-90°N, respectively.

A comparison of the STD and TROP_PERT experiments reveals a large reduction in PP and export over the low-latitude domain when local mesopelagic remineralization is disallowed (Fig. 2). For STD, PP and export are 21.5 PgC yr⁻¹ and 2.73 PgC yr⁻¹, respectively, when integrated over 30°S-30°N. For PERT_TROP, PP and export are 7.3 and 1.37 PgC yr⁻¹, respectively, over the same domain. Thus PERT_TROP represents a 66% drop in PP and a 50% drop in export relative to STD over 30°S-30°N. These changes are larger than the 17% reduction in PP and 38% decrease in export found for the PERT_SOUTH case (maps showing perturbation patterns for PERT_SOUTH and PERT_NORTH are shown in Extended Data Figs. 2 and 3, respectively). Proper attribution of the strong sensitivities under PERT_TROP relative to STD is facilitated by a coarse-graining of the modeled PO₄³⁻ budget in the LLMD spanning 30°S-30°N and 150m-870m (Fig. 3; the full budgets for all four simulations are shown in Extended Data Fig. 4). For STD (budgets in black) PP is 21.5 PgC yr⁻¹, the vertical flux of soft tissue and dissolved organic phosphorous (DOP) into the LLMD is 2.73 PgC yr⁻¹, while the soft tissue flux out of the LLMD at 870m is 0.88 PgC yr⁻¹ (DOP fluxes are negligible), with a net solubilization source within the LLMD (largest in the shallower thermocline layers) of 1.72 PgC yr⁻¹. As such, the regenerated PO₄ represents 63% of the net upward flux of PO₄³⁻ across the 150m horizon within the tropics. The net meridional fluxes (the sum of both northward and southward fluxes) of PO₄³⁻ across 30°S and 30°N, on the other hand, present a net divergence that depletes PO₄³⁻ over the LLMD. For the PERT_TROP case (red numbers) low-latitude remineralization was disallowed between 150m-1500m, and the -0.1 PgC yr⁻¹ of interior production corresponds to a degree of PP occurring just below 150m depth. The increase in the upward flux of PO₄³⁻ across 870m by 0.12 PgC yr⁻¹ and the net reduction of meridional divergence across 30°S and 30°N by 0.2 PgC yr⁻¹ indicate that there are feedbacks that reduce the loss of PO₄³⁻ and thereby dampen the drop in export for PERT_TROP.

The PERT_TROP perturbation relative to STD (Fig. 3) underscores that the dominant process sustaining elevated low-latitude PP and export is local resolubilization (mainly through remineralization) of PO₄³⁻ locally within the local LLMD, in conjunction with the subsequent local re-emergence into the low-latitude euphotic zone. The upward net flux of PO₄³⁻ across 870m into the LLMD plays a critical supporting role in balancing not only the downward particle flux across 870m, but also in compensating for the net meridional divergence of PO₄³⁻ out of the LLMD across 30°S and 30°N. A previous study¹⁸ considered a
role for remineralization in sustaining low latitude new production, but under the assumption that
regenerated nutrients are a downstream legacy of new/preformed nutrients supplied through SAMW,
thereby not accounting for a sizable low-latitude remineralized pool of PO₄³⁻ sustained through local low-
latitude processes. The same study also ruled out a role for supply from the deep ocean.

In our interpretation, the remineralized low-latitude pool is a legacy of the deeper low-latitude upwelling of preformed PO₄³⁻ that is originally sourced in the deep-water formation regions of the Southern Ocean. The net upwelling flux of 8 Sv across 870m in the global tropics (Extended Data Fig. 5) in the model circulation state is consistent with a deep upwelling cell in the low-latitudes that has been identified with observational constraints¹⁹ and is also characteristic of a broad range of climate models²⁰,²¹. In fact the main impact of our PERT_SOUTH experiment is to enhance the net divergence (0.4 PgC yr⁻¹ relative to STD) of the regenerated PO₄³⁻ pool from the low-latitudes (Extended Data Fig. 4).

