Abstract. Phytoplankton community composition is a key determinant of ocean biogeochemical cycles, yet its observation from 20 autonomous platforms remains challenging. In this study, we assessed the potential of in situ multispectral excitation fluorescence (MXF) to discriminate phytoplankton assemblages in the Northwestern Mediterranean Sea, with a view toward applications on Biogeochemical-Argo (BGC-Argo) profiling floats. Laboratory measurements on ten phytoplankton strains confirmed that MXF ratios at 440, 470, and 532 nm provide taxon-specific signatures, especially for picocyanobacteria and green algae. Field observations of phytoplankton pigments were clustered into four ecologically distinct phytoplankton communities along the seasonal cycle. A machine learning model was then trained to classify these clusters using MXF and additional bio-optical indices. Results show that existing BGC-Argo configurations (single-wavelength fluorescence, particulate backscattering and beam attenuation coefficients) reliably distinguish broad community structures such as pico- versus microphytoplankton dominance, but resolving finer pigment-based differences requires the added spectral information of MXF. The different excitation channels contributed unequally: 440 and 470 nm provided robust pigment sensitivity across communities, while 532 nm was particularly informative for detecting phycoerythrin- and chlorophyll b–rich taxa. Overall, combining MXF with bio-optical proxies improved classification performance by integrating pigment-specific and size-structure information, demonstrating the potential of MXF to enhance autonomous monitoring of phytoplankton community dynamics and their role in ocean biogeochemical cycles.

The Southern Ocean is a major sink of atmospheric carbon dioxide (CO2) and a key component of the global carbon cycle. Phytoplankton primary production modulates air-sea CO2 exchange, yet its response to ongoing climate-driven changes in storm intensity and storm-track position remains poorly constrained. A major challenge is that most primary production estimates rely on satellite observations restricted to the ocean surface, thereby missing subsurface production and limiting interpretation of storm-driven variability and long-term changes. Here we use the Copernicus Marine Service 3D biogeochemical product derived from in situ and satellite observations to reconstruct depth-resolved primary production over 1998–2023. Weekly three-dimensional fields of phytoplankton biomass and light-related variables are used as inputs to a depth- and phytoplankton-group-resolved bio-optical primary production model. Storm occurrence is characterized using the ERA5 atmospheric reanalysis. This approach allows us to examine how storms influence the vertical distribution of primary production and its partitioning among major phytoplankton groups. We evaluate how these effects vary across Southern Ocean regions and seasons.

Phytoplankton community composition is a key determinant of ocean biogeochemical cycles, yet its observation from autonomous platforms remains challenging. In this study, we assessed the potential of in situ multispectral excitation fluorescence (MXF) to predict phytoplankton community structure indices in the Northwestern Mediterranean Sea. With a view toward applications on Biogeochemical-Argo (BGC-Argo) profiling floats, we evaluated a miniaturised, three-excitation-channel fluorometer. Laboratory measurements on ten phytoplankton strains confirmed that MXF ratios at 440, 470, and 532 nm provide taxon-specific signatures, especially for picocyanobacteria and green algae. Field observations of phytoplankton pigments were clustered into four ecologically distinct phytoplankton communities across the seasonal cycle, which defined the targeted phytoplankton community structure indices. A machine learning model was then trained to classify these clusters using MXF and additional bio-optical indices. Results show that existing BGC-Argo configurations (single-wavelength fluorescence, particulate backscattering, and beam attenuation coefficients) reliably distinguish broad community structures, such as pico- versus microphytoplankton dominance, but resolving finer pigment-based differences requires the additional spectral information provided by MXF. The different excitation channels contributed unequally: 440 and 470 nm provided robust pigment sensitivity across communities, while 532 nm was particularly informative for detecting phycoerythrin-rich taxa. Overall, combining MXF with bio-optical proxies improved classification performance by integrating pigment-specific and size-structure information, demonstrating the potential of MXF to enhance autonomous monitoring of phytoplankton community dynamics and their role in ocean biogeochemical cycles.

