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The second field campaign of the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was conducted in the late spring of 2021 within the vicinity of the Porcupine Abyssal Plain (49.0°N, 16.5°W) in the North Atlantic Ocean. Observations from EXPORTS support previous characterizations of this system as highly productive and organic matter rich, with the majority of primary production occurring in large cells (≥ 5 µm) such as diatoms that are primarily utilizing nitrate. Rates of total euphotic zone depth-integrated net primary production ranged from 36.4 to 146.6 mmol C m−2d−1, with an observational period average f-ratio of 0.74, indicating predominantly new production. Substantial variability in the contribution of small (

Ocean physics and biology can interact in myriad and complex ways. Eddies, features found at many scales in the ocean, can drive substantial changes in physical and biogeochemical fields with major implications for marine ecosystems. Mesoscale eddies are challenging to model and difficult to observe synoptically at sea due to their fine-scale variability yet broad extent. In this work we observed a frontal eddy just north of Cape Hatteras via an intensive hydrographic, biogeochemical, and optical sampling campaign. Frontal eddies occur in western boundary currents around the globe and there are major gaps in our understanding of their ecosystem impacts. In the Gulf Stream, frontal eddies have been studied in the South Atlantic Bight, where they are generally assumed to shear apart passing Cape Hatteras. However, we found that the observed frontal eddy had different physical properties and phytoplankton community composition from adjacent water masses, in addition to continued cyclonic rotation. In this work we first synthesize the overall ecological impacts of frontal eddies in a simple conceptual model. This conceptual model led to the hypothesis that frontal eddies could be well timed to supply zooplankton to secondary consumers off Cape Hatteras where there is a notably high concentration and diversity of top predators. Towards testing this hypothesis and our conceptual model we report on the biogeochemical state of this particular eddy connecting physical and biological dynamics, analyze how it differs from Gulf Stream and shelf waters even in “death”, and refine our initial model with this new data.Key PointsIn-depth investigation of a frontal eddy in the Gulf Stream off Cape Hatteras, North CarolinaContinued physical and biogeochemical differences are observed between the eddy and adjacent water masses even as it begins to shear apartWe share a conceptual model of the ecological impact of frontal eddies with a hypothesis that they supply zooplankton to secondary consumersPlain Language SummaryFrontal eddies are spinning masses of water (~30km in diameter) that move along western boundary currents like the Gulf Stream. When they form they carry productive coastal water into the Gulf Stream and drive upwelling within their cores. Together this leads to an increase in the amount of phytoplankton within them - much higher compared to surrounding nutrient-limited Gulf Stream water. On the east coast of the United States one common area of frontal eddy formation is just off Charleston, SC. Eddies then travel up the coast and dissipate near Cape Hatteras, NC. In this work we measured a wide range of physical and biological properties of a frontal eddy just north of Cape Hatteras. We compared these properties within the eddy to the coastal water on one side and the Gulf Stream water on the other, finding clear differences in phytoplankton community composition and other physical and chemical properties. Using the results of these observations together with previous studies we share a simple model for how frontal eddies may impact phytoplankton, zooplankton, and fish – hypothesizing that they may contribute to the high diversity and density of top predators off Cape Hatteras.

International audience; In this study, we compare mechanistic and empirical approaches to reconstruct the air-sea flux of biological oxygen (F[O2]bio-as) by parameterizing the physical oxygen saturation anomaly (ΔO2[phy]) in order to separate the biological contribution from total oxygen. The first approach matches ΔO2[phy] to the monthly climatology of the argon saturation anomaly from a global ocean circulation model's output. The second approach derives ΔO2[phy] from an iterative mass balance model forced by satellite-based physical drivers of ΔO2[phy] prior to the sampling day by assuming that air-sea interactions are the dominant factors driving the surface ΔO2[phy]. The final approach leverages the machine-learning technique of Genetic Programming (GP) to search for the functional relationship between ΔO2[phy] and biophysicochemical parameters. We compile simultaneous measurements of O2/Ar and O2 concentration from 14 cruises to train the GP algorithm and test the validity and applicability of our modeled ΔO2[phy] and F[O2]bio-as). Among the approaches, the GP approach, which incorporates ship-based measurements and historical records of physical parameters from the reanalysis products, provides the most robust predictions (R2 = 0.74 for ΔO2[phy] and 0.72 for F[O2]bio-as); RMSE = 1.4% for ΔO2[phy] and 7.1 mmol O2 m-2 d-1 for F[O2]bio-as). We use the empirical formulation derived from GP approach to reconstruct regional, inter-annual, and decadal variability of F[O2]bio-as) based on historical oxygen records. Overall, our study represents a first attempt at deriving F[O2]bio-as) from snapshot measurements of oxygen, thereby paving the way toward using historical O2 data and a rapidly growing number of O2 measurements on autonomous platforms for independent insight into the biological pump.

