Global Ocean Research Articles

Heat waves related to Quasi-Biweekly variability over Southern China in the FGOALS-g3 model

In the present study, we focused on the heat waves (HW) associated with the quasi-biweekly (QBW, 10–20-day period) variability (QBW-HW) over Southern China (SC, 102º–120ºE, 21º–30ºN) in the Flexible Global Ocean–Atmosphere–Land System Model, GridPoint version 3 (FGOALS-g3), and the HW-associated structures and surface air temperature budget investigated by using model outputs from historical experiment of Coupled Model Intercomparison Project Phase 6 (CMIP6). We found that the anomalous circulations related to the QBW-HW events over SC are closely linked to the southeastward propagation of the wave train from mid-high latitudes and the northwestward-propagating disturbances from the tropics. The results also showed that adiabatic and diabatic heating play a key role in the QBW-HW over SC. These results are in good agreement with observations from previous studies. In addition, QBW-HWs are dry in the FGOALS-g3 model, while observed humid HWs occur over SC. The difference is mainly due to the new boundary layer scheme incorporated in the FGOALS-g3 model, which overestimates the entrainment process at the top of the boundary layer during the QBW-HW over SC, resulting in more and drier air into the boundary layer, and thus less moisture. It implies that the entrainment equation at the top of the boundary layer in the FGOALS-g3 model does need to be improved to be suitable for humid HW processes, although the boundary layer scheme can improve the model precipitation and radiative forcing.

Global marine conservation priorities for sustaining marine productivity, preserving biodiversity and addressing climate change

Marine primary productivity is a critical driver of functioning marine ecosystems, providing a foundation for biological diversity and associated economic productivity, and a key component of the oceanic carbon sink. However, it is largely under-represented within the global marine protected area estate and has been widely ignored in global priority assessments for marine conservation. Using global high-resolution data on marine primary productivity and human cumulative impact to marine systems, more than 18.6 million km² of high productivity-low impact areas in the global ocean were identified. These areas occur across all ocean basins and represent the vast majority of marine provinces and ecoregions. Over 80% of these highly productive waters with low levels of human impact lies within national jurisdictions and yet only 11% of the overall identified area is currently safeguarded within designated marine protected areas or sustainable management initiatives, leaving more than 16.5 million km2 of high productivity-low impact areas without those forms of formal protection. The multifaceted contribution of these areas to preserve biodiversity, support human welfare and help mitigating climate change suggest they are an essential, but currently overlooked, conservation priority for consideration in both global nature conservation and human wellbeing policy fora.

Advances in understanding of air–sea exchange and cycling of greenhouse gases in the upper ocean

The air–sea exchange and oceanic cycling of greenhouse gases (GHG), including carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), carbon monoxide (CO), and nitrogen oxides (NOx = NO + NO2), are fundamental in controlling the evolution of the Earth’s atmospheric chemistry and climate. Significant advances have been made over the last 10 years in understanding, instrumentation and methods, as well as deciphering the production and consumption pathways of GHG in the upper ocean (including the surface and subsurface ocean down to approximately 1000 m). The global ocean under current conditions is now well established as a major sink for CO2, a major source for N2O and a minor source for both CH4 and CO. The importance of the ocean as a sink or source of NOx is largely unknown so far. There are still considerable uncertainties about the processes and their major drivers controlling the distributions of N2O, CH4, CO, and NOx in the upper ocean. Without having a fundamental understanding of oceanic GHG production and consumption pathways, our knowledge about the effects of ongoing major oceanic changes—warming, acidification, deoxygenation, and eutrophication—on the oceanic cycling and air–sea exchange of GHG remains rudimentary at best. We suggest that only through a comprehensive, coordinated, and interdisciplinary approach that includes data collection by global observation networks as well as joint process studies can the necessary data be generated to (1) identify the relevant microbial and phytoplankton communities, (2) quantify the rates of ocean GHG production and consumption pathways, (3) comprehend their major drivers, and (4) decipher economic and cultural implications of mitigation solutions.