Phytoplankton are key components of ocean ecosystems that play a critical role in regulating Earth's climate. However, how climate-driven changes in light availability in the ocean will affect marine phytoplankton remains poorly understood. Here, we assess the impact of climate-induced shifts in the spectral quality of the underwater light field on the relative fitness of phytoplankton with distinct pigment traits using a global ecosystem model. We focus on Synechococcus pigment types, comparing light color specialists with a chromatic acclimator capable of adjusting its pigment composition. Under a high-emission scenario, the model simulation projected an increase in the average blue-to-green ratio across 76% of the ocean area by the end of the 21st century, while 24% of the simulated ocean showed a shift toward greener wavelengths. Regions characterized by larger seasonal variability in blue-to-green ratio values appeared to be reduced due to climate-driven spectral changes. We find that reduced variability in the ocean light field makes the chromatic acclimators' plasticity less advantageous, and this pigment type was most negatively affected. These findings highlight the potential of Synechococcus pigment types as functional bioindicators of ecosystem change and underscore the importance of incorporating functional diversity in global models to better predict phytoplankton responses to changing ocean conditions.

Theme 1 addresses the Southern Ocean’s (south of 30°S) critical role in regulating Earth’s climate through circulation patterns that mediate global exchanges of heat, carbon, freshwater, and nutrients. The region has absorbed over 70% of anthropogenic heat and has contributed to about 40% of the global ocean uptake of human-emitted carbon dioxide (CO₂) while also controlling ice shelf stability and sea level rise. However, fundamental gaps in year-round observations, particularly during austral autumn, winter, and spring, severely limit our understanding of these processes and their responses to rapid climate change. This white paper establishes key knowledge gaps and high-priority recommendations that are tractable within the InSync timeframe through coordinated program execution with strong engagement from national operators and funding agencies. Critical needs include seasonal observations of the marginal ice zone where carbon, heat and nutrient-rich waters upwell, year-round continental shelf measurements where dense water formation and ice-ocean interactions occur, standardized air-sea flux measurements, and strategic monitoring of regional choke points. Success requires international resource sharing, coordinated deployments, and sustained commitment to both process studies and long-term monitoring.

Sea spray is a large source of cloud condensation nuclei (CCN) over remote oceans, still the dependence of its emission number flux on marine biology is not often represented in models and is subject to high uncertainty. We implemented a recently developed parameterization which relates the number flux of sea spray aerosols to nanophytoplankton cell abundances in the mesoscale model WRF‐Chem and evaluated the impact on the prediction of sea spray aerosols (SSA) number concentrations and cloud properties over the Southern Ocean. Compared to SSA concentrations simulated using the whitecap function available in the model, the use of a function derived from recent observations in the Southern Ocean leads to a decrease in predicted SSA number concentrations (by ∼46%–47% over the domain); this further translates into a decrease in the cloud droplet number concentrations and a decrease (respectively an increase) of the liquid cloud water path (CWP) and rain water path (RWP). A strong increase in SSA number concentrations is predicted when the influence of marine biota is taken into account (by 172%–343%), with a concurrent clear increase of the number of cloud droplets and a subsequent increase (decrease) of the CWP (RWP). Detailed comparison of model outputs and measurements further demonstrates that accounting for the role of marine biological activity is essential to predict more realistic SSA number concentrations and spatio‐temporal variability. Comparisons of modeled vertical profiles of CWP and RWP with remote sensing measurements are also ameliorated with the account of marine biology.

Marine emission of the volatile gas dimethyl sulfide (DMS) is the most substantial source of natural sulfur in the global atmosphere. DMS is believed to play a significant role in the Earth's climate system as a precursor to new particle formation and cloud condensation nuclei in the pristine marine atmosphere. To simulate the global distribution of seawater DMS, atmospheric models use DMS climatologies. In this study, we tested the sensitivity of atmospheric DMS concentrations over the Southwest Pacific Ocean, simulated with the WRF‐Chem regional model, to five seawater DMS climatologies developed over the last two decades together with seawater DMS inferred from a recently developed relationship from nanophytoplankton satellite retrievals. Comparisons with in situ observations recently obtained in the Southwest Pacific and the Southern Ocean revealed that oceanic DMS climatologies are less accurate for latitudes south of 65°S than between 40°S and 65°S. In addition, in our study area, the spatial distribution of marine DMS is more accurately reproduced in the 40–65°S latitudes when deduced from a surface ocean biological variable, particularly when using a recently derived relationship from nanophytoplankton satellite retrievals, rather than from observation‐based climatologies. Simulated atmospheric DMS levels were sensitive to the oceanic DMS climatology used but the atmospheric DMS concentration variability was mostly dependent on the atmospheric dynamics. Atmospheric DMS concentrations and variability measured off the New‐Zealand coast are fairly well reproduced in WRF‐Chem using accurate phytoplanktonic assemblages and the nanophytoplankton‐related seawater DMS concentration.