Droughts and climate-change-driven warming lead to more frequent and intense wildfires, arguably contributing to the severe 2019–2020 Australian wildfires. The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols. Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively. Previous works suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity, but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019–2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find widespread and long-lasting phytoplankton blooms in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content, and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean.

Global climatological map of dissolved iron in the global ocean from publication: "Data-driven modeling of dissolved iron in the global ocean" by Huang et al. 2022. File Monthly_dFe.nc (NC_FORMAT_CLASSIC): 1 variable (excluding dimension variables): double dFe_RF [Longitude, Latitude, Depth, Month] units: nmol L-1 FillValue: NaN long_name: Monthly dissolved iron simulated from random forest algorithm coordinates: [Longitude, Latitude, Depth, Month] 4 dimensions: Longitude Size:357 units: degree_north long_name: Longitude Latitude Size:147 units: degree_east long_name: Latitude Depth Size:31 units: meter long_name: Depth Month Size:13 Units: "Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec", "Annuual mean" long_name: Month 4 global attributes: Author: Yibin Huang & Nicolas Cassar Correspond: nicolas.cassar@duke.edu Request_for_citation: If you use these data in publications or presentations, please cite: “Huang, Y., Tagliabue, A., & Cassar, N. (2022). Data-driven modeling of dissolved iron in the global ocean. Frontiers in Marine Science. doi:10.3389/fmars.2022.837183”. Creation date: March/20th/2022 &nbsp

The NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was established to better quantify the pathways of the biological carbon pump in order to gain a more comprehensive understanding of global carbon export efficiency. The summer 2018 field campaign in the vicinity of Ocean Station Papa (Station P; 50°N, 145°W) in the Northeast Pacific Ocean yielded evidence of low phytoplankton biomass and primary productivity dominated by small cells (

At sea, wind forcing is responsible for the formation and development of surface waves and represents an important source of near-surface turbulence. Therefore, processes related to near-surface turbulence and wave breaking, such as sea spray emission and air–sea gas exchange, are often parameterised with wind speed. Thus, shipborne wind speed measurements provide highly relevant observations. They can, however, be compromised by flow distortion due to the ship's structure and objects near the anemometer that modify the airflow, leading to a deflection of the apparent wind direction and positive or negative acceleration of the apparent wind speed. The resulting errors in the estimated true wind speed can be greatly magnified at low wind speeds. For some research ships, correction factors have been derived from computational fluid dynamic models or through direct comparison with wind speed measurements from buoys. These correction factors can, however, lose their validity due to changes in the structures near the anemometer and, thus, require frequent re-evaluation, which is costly in either computational power or ship time. Here, we evaluate if global atmospheric reanalysis data can be used to quantify the flow distortion bias in shipborne wind speed measurements. The method is tested on data from the Antarctic Circumnavigation Expedition onboard the R/V Akademik Tryoshnikov, which are compared to ERA-5 reanalysis wind speeds. We find that, depending on the relative wind direction, the relative wind speed and direction measurements are biased by −37 % to +22 % and -17∘ to +11∘ respectively. The resulting error in the true wind speed is +11.5 % on average but ranges from −4 % to +41 % (5th and 95th percentile). After applying the bias correction, the uncertainty in the true wind speed is reduced to ±5 % and depends mainly on the average accuracy of the ERA-5 data over the period of the experiment. The obvious drawback of this approach is the potential intrusion of model biases in the correction factors. We show that this problem can be somewhat mitigated when the error propagation in the true wind correction is accounted for and used to weight the observations. We discuss the potential caveats and limitations of this approach and conclude that it can be used to quantify flow distortion bias for ships that operate on a global scale. The method can also be valuable to verify computational fluid dynamic studies of airflow distortion on research vessels.