Our results also motivate us to reconsider previous modeling work showing substantial low-latitude PP
and export decreases under sustained warming to the year 2300 ⁸. Here we consider five CMIP6
models: IPSL-CM6A²², CESM2-WACCM²³, UKESM1²⁴, ACCESS-ESM1.5²⁵, and MIROC-ES2L²⁶, run to
2300 under high emissions forcing (SSP5-8.5)²⁷. Anomalies of primary production for each model
relative to the period 1850-1900 are shown in Fig. 4a (corresponding thermal fields are shown in
Extended Data Fig. 6), revealing a pronounced divergence in not only the amplitude of PP but also the
sign of the response. This divergence of the global response across CMIP6 models is best interpreted as
a measure of the very large uncertainty in PP, under the assumption that each of the models presents an
equally likely outcome under strong and persistent anthropogenic climate perturbations. The divergence
of the globally integrated primary production (Fig. 4a) is in fact largely sustained over 30°S-30°N (Fig.
4b), with the low-latitude response reflecting an underlying diversity of low-latitude mechanisms or
process-parameterizations²⁸,²⁹. As the increase in PP over 30°S-30°N for two of the models (IPSL-CM6A
and ACCESS-ESM1.5) is not at face value consistent with the Southern Ocean-triggered nutrient trapping
mechanism emphasized previously⁸, we address whether the low latitude nutrient recycling mechanism
highlighted in Figs. 2 and 3 may be contributing.

To this end, we consider the fractional changes in zonal mean ocean interior PO₄³⁻ concentrations for the
cases of the IPSL-CM6A (Fig. 4c) (increasing low-latitude PP) and CESM2-WACCM (Fig. 4d) (decreasing
low-latitude PP) models between the 1990s and 2290s. For a mean over the 1990s with both models, the
transfer efficiency defined as the ratio of the soft tissue flux at 780m relative to 150m, is 27% for IPSL-
CM6A and 29% for CESM2-WACCM over the tropics (Extended Data Tables 1 and 2). However, by the
2290s, transfer efficiency over the tropics has reduced to 21% for IPSL-CM6A (enhanced thermocline
nutrient retention) and has increased to 34% in CESM2-WACCM (decreased thermocline nutrient
retention), as is reflected in the perturbations to zonally averaged PO₄³⁻ between the 1990s and 2290s
for both models in Fig. 4. This difference between the two models reflects the fact that remineralization
in PISCES-v2 is temperature-dependent and thereby enhanced under sustained warming, whereas for the
MARBL biogeochemistry component of CESM2-WACCM the temperature dependence of remineralization remains static for CMIP6 transient simulations³⁰. As such, these differences in representation of remineralization and ensuing thermocline nutrient retention contribute to a modest increase in export for IPSL-CM6A of 0.3% by the year 2300, and a reduction by 38% for CESM2-WACCM. This underscores the importance of improved process-understanding and model representation of remineralization for projecting future changes in marine biological production and export³¹, and as a consequence the ability of marine ecosystems to sustain food provision and other services³².

# METHODS

## Observation-derived products

For the analysis of the resolubilized fractions of nutrients, we use GLODAPv2¹²,¹³ data product. The repeat hydrographic observational components of this global synthesis product have been consistently quality-controlled for comparisons between cruises conducted by different laboratories.