The South Pacific Ocean stands out as a dynamical region with contrasted biogeochemical (BGC) regimes. Among others, it encompasses mesotrophic areas as well as the most oligotrophic waters of the global ocean. Within these regions, the existing thousands of islands can locally or regionally disrupt the oceanic circulation and, thereby, thus the nutrient availability in the upper sunlit layer and the phytoplankton growth. Yet, the phytoplankton seasonal dynamics in these contrasted BGC regions remain largely unknown and understood. Indeed, most existing studies on phytoplankton seasonal variability from observations are either dedicated to the global ocean or based on remotely sensed data due to a lack of in-situ observations in the water column, preventing the consideration of 3D processes. Here we took advantage of in situ observations from 13 BGC-Argo profiling floats that have drifted from 2015 to 2023 in five subregions of the South Pacific Ocean: the Tasman and Coral Seas, the Fiji island region, the oligotrophic and equatorial mesotrophic areas. We used measurements of temperature, salinity, chlorophyll-a fluorescence (Chl), particulate backscattering at 700 nm (bbp) used as a proxy of particulate organic carbon and Photosynthetically Active Radiation. The seasonal variations of Chl and bbp vertical distributions are characterized among the subregions and physical and biogeochemical processes likely involved have been investigated. To do so, we considered isolume and nutricline depths, the Mixed Layer Depth (MLD) as well as the maximum Brunt-Vaissala depth as an indicator of the ocean stratification stability. The latter appears more suitable than the MLD when related to the phytoplankton seasonal dynamics.

The seasonal variability of phytoplankton vertical distribution is investigated in the South Pacific where observations are scarce and scattered. We used 13 BioGeoChemical-Argo floats deployed across diverse oceanic environments. The seasonal latitudinal displacement of the Tasman front induces transitions from mesotrophic to oligotrophic conditions. This shift results in Chlorophyll-a concentration vertical distribution changing from bloom types to Subsurface Chlorophyll Maxima (SCM) types, with intermediate hybrid types between these extremes. Such hybrid profiles frequently occur in the equatorial Pacific, highlighting a large-scale pattern rather than local island mass effect. In oligotrophic regions, seasonal variations of light availability and stratification dynamics below the mixed layer likely relate SCM to an increase in carbon biomass or photoacclimation. A biomass increase is frequently observed, contrary to previous studies, suggesting that subsurface phytoplankton biomass may have been largely underestimated. This calls for further observations of the water column in these remote undersampled open ocean areas.

Dimethyl sulphide (DMS) and methanethiol (MeSH) emissions from South Pacific surface seawater were determined in deck board Air-Sea Interface Tanks during the Sea2Cloud voyage in March 2020. The measured fluxes from water to headspace (F) varied with water mass type, with lowest fluxes observed with Subtropical and Subantarctic waters and highest fluxes from Frontal waters. Measured DMS fluxes were consistent with fluxes calculated using a two-layer model and seawater DMS concentrations. The MeSH:DMS flux ratio was 11%-18% across the three water mass types, confirming that MeSH may represent a significant unaccounted contribution to the atmospheric sulfur budget, with potentially important implications for marine aerosol formation and growth in models. Combining data from the ASITs and ambient surface seawater identified significant Spearman rank correlations for both dissolved DMS and MeSH with nanophytoplankton cell abundance (pvalue < 0.012), suggesting an important role for this phytoplankton size class in determining regional DMS and MeSH emissions. Applying a nanophytoplankton-based parameterization to estimate DMSw provided good agreement with a recent DMS climatology. Consequently, the observed relationship between DMSw, MeSHw and nanophytoplankton cell abundances may be applicable for modeling atmospheric fluxes.