Diazotrophs play a critical role in the biogeochemical cycling of nitrogen, carbon, and other elements in the global ocean. Despite their well‐recognized role, the diversity, abundance, and distribution of diazotrophs in the world's ocean remain poorly characterized largely due to limited observations. Here we update the database of diazotroph nifH gene abundances and assess how environmental factors may regulate diazotrophs at the global scale. Our meta‐analysis more than doubles the number of observations in the previous database. Using linear and nonlinear regressions, we find that the abundances of Trichodesmium, UCYN‐A, UCYN‐B, and Richelia relate differently to temperature, light, and nutrients. We further apply a random forest algorithm to estimate the global distributions of these diazotrophic groups, identifying undersampled potential hot spots of diazotrophy in the South Atlantic and southern Indian Ocean, and in coastal waters. The distinct ecophysiologies of diazotrophs highlighted here argue for separate parameterizations of different diazotrophs in model simulations. Plain Language Summary Microbial communities drive the cycling of critical elements like carbon and nitrogen in the ocean. By converting N2 into more bioavailable nitrogen, diazotrophs alleviate nitrogen limitation and support primary production. Despite their importance, their distributions are poorly characterized in great part due to limited observations. Here we compile from the literature observations to update the global database of marine diazotrophs. We also assess how the abundance and distribution of different types of diazotrophs at the global scale relate to environmental factors, including temperature, depth, and nutrients. Finally, we use a random forest machine learning method to predict the distribution of different types of diazotrophs in the world's ocean. Our results highlight the need for observations over broader oceanic regimes and a more granular representation of diazotrophy in models.

International audience; Picoplankton populations dominate the planktonic community in the surface oligotrophic ocean. Yet, their strategies in the acquisition and the partitioning of organic and inorganic sources of nitrogen (N) and carbon (C) are poorly described. Here, we measured at the single-cell level the uptake of dissolved inorganic C (C-fixation), C-leucine, N-leucine, nitrate (NO3−), ammonium (NH4+), and N-urea in pigmented and nonpigmented picoplankton groups at six low-N stations in the northwestern Atlantic Ocean. Our study highlights important differences in trophic strategies between Prochlorococcus, Synechococcus, photosynthetic pico-eukaryotes, and nonpigmented prokaryotes. Nonpigmented prokaryotes were characterized by high leucine uptake rates, nonsignificant C-fixation and relatively low NH4+, N-urea, and NO3− uptake rates. Nonpigmented prokaryotes contributed to 7% ± 3%, 2% ± 2%, and 9% ± 5% of the NH4+, NO3−, and N-urea community uptake, respectively. In contrast, pigmented groups displayed relatively high C-fixation rates, NH4+ and N-urea uptake rates, but lower leucine uptake rates than nonpigmented prokaryotes. Synechococcus and photosynthetic pico-eukaryotes NO3− uptake rates were higher than Prochlorococcus ones. Pico-sized pigmented groups accounted for a significant fraction of the community C-fixation (63% ± 27%), NH4+ uptake (47% ± 27%), NO3− uptake (62% ± 49%), and N-urea uptake (81% ± 35%). Interestingly, Prochlorococcus and photosynthetic pico-eukaryotes showed a greater reliance on C- and N-leucine than Synechococcus on average, suggesting a greater reliance on organic C and N sources. Taken together, our single-cell results decipher the wide diversity of C and N trophic strategies between and within marine picoplankton groups, but a clear partitioning between pigmented and nonpigmented groups still remains.

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires1,2,3, arguably contributing to the severe 2019–2020 Australian wildfires4. The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols5,6,7. Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively8,9,10. It has been suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity11,12, but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019–2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean. Climate models project more frequent and severe wildfires in many regions1,2,3. A greater appreciation of the links between wildfires, pyrogenic aerosols13, nutrient cycling and marine photosynthesis could improve our understanding of the contemporary and glacial–interglacial cycling of atmospheric CO2 and the global climate system. Analyses of satellite aerosol observations used in this study were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. We thank SeaWiFS and MODIS mission scientists and associated NASA personnel for the production of the data used in this research effort. The BGC-Argo data were collected and made freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System (https://doi.org/10.17882/42182). W.T. is supported by the Harry H. Hess Postdoctoral Fellowship from Princeton University. N.C. is supported by the “Laboratoire d’Excellence” LabexMER (ANR‐10‐LABX‐19) and co-funded by a grant from the French government under the program “Investissements d’Avenir”. S.B. acknowledges the AXA Research Fund for the support of the long-term research line on Sand and Dust Storms at the Barcelona Supercomputing Center (BSC) and CAMS Global Validation (CAMS-84). P.G.S., J.L., M.M.G.P. and A.R.B. are supported by the Australian Research Council Discovery Projects scheme (DP190103504). P.G.S. and J.W. are supported by the Australian Research Council Centre of Excellence for Climate Extremes (CLEX: CE170100023). J.L. is supported by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 754433. A.R.B. is supported by the Australian Research Council Future Fellowship scheme (FT130100037). R.M. is supported by the CSIRO Decadal Climate Forecasting Project. We thank M. Strzelec, M. East, T. Holmes, M. Corkill, S. Meyerink and the Wellington Park Management Trust for help with installation and sampling the Tasmanian aerosol time-series station; A. Townsend for iron aerosol analyses by ICPMS at the University of Tasmania; and A. Benedetti and S. Remy for providing insights on the validation of aerosol reanalysis. Peer Reviewed "Article signat per 15 autors/es: Weiyi Tang, Joan Llort, Jakob Weis, Morgane M. G. Perron, Sara Basart, Zuchuan Li, Shubha Sathyendranath, Thomas Jackson, Estrella Sanz Rodriguez, Bernadette C. Proemse, Andrew R. Bowie, Christina Schallenberg, Peter G. Strutton, Richard Matear & Nicolas Cassar"