Evaluation of three contrasting models in estimating primary production from ocean color remote sensing using long-term time-series data at oceanic and coastal sites

Accurate estimates of depth-integrated Net Primary Production (NPP, mg C m−2 d−1) and the creation of a robust climate data record of NPP for the global oceans are essential goals of the ocean color remote sensing community. Here, we take advantage of in situ NPP measurements from three long-term time-series sites, the HOT (Hawaii Ocean Time-series), BATS (Bermuda Atlantic Time-series Study) and CARIACO (Ocean Time-Series Program from the Cariaco basin), spanning over 30 years to evaluate three contrasting models in estimating NPP from ocean color remote sensing. These models for NPP estimation include the Absorption-based Model (AbPM), which relies on phytoplankton absorption coefficient, the Vertically Generalized Production Model (VGPM), which centers on chlorophyll-a concentration, and the Carbon-based Productivity Model (CbPM) centering on phytoplankton carbon. In addition to the accuracy of NPP estimation from these models, we laid great emphasis on evaluating their skills in capturing the monthly to seasonal variations and interannual trends in NPP at the three sites. Comparison with in situ NPP at all three long-term sites (∼20 years) showed that AbPM yielded the highest coefficient of determination (R2 = 0.67) and the lowest uncertainties (Bias = 0.03 and unbiased root mean square difference = 0.17). Seasonal and interannual variations apparent in the in situ NPP time-series records were best captured by AbPM. These results showcase the robust capabilities of AbPM and its superiority for global carbon cycling and climate change studies, largely because it takes into account optical and photosynthetic parameters of local phytoplankton.

CMEMS-LSCE: a global, 0.25°, monthly reconstruction of the surface ocean carbonate system

Abstract. Observation-based data reconstructions of global surface ocean carbonate system variables play an essential role in monitoring the recent status of ocean carbon uptake and ocean acidification, as well as their impacts on marine organisms and ecosystems. So far, ongoing efforts are directed towards exploring new approaches to describe the complete marine carbonate system and to better recover its fine-scale features. In this respect, our research activities within the Copernicus Marine Environment Monitoring Service (CMEMS) aim to develop a sustainable production chain of observation-derived global ocean carbonate system datasets at high space–time resolutions. As the start of the long-term objective, this study introduces a new global 0.25∘ monthly reconstruction, namely CMEMS-LSCE (Laboratoire des Sciences du Climat et de l'Environnement) for the period 1985–2021. The CMEMS-LSCE reconstruction derives datasets of six carbonate system variables, including surface ocean partial pressure of CO2 (pCO2), total alkalinity (AT), total dissolved inorganic carbon (CT), surface ocean pH, and saturation states with respect to aragonite (Ωar) and calcite (Ωca). Reconstructing pCO2 relies on an ensemble of neural network models mapping gridded observation-based data provided by the Surface Ocean CO2 ATlas (SOCAT). Surface ocean AT is estimated with a multiple-linear-regression approach, and the remaining carbonate variables are resolved by CO2 system speciation given the reconstructed pCO2 and AT; 1σ uncertainty associated with these estimates is also provided. Here, σ stands for either the ensemble standard deviation of pCO2 estimates or the total uncertainty for each of the five other variables propagated through the processing chain with input data uncertainty. We demonstrate that the 0.25∘ resolution pCO2 product outperforms a coarser spatial resolution (1∘) thanks to higher data coverage nearshore and a better description of horizontal and temporal variations in pCO2 across diverse ocean basins, particularly in the coastal–open-ocean continuum. Product qualification with observation-based data confirms reliable reconstructions with root-mean-square deviation from observations of less than 8 %, 4 %, and 1 % relative to the global mean of pCO2, AT (CT), and pH. The global average 1σ uncertainty is below 5 % and 8 % for pCO2 and Ωar (Ωca), 2 % for AT and CT, and 0.4 % for pH relative to their global mean values. Both model–observation misfit and model uncertainty indicate that coastal data reproduction still needs further improvement, wherein high temporal and horizontal gradients of carbonate variables and representative uncertainty from data sampling would be taken into account as a priority. This study also presents a potential use case of the CMEMS-LSCE carbonate data product in tracking the recent state of ocean acidification. The data associated with this study are available at https://doi.org/10.14768/a2f0891b-763a-49e9-af1b-78ed78b16982 (Chau et al., 2023).