Phytoplankton biomass, the foundation of the oceanic food web, is predominantly estimated from chlorophyll-a (Chla) concentration. In vivo chlorophyll-a fluorescence (fluo), a key proxy for Chla, has become one of the most widely measured biogeochemical parameters in the ocean. This advancement is largely due to the integration of fluorometers onto BioGeoChemical-Argo (BGC-Argo) profiling floats, a key component of the multidisciplinary OneArgo array. By significantly expanding the number of fluo profiles compared to historical ship-based observations, this development has solidified OneArgo's role as a cornerstone of the global biogeochemical observing system.However, converting fluo into Chla is not straightforward, as it is influenced by various factors, including the composition and physiological state of phytoplankton communities. Accurate calibration of fluo into Chla is therefore both challenging and essential for fully utilizing the rapidly growing volume of fluo data. The Argo Data Management Team (ADMT) has made significant efforts to calibrate and validate fluo measurements from OneArgo floats, aiming to deliver Chla estimates with the highest possible accuracy. Despite these efforts, the current OneArgo Chla dataset still exhibits substantial regional biases in real-time (RT), particularly in high-latitude regions such as the Southern Ocean.Recent advances in observation-based products have introduced innovative solutions to address these challenges, including new delayed-mode (DM) correction methods that significantly reduce regional biases in Chla estimates. However, a key issue persists: DM and real-time (RT) datasets often differ considerably depending on the location, resulting in inconsistencies that compromise the homogeneity and interoperability of the OneArgo database. To address this, we propose a new RT correction method, based on observation-based products, to improve Chla accuracy and better align RT data with DM-calibrated values. This advancement is expected to be implemented soon, enabling a more seamless integration of RT and DM datasets and ultimately enhancing the overall quality and utility of the OneArgo Chla dataset.This study underscores the potential of new observation-based products to enhance the accuracy and coherence of the OneArgo Chla dataset. High-quality OneArgo data are critical for both scientific research and operational oceanography, including the assimilation of data into biogeochemical models.

Marine photoautotrophs have evolved to exploit the ocean's variable light conditions, with chromatic acclimators being able to adjust their pigment content to better match the ambient light color. The impact of chromatic acclimation on phytoplankton distribution and competition is not well understood despite its global importance. This study explores chromatic acclimation's role in shaping the biogeography of Synechococcus, a widespread cyanobacterium. We integrated three pigment types into a global ecosystem model: a green-light specialist, a blue-light specialist, and a chromatic acclimator. Laboratory studies defined each type's specific absorption properties. Our results indicate that chromatic acclimation offers an evolutionary advantage by enabling Synechococcus to adapt to varying light environments. This ability to mimic blue- and green-light specialists and enhance absorption at intermediate states, particularly in areas with high seasonal light variations, increases Synechococcus distribution and biomass. Thus, chromatic acclimation affects ecosystem functioning and biogeochemical processes in the ocean.

1 LOPS, IUEM, IRD, Ifremer, CNRS, Univ. Brest, Brest, France, 2 EIO, IRD, Ifremer, UPF and ILM, Tahiti, French Polynesia, 3 LOCEAN-IPSL, Sorbonne Universités/UPMC-CNRS-IRD-MNHN, Paris, France, 4 ENTROPIE, IRD, Univ. de la Réunion, CNRS, Univ. de la Nouvelle Calédonie, Ifremer, Noumea, New Caledonia, 5 Laboratoire d’Océanographie de Villefranche, CNRS and Sorbonne Université, Villefranche-sur-Mer, France, Georgia Institute of Technology, Atlanta, GA, United States, 7 IMT Atlantique, Lab-STICC, UMR CNRS 6285, Brest, France

Dans l’océan, des organismes microscopiques sont à la fois les peintres et la teinte d’un grand tondo1 planétaire. Ces organismes, le phytoplancton, de couleur verte, sous des conditions marines favorables, colorent d’énormes zones de l’océan. Tellement énormes que leur extension ne peut être observée que depuis l’espace. Nous présentons ici quelques images de la Méditerranée nord-occidentale, prises par un satellite scientifique, qui nous montrent la beauté de ce tondo visible seulement depuis une altitude de 700 km.