At depth, O2 depleted because of organic matter remineralization is generally estimated based on apparent oxygen utilization (AOU). However, AOU is an imperfect measure of oxygen utilization because of O2 air-sea disequilibrium at the site of deep-water formation. Recent methodological and instrumental advances have paved the way to further deconvolve processes driving the O2 signature. Using numerical model simulations of the global ocean, we show that measurements of the dissolved O2/Ar ratio, which so far have been confined to the ocean surface, can provide improved estimates of oxygen utilization, especially in regions where the disequilibrium at the site of deep-water formation is associated with physical processes. We discuss applications of this new approach and implications for current tracers relying on O2 such as remineralization ratios, respiratory quotients and preformed nutrients. Finally, we propose a new composite geochemical tracer, [O2]*bio combining dissolved O2/Ar and phosphate concentration. Being insensitive to photosynthesis and respiration, the change in this new tracer reflects gas exchange at the air-sea interface at the sites of deep-water formation. Plain Language Summary Oxygen utilization at depth offers insight into organic matter remineralization and the strength of the biological carbon pump. However, the oxygen concentration in the ocean interior is also impacted by additional biotic and abiotic processes occurring at the sites of deep-water formation and during transit in the ocean interior. In this study, we summarize, formalize, and model these processes to explore how decomposing the O2 signal at depth with new tools can provide new insight and more refined budgets of the broad-scale oceanic biogeochemical cycling of oxygen and nutrients. This is particularly important in light of the recent evidence that the role of physical processes in regions of convective deep-water formation may currently be underestimated.

International audience; The goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign is to develop a predictive understanding of the export, fate, and carbon cycle impacts of global ocean net primary production. To accomplish this goal, observations of export flux pathways, plankton community composition, food web processes, and optical, physical, and biogeochemical (BGC) properties are needed over a range of ecosystem states. Here we introduce the first EXPORTS field deployment to Ocean Station Papa in the Northeast Pacific Ocean during summer of 2018, providing context for other papers in this special collection. The experiment was conducted with two ships: a Process Ship, focused on ecological rates, BGC fluxes, temporal changes in food web, and BGC and optical properties, that followed an instrumented Lagrangian float; and a Survey Ship that sampled BGC and optical properties in spatial patterns around the Process Ship. An array of autonomous underwater assets provided measurements over a range of spatial and temporal scales, and partnering programs and remote sensing observations provided additional observational context. The oceanographic setting was typical of late-summer conditions at Ocean Station Papa: a shallow mixed layer, strong vertical and weak horizontal gradients in hydrographic properties, sluggish sub-inertial currents, elevated macronutrient concentrations and low phytoplankton abundances. Although nutrient concentrations were consistent with previous observations, mixed layer chlorophyll was lower than typically observed, resulting in a deeper euphotic zone. Analyses of surface layer temperature and salinity found three distinct surface water types, allowing for diagnosis of whether observed changes were spatial or temporal. The 2018 EXPORTS field deployment is among the most comprehensive biological pump studies ever conducted. A second deployment to the North Atlantic Ocean occurred in spring 2021, which will be followed by focused work on data synthesis and modeling using the entire EXPORTS data set.