A synthesis of ocean total alkalinity and dissolved inorganic carbon measurements from 1993 to 2022: the SNAPO-CO2-v1 dataset

Abstract. Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties with respect to understanding the ocean carbon cycle and its link to global change (ocean carbon sinks and sources, ocean acidification) and ultimately finding carbon-based solutions or mitigation procedures (marine carbon removal). We present a database of more than 44 400 AT and CT observations along with basic ancillary data (spatiotemporal location, depth, temperature and salinity) from various ocean regions obtained, mainly in the framework of French projects, since 1993. This includes both surface and water column data acquired in the open ocean, coastal zones and in the Mediterranean Sea and either from time series or dedicated one-off cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The data are provided in two separate datasets – for the Global Ocean and the Mediterranean Sea (https://doi.org/10.17882/95414, Metzl et al., 2023), respectively – that offer a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, and constraint and validation of diagnostic CT and AT reconstructed fields or ocean carbon and coupled climate–carbon models simulations as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. When associated with other properties, these data can also be used to calculate pH, the fugacity of CO2 (fCO2) and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.

Arctic Ocean acidification dynamics during 1993–2021 and its projections for the rest of this century

Dynamics of acidification of the Arctic Ocean through 1993–2021 and predictions of further tendencies of this process until the end of 2100 were assessed making use of both the GLODAPv.2021 and the Global Ocean Biogeochremistry Hindcast (GOBH) reanalysis data on pH. The projections of pH were performed by CMIP6 models for four scenarios of rates of socio-economic and agricultural development and emissions of greenhouse gases: SSP1–2.6, SSP2–4.5, SSP3–7.0 and SSP5–8.5.The tendencies of pH decline over the last 27 years (1993–2019) as determined from the GLODAP in situ and the reanalysis data over 1993–2021 proved to be, respectively –0.9% (from 8.18–8.11) and –0.7% (from 8.10–8.05). Thus, the annual acidification rate as assessed from both data sources proved to be –0.03%.Through the percentile method-based comparison of consistency of historical observation data on pH with GBH model hindcast four best models were identified: MPI-ESM1–2-LR, NorESM2-MM, NorESM2-LM, and CMCC-ESM2. The projection results strongly indicate that the Arctic Ocean acidification will continue till the end of this century. The highest rates of pH decrease (–4.9% and –6.2%) were forecasted, respectively, for scenarios SSP3–7.0 and SSP5–8.5 that implied the global mean temperature increases by 3.6 °C and 4.4 °C, respectively. A comparison of the results obtained with the previously made assessments is indicative that by the end of the current century the rate of acidification (i. e. pH decrease) in the Arctic should be expected to be higher than that averaged over the World Oceans: the difference for each of the SSP scenarios proved to be –0.1.

Seawater physics and chemistry along the Med-SHIP transects in the Mediterranean Sea in 2016

The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises.

Age, metabolisms, and potential origin of dominant anammox bacteria in the global oxygen-deficient zones.

Anammox bacteria inhabiting oxygen-deficient zones (ODZs) are a major functional group mediating fixed nitrogen loss in the global ocean. However, many basic questions regarding the diversity, broad metabolisms, origin, and adaptive mechanisms of ODZ anammox bacteria remain unaddressed. Here we report two novel metagenome-assembled genomes of anammox bacteria affiliated with the Scalindua genus, which represent most, if not all, of the anammox bacteria in the global ODZs. Metagenomic read-recruiting and comparison with historical data show that they are ubiquitously present in all three major ODZs. Beyond the core anammox metabolism, both organisms contain cyanase, and the more dominant one encodes a urease, indicating most ODZ anammox bacteria can utilize cyanate and urea in addition to ammonium. Molecular clock analysis suggests that the evolutionary radiation of these bacteria into ODZs occurred no earlier than 310 million years ago, ~1 billion years after the emergence of the earliest modern-type ODZs. Different strains of the ODZ Scalindua species are also found in benthic sediments, and the first ODZ Scalindua is likely derived from the benthos. Compared to benthic strains of the same clade, ODZ Scalindua uniquely encodes genes for urea utilization but has lost genes related to growth arrest, flagellum synthesis, and chemotaxis, presumably for adaptation to thrive in the global ODZ waters. Our findings expand the known metabolisms and evolutionary history of the bacteria controlling the global nitrogen budget.

