Olivier Aumont

The majority of particulate organic carbon in the ocean’s mesopelagic zone is contained in particles smaller than 100 µm. However, the challenges of measuring their abundance and carbon content using shipboard techniques has limited our understanding of their role in the ocean carbon cycle. Here we use a 4D Biogeochemical-Argo based data product to investigate the spatial distribution and seasonality in small particles, as well as their loss in the mesopelagic zone. We find that small particle loss in the mesopelagic is greater at high latitudes than at low latitudes. We examine several hypotheses that might explain this observed pattern, although limitations in the extant data prevent us drawing definitive conclusions. Contrary to expectations, we find that temperature-driven remineralisation is unlikely to be the primary driver of the observed patterns, and instead find that particle lability may be a significant factor in explaining this unexpected result.

Plankton communities are an essential component of ocean biogeochemistry and play a key role in making oceans an important climatic buffer. In the oceans, the environmental control of planktonic activity is modulated by the composition and diversity of plankton physiological traits (e.g., size, temperature and light preferences, stoichiometry, etc.). Yet, very little is known about how plankton communities assemble in the ocean under the combined influence of biological (eco-evolutionary dynamics) and physical mechanisms (mixing, transport). Moreover, this key process is very crudely represented for in current ocean models. Here, I show how integrating Darwinian adaptation into ocean models allows simulating how the functional composition and diversity of plankton communities is shaped by adaptation and ocean physics, how it feeds back on ocean biogeochemistry, and what the implications are for the resilience of marine ecosystems under climate change.

Earth system models (ESMs) highly simplify their representation of biological processes, leading to major uncertainty in the impacts of climate change. Despite a growing understanding of molecular networks from genomic data, describing how changing phytoplankton physiology affects biogeochemical processes remains elusive. Here, we embed genome-scale models within a state-of-the-art ESM to deliver an integrated understanding of how gradients of nutrients modulate the molecular physiology of various plankton. In particular, when applied to Prochlorococcus , we find that glycogen and lipid management can be interpreted in terms of acclimation to different environments. Generalized to other phytoplankton such as the diatom Thalassiosira , we estimate the production of 39 metabolites that constitute hot spots of dissolved organic carbon described by their amount of carbon produced and their diversity of associated metabolites in ESMs. This modeling approach shows how genome scale–enabled ESMs have the potential to advance our understanding of microbial ecosystem functioning in ocean biogeochemical processes.

The Amazon River is an important freshwater and nutrient source for the tropical Atlantic Ocean, sustaining large phytoplankton blooms that extend hundreds of kilometers from the river mouth. However, the main drivers of the productivity in the Amazon plume are still poorly understood. To address this question, a regional coupled ocean‐biogeochemical configuration of the tropical Atlantic at 1/4° is used, as well as remote sensing observations of sea surface salinity and ocean color, which allow an improved model treatment of the organic matter of riverine origin. Results reveal that the offshore extension of the productive plume is mainly driven by the large fluxes of terrestrial dissolved organic matter (TDOM) supplied by the Amazon River. Two mechanisms by which TDOM influences plume extension have been identified. First, the colored fraction limits the light available for phytoplankton growth, thereby delaying nutrient consumption. Second, TDOM supplied by rivers has low lability and is therefore slowly remineralized, providing nutrients throughout the plume until far offshore. The inclusion of these two effects (shading and specific remineralization of TDOM) in the model allows for a better representation of Amazon plume productivity and offshore extension, something that is generally lacking in regional or global biogeochemical models.

A document by Manon Gévaudan. Click on the document to view its contents.

Marine mesozooplankton play an important role for marine ecosystem functioning and global biogeochemical cycles. Their size structure, varying spatially and temporally, heavily impacts biogeochemical processes and ecosystem services. Mesozooplankton exhibit size changes throughout their life cycle, affecting metabolic rates and functional traits. Despite this variability, many models oversimplify mesozooplankton as a single, unchanging size class, potentially biasing carbon flux estimates. Here, we include mesozooplankton ontogenetic growth and reproduction into a 3‐dimensional global ocean biogeochemical model, PISCES‐MOG, and investigate the subsequent effects on simulated mesozooplankton phenology, plankton distribution, and organic carbon export. Utilizing an ensemble of statistical predictive models calibrated with a global set of observations, we generated monthly climatologies of mesozooplankton biomass to evaluate the simulations of PISCES‐MOG. Our analyses reveal that the model and observation‐based biomass distributions are consistent ( = 0.40, total epipelagic biomass: 137 TgC from observations vs. 232 TgC in the model), with similar seasonality (later bloom as latitude increases poleward). Including ontogenetic growth in the model induced cohort dynamics and variable seasonal dynamics across mesozooplankton size classes and altered the relative contribution of carbon cycling pathways. Younger and smaller mesozooplankton transitioned to microzooplankton in PISCES‐MOG, resulting in a change in particle size distribution, characterized by a decrease in large particulate organic carbon (POC) and an increase in small POC generation. Consequently, carbon export from the surface was reduced by 10%. This study underscores the importance of accounting for ontogenetic growth and reproduction in models, highlighting the interconnectedness between mesozooplankton size, phenology, and their effects on marine carbon cycling.

Low-latitude (LL) oceans account for up to half of global net primary production and export^1,2,3,4,5. It has been argued that the Southern Ocean dominates LL primary production and export⁶, with implications for the response of global primary production and export to climate change⁷. Here we applied observational analyses and sensitivity studies to an individual model to show, instead, that 72% of LL primary production and 55% of export is controlled by local mesopelagic macronutrient cycling. A total of 34% of the LL export is sustained by preformed macronutrients supplied from the Southern Ocean via a deeper overturning cell, with a shallow preformed northward supply, crossing 30° S through subpolar and thermocline water masses, sustaining only 7% of the LL export. Analyses of five Coupled Model Intercomparison Project Phase 6 (CMIP6) models, run under both high-emissions low-mitigation (shared socioeconomic pathway (SSP5-8.5)) and low-emissions high-mitigation (SSP1-2.6) climate scenarios for 1850–2300, revealed significant across-model disparities in their projections of not only the amplitude, but also the sign, of LL primary production. Under the stronger SSP5-8.5 forcing, with more substantial upper-ocean warming, the CMIP6 models that account for temperature-dependent remineralization promoted enhanced LL mesopelagic nutrient retention under warming, with this providing a first-order contribution to stabilizing or increasing, rather than decreasing, LL production under high emissions and low mitigation. This underscores the importance of a mechanistic understanding of mesopelagic remineralization and its sensitivity to ocean warming for predicting future ecosystem changes.