The Southern Ocean is a critical component of Earth’s climate system, but its remoteness makes it challenging to develop a holistic understanding of its processes from the small to the large scale. As a result, our knowledge of this vast region remains largely incomplete. The Antarctic Circumnavigation Expedition (ACE, austral summer 2016/2017) surveyed a large number of variables describing the dynamic state of the ocean and the atmosphere, the freshwater cycle, atmospheric chemistry, ocean biogeochemistry and microbiology. This circumpolar cruise included visits to twelve remote islands, the marginal ice zone, and the Antarctic coast. Here, we use 111 of the observed variables to study the latitudinal gradients, seasonality, shorter term variations, the geographic setting of environmental processes, and interactions between them over the duration of 90 days. To reduce the dimensionality and complexity of the dataset and make the relations between variables interpretable, we applied a sparse Principal Component Analysis (sPCA), which describes environmental processes through 14 latent variables. To derive a robust statistical perspective on these processes and to estimate the uncertainty in the sPCA decomposition, we have developed a bootstrap approach. We identified temporal patterns from diurnal to seasonal cycles, as well as geographical gradients and “hotspots” of interaction. Our results establish connections of oceanic, atmospheric, biological and terrestrial processes in an innovative way, while confirming many well known relations of the Southern Ocean system. More specifically, we identify: the important role of the oceanic circulations, frontal zones, and islands in shaping the nutrient availability that controls biological community composition and productivity; that sea ice predominantly controls sea water salinity, dampens the wave field, and is associated with increased phytoplankton growth and net community productivity possibly due to iron fertilization and reduced light limitation; and clear regional patterns of aerosol characteristics emerged, stressing the role of the sea state, atmospheric chemical processing, as well as source processes near “hotspots” for the availability of cloud condensation nuclei and hence cloud formation. A set of key variables and their combinations, such as the difference between the air and sea surface temperature, atmospheric pressure, sea surface height, geostrophic currents, upper ocean layer light intensity, surface wind speed and relative humidity, played an important role in the majority of latent variables, highlighting their importance for a large variety of processes and the necessity for Earth System Models to represent them adequately. In conclusion, our study highlights the use of sPCA to identify key ocean-atmosphere interactions across physical, chemical, and biological processes and their associated spatio-temporal scales. The sPCA processing code is available as open-access and we believe that our approach is widely applicable to other environmental field studies.

International audience; The significance of the water-side gas transfer velocity for air–sea CO2 gas exchange (k) and its non-linear dependence on wind speed (U) is well accepted. What remains a subject of inquiry are biases associated with the form of the non-linear relation linking k to U (hereafter labeled as f(U), where f(.) stands for an arbitrary function of U), the distributional properties of U (treated as a random variable) along with other external factors influencing k, and the time-averaging period used to determine k from U. To address the latter issue, a Taylor series expansion is applied to separate f(U) into a term derived from time-averaging wind speed (labeled as ⟨U⟩, where ⟨.⟩ indicates averaging over a monthly time scale) as currently employed in climate models and additive bias corrections that vary with the statistics of U. The method was explored for nine widely used f(U) parameterizations based on remotely-sensed 6-hourly global wind products at 10 m above the sea-surface. The bias in k of monthly estimates compared to the reference 6-hourly product was shown to be mainly associated with wind variability captured by the standard deviation σσU around ⟨U⟩ or, more preferably, a dimensionless coefficient of variation Iu= σσU/⟨U⟩. The proposed correction outperforms previous methodologies that adjusted k when using ⟨U⟩ only. An unexpected outcome was that upon setting Iu2 = 0.15 to correct biases when using monthly wind speed averages, the new model produced superior results at the global and regional scale compared to prior correction methodologies. Finally, an equation relating Iu2 to the time-averaging interval (spanning from 6 h to a month) is presented to enable other sub-monthly averaging periods to be used. While the focus here is on CO2, the theoretical tactic employed can be applied to other slightly soluble gases. As monthly and climatological wind data are often used in climate models for gas transfer estimates, the proposed approach provides a robust scheme that can be readily implemented in current climate models.

While it has been known that wave breaking and bubble generation at high wind speeds enhance air‐sea CO2 exchange rates (F), quantification of their contribution at the global scale remains a formidable challenge. There is urgency to make progress on this issue as a significant uptick in both magnitude and frequency of high wind events (HW) has been documented over the last three decades. Using a wind‐wave dependent expression for gas transfer velocity (k) that explicitly considers bubbles and a widely used wind‐only parameterization, the spatial pattern of k at high winds can be explained by sea surface temperature distribution. The HW, which represent some 3% of wind conditions, contribute disproportionally to the global F (18%) with an increasing trend. Approximately 50% of the global F at high winds is attributed to bubble contribution. The findings are of significance to quantifying CO2 transfer to the ocean interior. Plain Language Summary Studies on air‐sea carbon dioxide (CO2) exchange seek to determine how rapidly CO2 molecules traverse the air‐water interface. This exchange impacts a plethora of processes related to ocean biogeochemistry, oceanic carbon cycling, and CO2 buildup in the atmosphere. At high wind speeds, both conventional aerodynamic transfer processes and bubbles generated by wave breaking are expected to enhance air‐sea CO2 gas exchange. Yet the impact of bubbles in isolation on global and regional CO2 exchange remains a subject of inquiry and speculation. There is some urgency to make progress on this topic because the magnitude and frequency of high wind events (HW) have been steadily increasing over the last three decades. The work here demonstrates that bubbles contribute as much as 50% of CO2 gas exchange under high wind speeds conditions, which rarely occur over the ocean (less than 3% of the time). Yet, HW contribute disproportionally to the global air‐sea CO2 gas exchange (about 18%).