A database of mapped global fishing activity 1950–2017

A new database on historical country-level fishing fleet capacity and effort is described, derived from a range of publicly available sources that were harmonized, converted to fishing effort, and mapped to 30-min spatial cells. The resulting data is comparable with widely used but more temporally-limited satellite-sourced Automatic Identification System (AIS) datasets for large vessels, while also documenting important smaller fleets and artisanal segments. It ranges from 1950 to 2017, and includes information on number of vessels, engine power, gross tonnage, and nominal effort, categorized by vessel length, gear type and targeted functional groups. The data can be aggregated to Large Marine Ecosystem, region and/or fishing country scales and provides a temporally and spatially explicit source for fishing effort and fleet capacity for studies aimed at understanding the implications of long-term changes in fishing activity in the global ocean.

Transcriptomic insights into the shift of trophic strategies in mixotrophic dinoflagellate Lepidodinium in the warming ocean.

The shift between photoautotrophic and phagotrophic strategies in mixoplankton significantly impacts the planktonic food webs and biogeochemical cycling. Considering the projected global warming, studying how temperature impacts this shift is crucial. Here, we combined the transcriptome of in-lab cultures (mixotrophic dinoflagellate Lepidodinium sp.) and the metatranscriptome dataset of the global ocean to investigate the mechanisms underlying the shift of trophic strategies and its relationship with increasing temperatures. Our results showed that phagocytosis-related pathways, including focal adhesion, regulation of actin cytoskeleton, and oxidative phosphorylation, were significantly stimulated in Lepidodinium sp. when cryptophyte prey were added. We further compared the expression profiles of photosynthesis and phagocytosis genes in Lepidodinium sp. in the global sunlit ocean. Our results indicated that Lepidodinium sp. became more phagotrophic with increasing temperatures when the ambient chlorophyll concentration was >0.3mg.m-3 (~20.58% of the ocean surface) but became more photoautotrophic with increasing temperatures when the chlorophyll concentration was between 0.2 and 0.3mg.m-3 (~11.47% of the ocean surface). Overall, we emphasized the crucial role of phagocytosis in phago-mixotrophy and suggested that the expression profile of phagocytosis genes can be a molecular marker to target the phagotrophic activity of mixoplankton in situ.

Iron limitation of heterotrophic bacteria in the California Current System tracks relative availability of organic carbon and iron.

Iron is an essential nutrient for all microorganisms of the marine environment. Iron limitation of primary production has been well documented across a significant portion of the global surface ocean, but much less is known regarding the potential for iron limitation of the marine heterotrophic microbial community. In this work, we characterize the transcriptomic response of the heterotrophic bacterial community to iron additions in the California Current System, an eastern boundary upwelling system, to detect in situ iron stress of heterotrophic bacteria. Changes in gene expression in response to iron availability by heterotrophic bacteria were detected under conditions of high productivity when carbon limitation was relieved but when iron availability remained low. The ratio of particulate organic carbon to dissolved iron emerged as a biogeochemical proxy for iron limitation of heterotrophic bacteria in this system. Iron stress was characterized by high expression levels of iron transport pathways and decreased expression of iron-containing enzymes involved in carbon metabolism, where a majority of the heterotrophic bacterial iron requirement resides. Expression of iron stress biomarkers, as identified in the iron-addition experiments, was also detected insitu. These results suggest iron availability will impact the processing of organic matter by heterotrophic bacteria with potential consequences for the marine biological carbon pump.

The active free-living bathypelagic microbiome is largely dominated by rare surface taxa.

A persistent microbial seed bank is postulated to sustain the marine biosphere, and recent findings show that prokaryotic taxa present in the ocean's surface dominate prokaryotic communities throughout the water column. Yet, environmental conditions exert a tight control on the activity of prokaryotes, and drastic changes in these conditions are known to occur from the surface to deep waters. The simultaneous characterization of the total (DNA) and active (i.e. with potential for protein synthesis, RNA) free-living communities in 13 stations distributed across the tropical and subtropical global ocean allowed us to assess their change in structure and diversity along the water column. We observed that active communities were surprisingly more similar along the vertical gradient than total communities. Looking at the vertical connectivity of the active vs. the total communities, we found that taxa detected in the surface sometimes accounted for more than 75% of the active microbiome of bathypelagic waters (50% on average). These active taxa were generally rare in the surface, representing a small fraction of all the surface taxa. Our findings show that the drastic vertical change in environmental conditions leads to the inactivation and disappearance of a large proportion of surface taxa, but some surface-rare taxa remain active (or with potential for protein synthesis) and dominate the bathypelagic active microbiome.