For the analysis in Fig. 1, where the large pool of regenerated $PO_4^{3-}$ is identified for the low-latitude mesopelagic domain, the identification of this regenerated pool makes use of a deconvolution of preformed and regenerated fractions of $PO_4^{3-}$ using a long-standing method¹⁴ for the case of $NO_3^-$, through the use of apparent oxygen utilization (AOU). We define the regenerated fraction of $PO_4^{3-}$ to be

$$PO4\_regen = (PO_4^{3-} - PO_4^{3-*})/ PO_4^{3-}$$

Where $PO_4^{3-}$ is the preformed $PO_4^{3-}$ concentration inferred as:

$$PO_4^* = PO_4^{3-} - AOU*170.$$

Where the 170 multiplier in the latter expression represents a widely-used stochiometric ratio³³. The uncertainty in the amplitude of $PO_4\_regen$ due to uncertainty in the stochiometric ratio is described in Extended Data Fig. 1, where it is seen that the identification of the large pool of regenerated $PO_4^{3-}$ ($PO_4\_regen>50\%$) is robust to uncertainty in the values for the stochiometric ratio, where we have used the value of 138³⁴ for comparison. When applying this framework to ocean interior fields, one usually makes two assumptions: (i) $O_2$ concentrations are at saturation with atmospheric $O_2$ in the mixed layer of the ocean at the time of subduction, and (ii) ocean interior circulation is to first order a largely adiabatic process along constant density (isopycnal) horizons, and (iii) there is a relatively constrained relationship between the rate of $O_2$ consumption and $PO_4^{3-}$ production through respiration, such that a
deconvolution of the preformed and regenerated fractions can be identified using temperature, salinity, and $O_2$ concentrations, along with $PO_4^{3-}$ concentrations.

## NEMO-PISCES model simulations

The biogeochemical model considered here is PICES-v2 (Pelagic Interactions Scheme for Carbon and Ecosystem Studies, version 2)¹⁷. The model is designed to simulate the lower trophic levels of ocean ecosystems (phytoplankton, microzooplankton, and mesozooplankton), as well as the marine biogeochemical cycling of carbon and the major nutrients (P, N, Fe, and Si). The runs considered here represent the coupling of PICES-v2 to the ORCA2 configuration of the Nucleus for European Modelling of the Ocean (NEMO) model version 3.6. The ORCA2 configuration has nominally two-degree horizontal resolution with meridional refinement to 0.5° near the equator, with 30 vertical levels. As with the presentation study for PICES-v2¹⁷, the PICES-v2 simulations use identical five-day averaged climatological physical state variables to drive the biogeochemical model offline. Each of the four runs described in the main text was run for more than 5000 years.

In addition to the control (STD) simulation for which the PICES_v2¹⁷, the three perturbation simulations were conducted by disallowing solubilization of organic carbon, $PO_4^{3-}$, and $NO_3^-$ in the ocean interior (below 150m depth) within three specific latitude bands. The horizon of 150m was chosen to define the base of the euphotic zone. For PERT_NORTH this is applied over 30°N-90°N, for PERT_SOUTH over 90°S-30°S, and for PERT_TROP over 30°S-30°N. For PERT_NORTH and PERT_SOUTH is deactivated between 150m and the bottom model grid cell in the model. For the PERT_TROP simulation, it was decided to disallow remineralization over the depth range 150m-1500m, with this depth range including all water masses comprised by the wind-driven circulation, ranging from thermocline to subpolar mode waters to intermediate waters.

## Declarations

### Acknowledgments:

The authors would like to thank Jorge Sarmiento for his iterative feedback and constructive comments through the development of this project. We would also like to thank Lester Kwiatkowski for his feedback and constructive comments.

### Funding:

KBR was supported by the Institute for Basic Sciences (IBS), Republic of Korea, under IBS-R028-D1 (KR).

DB acknowledges support from US NSF grant OCE-1847687.

KT and MI received support from the Japan Meteorological Research Institute research fund C4 for the study of ocean biogeochemistry and acidification.
**Author contributions:**

Conceptualization: KBR, OA

Methodology: KBR, OA

Investigation: KBR, OA, KT, DS

Visualization: KBR, OA, KT, RY

Writing – original draft: KBR, OA

Writing – review & editing: KBR, OA, KT, LR, MI, TN, DS, RY, DB

**Competing interests:** Authors declare that they have no competing interests.

**Data and materials availability:** The data and code used in the study will be made publicly available upon publication of the article.