International audience; Approximately half of global primary production occurs in the ocean. While the large-scale variability in net primary production (NPP) has been extensively studied, ocean gross primary production (GPP) has thus far received less attention. In this study, we derived two satellite-based GPP models by training machine learning algorithms (Random Forest) with light-dark bottle incubations (GPPLD) and the triple isotopes of dissolved oxygen (GPP17Δ). The two algorithms predict global GPPs of 9.2 ± 1.3 × 1015 and 15.1 ± 1.05 × 1015 mol O2 yr−1 for GPPLD and GPP17Δ, respectively. The projected GPP distributions agree with our understanding of the mechanisms regulating primary production. Global GPP17Δ was higher than GPPLD by an average factor of 1.6 which varied meridionally. The discrepancy between GPP17Δ and GPPLD simulations can be partly explained by the known biases of each methodology. After accounting for some of these biases, the GPP17Δ and GPPLD converge to 9.5 ∼ 12.6 × 1015 mol O2 yr−1, equivalent to 103 ∼ 150 Pg C yr−1. Our results suggest that global oceanic GPP is 1.5–2.2 fold larger than oceanic NPP and comparable to GPP on land. © 2021. American Geophysical Union. All Rights Reserved.

*********** IGNORE VERSION 1 AND USE VERSION 2 *********** Version 2: https://doi.org/10.5281/zenodo.5768050 ************************************************************************* File Monthly_GPP.nc (NC_FORMAT_CLASSIC): 4 variables (excluding dimension variables): double GPP_LD_MLD_RF[Lon,Lat,Month] units: mmol O2 m-2 d-1 _FillValue: NaN long_name: Monthly mixed_layer integration of gross primary production trained from the dataset determined by the light-dark bottle incubation using Random Forest algorithm coordinates: Longitude Latitude Month double GPP_LD_ZEU_RF[Lon,Lat,Month] units: mmol O2 m-2 d-1 _FillValue: NaN long_name: Monthly euphotic-zone integration of gross primary production trained from the dataset determined by the light-dark bottle incubation using Random Forest algorithm coordinates: Longitude Latitude Month double GPP_Triple_MLD_RF[Lon,Lat,Month] units: mmol O2 m-2 d-1 _FillValue: NaN long_name: Monthly mixed_layer integration of gross primary production trained from the dataset determined by the triple isotopes of dissolved oxygen using Random Forest algorithm coordinates: Longitude Latitude Month double GPP_Triple_ZEU_RF[Lon,Lat,Month] units: mmol O2 m-2 d-1 _FillValue: NaN long_name: Monthly euphotic-zone integration of gross primary production trained from the dataset determined by the triple isotopes of dissolved oxygen using Random Forest algorithm 3 dimensions: Lon Size:181 units: degree_north long_name: Longitude Lat Size:91 units: degree_east long_name: Latitude Month Size:13 units: Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec","Annuual mean long_name: Month 4 global attributes: Author: Yibin Huang & Nicolas Cassar Correspondence: nicolas.cassar@duke.edu Request_for_citation: If you use these data in publications or presentations, please cite: Huang, Y., Nicholson, D., Huang, B., & Cassar, N. (2021). Global estimates of marine gross primary production based on machine‐learning upscaling of field observations. Global Biogeochemical Cycles, 35, e2020GB006718. https://doi.org/10.1029/2020GB006718 Creation date: Jan/14th/2021 *********** IGNORE VERSION 1 AND USE VERSION 2 *********** Version 2: https://doi.org/10.5281/zenodo.5768050 *************************************************************************

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires

Since the middle of the past century, the Western Antarctic Peninsula has warmed rapidly with a significant loss of sea ice but the impacts on plankton biodiversity and carbon cycling remain an open question. Here, using a 5-year dataset of eukaryotic plankton DNA metabarcoding, we assess changes in biodiversity and net community production in this region. Our results show that sea-ice extent is a dominant factor influencing eukaryotic plankton community composition, biodiversity, and net community production. Species richness and evenness decline with an increase in sea surface temperature (SST). In regions with low SST and shallow mixed layers, the community was dominated by a diverse assemblage of diatoms and dinoflagellates. Conversely, less diverse plankton assemblages were observed in waters with higher SST and/or deep mixed layers when sea ice extent was lower. A genetic programming machine-learning model explained up to 80% of the net community production variability at the Western Antarctic Peninsula. Among the biological explanatory variables, the sea-ice environment associated plankton assemblage is the best predictor of net community production. We conclude that eukaryotic plankton diversity and carbon cycling at the Western Antarctic Peninsula are strongly linked to sea-ice conditions., Over the past century, the Western Antarctic Peninsula has experienced rapid warming and a substantial loss of sea ice with important implications for plankton biodiversity and carbon cycling. Using a 5-year DNA metabarcoding dataset, this study assesses how interannual variability in sea-ice conditions impacts biodiversity and biological carbon fluxes in this region.