A Reconstructing Model Based on Time–Space–Depth Partitioning for Global Ocean Dissolved Oxygen Concentration

Dissolved oxygen (DO) is essential for assessing and monitoring the health of marine ecosystems. The phenomenon of ocean deoxygenation is widely recognized. Nevertheless, the limited availability of observations poses a challenge in achieving a comprehensive understanding of global ocean DO dynamics and trends. The study addresses the challenge of unevenly distributed Argo DO data by developing time–space–depth machine learning (TSD-ML), a novel machine learning-based model designed to enhance reconstruction accuracy in data-sparse regions. TSD-ML partitions Argo data into segments based on time, depth, and spatial dimensions, and conducts model training for each segment. This research contrasts the effectiveness of partitioned and non-partitioned modeling approaches using three distinct ML regression methods. The results reveal that TSD-ML significantly enhances reconstruction accuracy in areas with uneven DO data distribution, achieving a 30% reduction in root mean square error (RMSE) and a 20% decrease in mean absolute error (MAE). In addition, a comparison with WOA18 and GLODAPv2 ship survey data confirms the high accuracy of the reconstructions. Analysis of the reconstructed global ocean DO trends over the past two decades indicates an alarming expansion of anoxic zones.

Assessment of global detection capability of oceanographic lidar

The profiling of marine bio-optical properties by oceanographic lidar is of great significance to the monitoring of the marine environment and the study of the global carbon cycle. In this study, the global detection capability of oceanographic lidar is analyzed with different specifications. To evaluate the detection performance, a simulator on multi-platform is developed. Based on the quasi-single elastic scattering approximation lidar equation and the bio-optical models, the simulator can generate different kinds of outputs, such as lidar signal profiles, signal-to-noise ratio profiles, maximum detectable depth distributions. The optical properties profiles of the global ocean could be derived from the global chlorophyll-a concentration profile data set, which is used as the input to the simulator. The simulation results are compared with the measurements from airborne lidar in the South China Sea, which verifies the reliability of the simulator. The global detection capability of spaceborne lidar with different power aperture products is simulated, and the influence of laser wavelength is analyzed. The results show that, for the simulated spaceborne lidar with typical specifications at 486 nm, there are probabilities ranging from 0.66 to 0.85 that the upper boundary of the thermocline can be detected on a global scale. In the cleanest open ocean, the lidar has a good capability to penetrate the upper boundary of the thermocline, while it cannot penetrate the subsurface chlorophyll maximum layers, in which the global penetration ratio is 0.93 under the simulation specifications at 486 nm. Finally, the influence of repetition frequency is analyzed, and the results show that reducing the pulse repetition frequency at the same average power can effectively reduce the background light of cumulative profiles and improve the signal-to-noise ratio.

Extremely high resolution XRF core scanning reveals the Early Triassic depositional history of the Montney Formation in northeastern British Columbia, Canada

The Montney Formation is a laterally extensive, fine-grained sedimentary unit in western Alberta and northeastern British Columbia, and a major target of hydrocarbon exploration. It also retains a complete record of the Early Triassic epoch, a time when the biosphere was recovering from the effects of the end-Permian mass extinction. An industry well provides a full-diameter core of the complete distal Montney Formation in northeastern British Columbia (over 350 m thick), with stratigraphic age control provided by conodont biostratigraphy and regional correlations. For this study, the core was scanned using an ITRAX X-ray fluorescence spectrometer, at centimeter-scale resolution. The resulting dataset (over 37,000 individual measurements) provides an extremely high level of resolution for a deep-time geochemical study. While the data generated by this instrument are semiquantitative, they can be calibrated by correlating them with more conventional ICP-MS and XRF measurements of select samples. Using this method, we assessed the reliability of each of the elemental data series, and where possible converted them to quantitative geochemical measurements. These calibrated measurements serve as the basis for our detailed reconstruction of the Montney Formation's depositional history.The distal Montney depositional basin, including the location of the c-65-F core, was euxinic immediately following the end-Permian mass extinction, and accumulated sediments from the rapidly eroding margin of western North America. Conditions stabilized somewhat in the Dienerian, with oxic bottom water and a trend toward more compositionally mature sediment, but conditions of widespread euxinia and rapid erosion of the North American craton recurred in the early Smithian, earlier than this global crisis has been recognized elsewhere. Above the middle-upper Montney sequence boundary, in the overlying Spathian succession, local effects predominate. Coastal upwelling resumed in the basal Spathian, though the Montney depositional basin was soon cut off from the global ocean by uplift of the Yukon-Tanana arc bounding the basin to the west. Restricted conditions, with euxinic bottom water, were punctuated by oxic intervals, when rising sea level allowed the basin to freely exchange water with the open ocean. Intervals of restriction became progressively less acute in the upper Spathian.