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Figures
<figure><img src="image_2.png" /><figcaption>a</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>PO4_regen at 150°W (P16)</figcaption></figure>

<caption>Figure 1</caption>

**Regenerated fraction of phosphate (PO₄³⁻) from observation-based products.** From the GLODAPv2 observational product¹²,¹³: **a.** Phosphate (PO₄³⁻) concentrations on a mid-thermocline density horizon s₀=25.5 kg m⁻³, **b.** regenerated fraction of PO₄³⁻ (PO4_regen) on the s₀=25.5 kg m⁻³ horizon defined as PO4_regen=( PO₄³⁻ - PO₄³⁻*)/ PO₄³⁻, and a stochiometric ratio (O₂: PO₄³⁻) of 170, **c.** PO₄³⁻ concentrations at the upper boundary of Subantarctic Mode Water at s₀=25.5 kg m⁻³, **d.** PO4_regen on s₀=26.8 kg m⁻³, **e.** PO₄³⁻ concentrations at 150°W (P16) in the Pacific, and **f.** PO4_regen along 150°W in the Pacific. The sensitivity of PO4_regen to the choice of the stochiometric ratio of O₂: PO₄³⁻ is documented in Extended Data Fig. 1. PO₄³⁻ is in units of mmol kg⁻¹, while PO4_regen is unitless.
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

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<figure><img src="image_13.png" /><figcaption>Figure 2</figcaption></figure>

**Model-derived sensitivity of primary production (PP) and export to suppression of nutrient regeneration over the low-latitudes.** PISCES-v2¹⁷ model sensitivity for primary production (PP) and export at 150m to suppression of nutrient regeneration over the low-latitude region spanning 30°S-30°N, 150m-1500m. **a.** Annual mean PP for control (STD) experiment, **b.** annual mean export for STD experiment; **c.** PP for PERT_TROP experiment; **d.** export for PERT_TROP experiment; **e.** difference in PP between PERT_TROP and STD; **f.** difference in export between PERT_TROP and STD. PP is considered for full-depth integrals, and export is considered at 150m, with this being the upper boundary defined for the LLMD shown in Fig. 3. Units are in molesC m⁻² yr⁻¹.
<figure><img src="image_14.png" /><figcaption>Figure 3</figcaption></figure>

Coarse-grained view of modeled PO₄³⁻ fluxes about the low-latitude mesopelagic domain (LLMD). The boundary and interior sources of PO₄³⁻ shown as a coarse-graining of the domain bounded by 30°S-30°N and 150m-870m comparing the STD (black) and PERT_TROP (red) cases with PISCES-v2¹⁷ shown in Fig. 2. The lateral boundary fluxes for the domain represent net fluxes, thereby accounting for the effects of gyre recirculation across 30°S and 30°N. The transfer efficiency, defined as the ratio of export flux across 870m to the export flux across 150m, is 32%. Fluxes are for PO₄³⁻, but shown in carbon units of PgC yr⁻¹ to be consistent with units typically used for export and the biological pump.
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>Figure 4</figcaption></figure>

**Response of CMIP6 Earth system models to sustained strong forcing to 2300.** Evolution of changes (each relative to their respective 1850-1899 mean) of **a.** globally integrated PP and **b.** PP integrated over 30°S-30°N for five CMIP6 models [IPSL-CM6A<sup>22</sup>, CESM2-WACCM<sup>23</sup>, UKESM1<sup>24</sup>, ACCESS-ESM1.5<sup>25</sup>, and MIROC-ES2L<sup>26</sup>] run to 2300 under historical/SSP5-8.5 forcing<sup>27</sup>. Fractional changes in zonally averaged PO<sub>4</sub><sup>3-</sup> concentrations between a 1990s climatology and a 2290s climatology for two of the models **c.** IPSL-CM6A and **d.** CESM2-WACCM.

**Supplementary Files**

This is a list of supplementary files associated with this preprint. Click to download.

* ExtendedData.docx