Upwelling of nutrient rich waters along continental shelves generates highly productive marine ecosystems affectingplanktonic communities from coastal to offshore domains. Methods to constrain pela...

International audience; Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N-2) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N-2 fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in the global ocean. However, even in the North Atlantic where most research on diazotrophs and N-2 fixation has taken place, environmental controls remain contentious. Here we measure diazotroph composition, abundance, and activity at high resolution using newly developed underway sampling and sensing techniques. We capture a diazotrophic community shift from Trichodesmium to UCYN-A between the oligotrophic, warm (25-29 degrees C) Sargasso Sea and relatively nutrient-enriched, cold (13-24 degrees C) subpolar and eastern American coastal waters. Meanwhile, N-2 fixation rates measured in this study are among the highest ever recorded globally and show significant increase with phosphorus availability across the transition from the Gulf Stream into subpolar and coastal waters despite colder temperatures and higher nitrate concentrations. Transcriptional patterns in both Trichodesmium and UCYN-A indicate phosphorus stress in the subtropical gyre. Over this iron-replete transect spanning the western North Atlantic, our results suggest that temperature is the major factor controlling the diazotrophic community structure while phosphorous drives N-2 fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N-2 fixation rates in the cold coastal region where nitrate concentrations are highest (similar to 200 nM) challenges current paradigms on what drives the distribution of diazotrophs and N-2 fixation.

Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open-ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in delta C-13 values of 0.8 parts per thousand-2.5 parts per thousand were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel-derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in C-13-rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna delta C-13 values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota.

Diatoms play a fundamental role at the base of the polar marine food web. In the Southern Ocean, low iron concentrations and light levels control diatom abundance and distribution. Diatoms must therefore employ strategies that allow them to cope when iron and/or light availability is growth limiting. Through a combination of physiological and molecular-based approaches, we have investigated the physiological response to variable iron concentrations and light levels along with the expressed gene repertoires of nine newly isolated diatoms from the Western Antarctic Peninsula (WAP) region of the Southern Ocean. The diatoms ranged across five orders of magnitude in biovolume and displayed various degrees of susceptibility to low iron and light availability. Under the performed laboratory culture conditions, the growth rates of most diatoms decreased more due to low light level rather than low iron concentrations. Additionally, most diatoms were not subject to further reductions in growth rates when grown under combined low-light and iron-limiting conditions, indicating they are less likely to be co-limited by an additive effect. By sequencing the transcriptomes of these diatoms, we identified genes that likely facilitate their growth under variable iron and light conditions commonly present in the Southern Ocean. Specifically, we investigated the presence of 20 key genes involved in iron acquisition and homeostasis, iron usage in photosynthesis and nitrogen assimilation, and protection from reactive oxygen species. When comparing gene repertoires of recently sequenced transcriptomes of diatoms isolated from around the globe, the prevalence of certain genes exhibited biogeographical patterns that clearly distinguish Southern Ocean diatoms from those isolated from other regions.

Export production reflects the amount of organic matter transferred from the surface ocean to depth through biological processes. This export is in great part controlled by nutrient and light availability, which are conditioned by mixed layer depth (MLD). In this study, building on Sverdrup’s critical depth hypothesis, we derive a mechanistic model of an upper bound on carbon export based on the metabolic balance between photosynthesis and respiration as a function of MLD and temperature. We find that the upper bound is a positively skewed bell-shaped function of MLD. Specifically, the upper bound increases with deepening mixed layers down to a critical depth, beyond which a long tail of decreasing carbon export is associated with increasing heterotrophic activity and decreasing light availability. We also show that in cold regions the upper bound on carbon export decreases with increasing temperature when mixed layers are deep, but increases with temperature when mixed layers are shallow. A metaanalysis shows that our model envelopes field estimates of carbon export from the mixed layer. When compared to satellite export production estimates, our model indicates that export production in some regions of the Southern Ocean, most particularly the Subantarctic Zone, is likely limited by light for a significant portion of the growing season.