AbstractThere is growing evidence that marine microorganisms may influence cloud cover over the ocean through their impact on sea spray and trace gas emissions, further forming cloud droplets or ice crystals. However, evidence of a robust causal relationship based on observations is still pending. In this study, we use 4 years of multi‐instrument satellite data to segregate low‐level clouds into ice‐containing and liquid‐water clouds to obtain clear relationships between cloud types and ocean biological tracers, especially with nanophytoplankton cell abundances. Results suggest that microorganisms may be involved in compensating effects on cloud properties, increasing the frequency of occurrence of warm‐liquid clouds, and decreasing the occurrence of ice‐containing clouds in most regions during springtime. The relationships observed in most regions do not apply to the South Pacific Ocean in the 40°S–50°S latitude band. These results shed light on overlooked potential compensating effects of ocean microorganisms on cloud cover.

The Southern Ocean (SO) is known for its atypical bio-optical regime. This complicates the interpretation of proxies measured from satellite and in situ platforms equipped with optical sensors, which occupy an important niche for monitoring the vast and remote SO. A ship-based field study in concert with time series observations from BioGeoChemical-Argo (BGC-Argo) profiling floats were used to investigate spatial and temporal variations in bio-optical relationships in the open ocean waters surrounding the Kerguelen Plateau in the Indian sector of the SO. Compared to other regions with similar chlorophyll concentrations, chlorophyll-specific phytoplankton absorption in the blue waveband presented a consistent negative anomaly. The anomaly was uniform over deep mixed layers and correlated with phytoplankton size, photoacclimation and atypically high concentrations of fucoxanthin. The BGC-Argo observation-based proxies revealed that the blue absorption anomaly increased with chlorophyll concentration both spatially and temporally and, while particularly pronounced in the naturally iron-fertilized waters, was also found in the High Nutrient Low Chlorophyll region. While phytoplankton size was an important driver of the anomaly, photoacclimation associated with self-shading of phytoplankton cells was also involved during intense booms. The backscattering coefficient exhibited negative and positive anomalies in the low and high biomass regimes, respectively. The large positive anomaly in high biomass regimes was attributed to the variable non-algal particles characteristics associated with a relatively high production of bloom by-products. With clear understanding of the bio-optical anomalies, BGC-Argo floats stand as unique tools for monitoring the bio-optical spatio-temporal complexity of the SO. The Southern Ocean (SO) plays a key role in Earth's climate. However, its remoteness and harsh climate necessitate remote sensing approaches such as optical sensors on profiling floats and satellites. These require optical measurements and shipboard samples to relate optical signals to biological properties, called optical proxies. The SO optical proxies have been found to be very different from those measured in other oceanic regions. Here the proxies in the Kerguelen region are investigated along a ship's track crossing from subtropical to subpolar latitudes. The Kerguelen Plateau impedes both wind and currents such that deep waters upwell to the surface providing nutrients to primary producers. This region in the Indian sector of the SO provides a tremendous range in ecosystem properties from nutrient-starved to nutrient-rich, making these results applicable to other SO areas. The authors find that the differences in optical proxies can be explained by the presence of different phytoplankton communities and their responses to being mixed to depths where light is limited. This explains why the proxies vary so significantly across this region and change over the seasons. This knowledge makes the observations obtained by floats and satellites invaluable for understanding the entire SO. Contribution by large diatoms and photoacclimation are major drivers of a negative anomaly in the blue chlorophyll-specific phytoplankton absorption coefficientBioGeoChemical-Argo observations indicate a bio-optical anomaly exists over the entire year with maximal values associated with seasonal bloomsThe Indian Southern Ocean shows a large seasonal and regional variability in bio-optical regimes that lie outside the global means

Measuring plankton and associated variables as part of ocean time-series stations has the potential to revolutionize our understanding of ocean biology and ecology and their ties to ocean biogeochemistry. It will open temporal scales (e.g., resolving diel cycles) not typically sampled as a function of depth. In this review we motivate the addition of biological measurements to time-series sites by detailing science questions they could help address, reviewing existing technology that could be deployed, and providing examples of time-series sites already deploying some of those technologies. We consider here the opportunities that exist through global coordination within the OceanSITES network for long-term (climate) time series station in the open ocean. Especially with respect to data management, global solutions are needed as these are critical to maximize the utility of such data. We conclude by providing recommendations for an implementation plan.