Impact of Data Assimilation on KMA's Global and Regional Ocean Wave Predictions

Impact of Data Assimilation on KMA's Global and Regional Ocean Wave Predictions

Accelerated marine carbon cycling forced by tectonic degassing over the Miocene Climate Optimum

Global warming during the Miocene Climate Optimum (MCO, ∼17–14 million years ago) is associated with massive carbon emissions sourced from the flood basalt volcanism and ocean crustal production. However, the perturbation of tectonic carbon degassing on the interaction between climate change and carbon cycle remains unclear. Here, through time-evolutive phase analysis of new and published high-resolution benthic foraminiferal oxygen (δ18O) and carbon (δ13C) isotope records from the global ocean, we find that variations in the marine carbon cycle lead the climate-cryosphere system (δ13C-lead-δ18O) on 405,000-year eccentricity timescales during the MCO. This is in contrast to the previously reported climate-lead-carbon (δ18O-lead-δ13C) scenario during most of the Oligo-Miocene (∼34–6 million years ago). Further sensitivity analysis and model simulations suggest that the elevated atmospheric CO2 concentrations and the resulting greenhouse effect strengthened the low-latitude hydrological cycle during the MCO, accelerating the response of marine carbon cycle to eccentricity forcing. Tropical climate processes played a more important role in regulating carbon-cycle variations when Earth’s climate was in a warm regime, as opposed to the dominant influence of polar ice-sheet dynamics during the Plio-Pleistocene (after ∼6 million years ago).

Disentangling microbial networks across pelagic zones in the tropical and subtropical global ocean

Microbial interactions are vital in maintaining ocean ecosystem function, yet their dynamic nature and complexity remain largely unexplored. Here, we use association networks to investigate possible ecological interactions in the marine microbiome among archaea, bacteria, and picoeukaryotes throughout different depths and geographical regions of the tropical and subtropical global ocean. Our findings reveal that potential microbial interactions change with depth and geographical scale, exhibiting highly heterogeneous distributions. A few potential interactions were global, meaning they occurred across regions at the same depth, while 11-36% were regional within specific depths. The bathypelagic zone had the lowest proportion of global associations, and regional associations increased with depth. Moreover, we observed that most surface water associations do not persist in deeper ocean layers despite microbial vertical dispersal. Our work contributes to a deeper understanding of the tropical and subtropical global ocean interactome, which is essential for addressing the challenges posed by global change.

New Insights into the Organic Complexation of Bioactive Trace Metals in the Global Ocean from the GEOTRACES Era

The GEOTRACES program has greatly expanded measurements of dissolved trace metal concentrations across ocean basins, but to understand the behavior and cycling of metals and their impacts on primary productivity, we must understand the chemical forms in which they are present in the environment. Organic ligands play a central role in the speciation and cycling of trace metals in the marine environment, controlling their chemical reactivity and bioavailability. Here, we present an overview of the contributions the GEOTRACES program has made to understanding ocean metal speciation through advancing our knowledge of the distribution, sources, and sinks of metal-binding organic ligands across the global ocean, particularly for iron. Detailed assessments and intercalibration of the speciation methods most commonly applied have allowed integration of metal-binding ligand measurements across datasets. Work to characterize specific ligand groups within the wider pool of dissolved organic matter, along with their sources and sinks, is starting to unravel the role of metal-binding organic ligands in global biogeochemical cycles. Recent advances in complementary analytical techniques using liquid chromatography and mass spectrometry present a molecular picture of metal speciation and bioavailability—and also pose new questions. Moving forward, we need to address knowledge gaps in our understanding of how metal speciation and complexation relates to bioavailability in order to recognize the impacts of ocean metal distributions and cycling on marine productivity and the global carbon cycle.