The Western Antarctic Peninsula (WAP) is a highly productive marine environment that is undergoing rapid change, with consequences for productivity and total ecosystem carbon cycling. We present continuous underway O2/Ar estimates of net community production (NCPO2Ar) in austral summer 2012, 2013 and 2014 at sub-kilometer horizontal resolution within the Palmer Long-Term Ecological Research (Pal-LTER) grid region of the WAP. Substantial spatial variability is observed with NCPO2Ar ranging from 0 to 790 mmol O2 m−2 d−1 and considerable interannual variability with mean values in the grid region of 54.4±48.5, 44.6±40.5, and 85.6±75.9 mmol O2 m−2 d−1 in 2012, 2013 and 2014 respectively. Based on a strong correlation (r2=0.83) between residence time integrated NCPO2Ar and NCPDIC derived from seasonal DIC drawdown, we find the observed NCPO2Ar spatial and interannual variability to be consistent with the December–January NCPDIC magnitude. Seeking to explain the mechanistic drivers of NCP in the WAP, we observe a linear relationship between NCPO2Ar and meteoric water content derived from δ18O and salinity. This correlation may be due to Fe supply from glacial melt and/or strengthening of stratification and relief of light limitation. Elevated surface Fe availability, as indicated by Fv/Fm and measurements of surface water dissolved Fe and Mn (a rough proxy for recent potential Fe availability), and shallower, more stable mixed layers are present where meteoric water and/or sea ice melt is high near the coast. Light limitation is evident in the WAP when mixed layer depths are greater than ~40 m. Additionally we document hotspots of NCP associated with submarine canyons along the WAP. While it is difficult to predict how the physical-biological system might evolve under changing climatic conditions, it is evident that NCP, and potentially carbon flux out of the mixed layer, along the WAP will be sensitive to shifts in meltwater input and timing.

Using simultaneous sub-kilometer resolution underway measurements of surface O2/Ar, total O2 and pCO2 from annual austral summer surveys in 2012, 2013 and 2014, we explore the impacts of biological and physical processes on the O2 and pCO2 system spatial and interannual variability at the Western Antarctic Peninsula (WAP). In the WAP, mean O2/Ar supersaturation was (7.6±9.1)% and mean pCO2 supersaturation was (−28±22)%. We see substantial spatial variability in O2 and pCO2 including sub-mesoscale/mesoscale variability with decorrelation length scales of ~4.5 km, consistent with the regional Rossby radius. This variability is embedded within onshore–offshore gradients. O2 in the LTER grid region is driven primarily by biological processes as seen by the median ratio of the magnitude of biological oxygen (O2/Ar) to physical oxygen (Ar) supersaturation anomalies (%) relative to atmospheric equilibrium (2.6), however physical processes have a more pronounced influence in the southern onshore region of the grid where we see active sea-ice melting. Total O2 measurements should be interpreted with caution in regions of significant sea-ice formation and melt and glacial meltwater input. pCO2 undersaturation predominantly reflects biological processes in the LTER grid. In contrast we compare these results to the Drake Passage where gas supersaturations vary by smaller magnitudes and decorrelate at length scales of ~12 km, in line with latitudinal changes in the regional Rossby radius. Here biological processes induce smaller O2/Ar supersaturations (mean (0.14±1.3)%) and pCO2 undersaturations (mean (−2.8±3.9)%) than in the WAP, and pressure changes, bubble and gas exchange fluxes drive stable Ar supersaturations.

Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles. The geographical distribution and controlling factors of marine N2 fixation are understudied. Here the authors find increasing rates of N2 fixation from the Sargasso Sea to the coastal waters of North America, driven primarily by cyanobacterial diazotrophs and best correlated with phosphorus availability and chlorophyll-a concentrations.

WOS:000497952500008; International audience; Despite generally low primary productivity and diatom abundances in oligotrophic subtropical gyres, the North Atlantic Subtropical Gyre (NASG) exhibits significant diatom-driven carbon export on an annual basis. Subsurface pulses of nutrients likely fuel brief episodes of diatom growth, but the exact mechanisms utilized by diatoms in response to these nutrient injections remain understudied within near-natural settings. Here we simulated delivery of subsurface nutrients and compare the response among eukaryotic phytoplankton using a combination of physiological techniques and metatranscriptomics. We show that eukaryotic phytoplankton groups exhibit differing levels of transcriptional responsiveness and expression of orthologous genes in response to release from nutrient limitation. In particular, strategies for use of newly delivered nutrients are distinct among phytoplankton groups. Diatoms channel new nitrate to growth-related strategies while physiological measurements and gene expression patterns of other groups suggest alternative strategies. The gene expression patterns displayed here provide insights into the cellular mechanisms that underlie diatom subsistence during chronic nitrogen-depleted conditions and growth upon nutrient delivery that can enhance carbon export from the surface ocean.