Giorgio Dall'OLmo

Phytoplankton in the Southern Ocean strongly influence global biogeochemical cycles, but the region's remoteness limits in situ monitoring. Satellite ocean color enables synoptic monitoring of phytoplankton dynamics by retrieving chlorophyll‐a concentration (Chl‐a). However, Southern Ocean optical properties deviate from global patterns, causing persistent ocean‐color anomalies and uncertain Chl‐a retrievals. These anomalies have been attributed to differences in either phytoplankton composition or non‐algal constituents relative to low‐latitude waters, but a unified explanation remains lacking. Using an extensive in situ data set, we show that phytoplankton size structure in Antarctic waters is fundamentally distinct from that in other ocean basins. Incorporating this distinct size distribution into an ocean‐color model, with the same underlying bio‐optical parameterizations, successfully reproduces the observed bio‐optical anomaly. Simulations further show that changes in size structure alter the bulk optical properties of both phytoplankton and non‐algal constituents. Tracking phytoplankton size structure is therefore essential for improving satellite Chl‐a trends and monitoring Southern Ocean biogeochemical cycles.

Phytoplankton in the Southern Ocean play a major role in regulating global biogeochemical cycles, but the region's remoteness makes in-situ monitoring challenging. Satellite ocean colour is among the best tools to monitor phytoplankton dynamics synoptically through retrievals of the chlorophyll-a concentration (Chl-a). However, optical properties in the Southern Ocean deviate from global patterns, causing persistent ocean-colour anomalies and biased Chl-a retrievals. Theories have attributed these anomalies to differences in either phytoplankton composition or non-algal constituents relative to global norms, each capturing important facets but without a unified explanation. Here, using an extensive set of in-situ measurements, we demonstrate that phytoplankton size structure in the Southern Ocean is fundamentally different to other ocean basins. Incorporating this distinct size distribution into an ocean-colour model, with the same set of optical parameters, successfully reproduces the observed bio-optical anomaly. Model simulations further reveal that these size shifts affect bulk optical properties of both phytoplankton and non-algal constituents, providing a unified explanation that reconciles the observed regional anomalies within a global framework. Tracking changes in phytoplankton size structure is key to improving satellite Chl-a trends in the Southern Ocean, with important implications for improved monitoring of the regional biogeochemical cycles that sustain Earth's climate system.

The contribution of the ocean biological carbon pump to the export of organic carbon at depth has predominantly been assessed by considering sinking particulate matter and vertically migrating organisms. Despite growing recognition of the importance of dynamical pathways that export carbon through upper-ocean mixing and advection, observation-based estimates of their global impact are still lacking. Here, we quantify the values and uncertainties of the export driven by the physical injection pump (PIP) and its interannual variability by leveraging a 4D data-driven time series (1997-2018) of particulate organic carbon concentration (POC) and ocean circulation, as well as 3D fields of climatological dissolved organic carbon (DOC). Vertical diffusion dominates our POC export estimates, but remains the most uncertain process. Assuming maximal diffusivity estimates that are consistent with observations, POC and DOC export amount to 0.37 Pg C yr⁻¹ and 0.48 Pg C yr⁻¹, respectively. The contribution from entrainment and advection is strongly modulated by the POC annual cycle, revealing the critical coupling between biological production and upper-layer mixing in driving the net annual export. Observed interannual signals correlate with a linear combination of El Niño–Southern Oscillation and Southern Annular Mode indices, suggesting that the PIP is connected to intermediate- and mode-water formation dynamics in the Southern Ocean.

Sea-air exchange of carbon dioxide in the Southern Ocean is strongly seasonal, with ocean uptake in summer, which is partly offset by carbon dioxide outgassing in winter. This seasonal balance can shift due to sea ice conditions, inducing interannual variability in the Southern Ocean carbon sink. A decade (2010–2020) of unique, year-round marine carbonate chemistry observations from the Rothera Time Series (West Antarctic Peninsula) reveals that interannual variability in seawater fugacity of carbon dioxide depends on wintertime processes. Sea ice duration controls ocean stratification, which acts as a gateway to the carbon-rich ocean interior. Consequently, years with persistent sea ice cover and high mean winter stratification absorb, on average, 20% more carbon dioxide than years with less sea ice and weaker stratification in winter. Wintertime marine observations are therefore essential to resolve critical processes and reliably quantify interannual variability of the sea-air carbon dioxide flux in seasonally sea ice-covered regions.

The ocean is a large but uncertain sink of tropospheric ozone. Ozone deposition is controlled partly by its reactions with marine substances, but in situ evidence of this marine biogeochemical control remains sparse. Here we report a novel measurement of ozone uptake efficiency (OUE) from a trans‐Atlantic cruise (50°N to 45°S). Observed OUE for surface waters varied two‐fold and the implied chemical deposition velocity varied from 0.012 to 0.034 cm s ⁻¹ . Iodide accounted for on average 2/3 of total OUE, with generally higher contributions in tropical waters. The residual OUE, generally higher in temperate waters and positively correlated with biological proxies, was likely due to marine organics. OUE was also measured for 1,000 m waters, which were likely devoid of iodide but contained biologically refractory organics. Unexpectedly, these waters were rather reactive toward ozone, suggesting that surface organics that affect ozone uptake are not all freshly produced by marine biota.

In situ measurements of particulate inherent optical properties (IOPs) – absorption (a_p(λ)), scattering (b_p(λ)), and beam attenuation (c_p(λ)) – are crucial for the development of optical algorithms that retrieve biogeochemical quantities such as chlorophyll a, particulate organic carbon (POC), and total suspended matter (TSM). Here we present a compilation of particulate absorption–attenuation spectrophotometric data measured underway on nine Atlantic Meridional Transect (AMT) cruises between 50° N and 50° S from 2009–2019. The compilation includes coincident high-performance liquid chromatography (HPLC) phytoplankton pigment concentrations, which are used to calibrate transects of total chlorophyll a (Tot_Chl_a) concentrations derived from the a_p(λ) line-height method. The IOP data are processed using a consistent methodology and include propagated uncertainties for each IOP variable, uncertainty quantification for the Tot_Chl_a concentrations based on HPLC match-ups, application of consistent quality-control filters, and standardization of output data fields and formats. The total IOP dataset consists of ∼310 000 measurements at a 1 min binning (∼270 000 hyper-spectral) and >700 coincident HPLC pigment surface samples (∼600 of which are coincident with hyper-spectral IOPs). We present the geographic variation in the IOPs, HPLC phytoplankton pigments, and a_p-derived Tot_Chl_a concentrations which are shown to have uncertainties between 8 % and 20 %. Additionally, to stimulate further investigation of accessory pigment extraction from a_p(λ), we quantify pigment correlation matrices and identify spectral characteristics of end-member a_p(λ) spectra, where accessory pigment groupings are present in higher concentrations relative to Tot_Chl_a. All data are made publicly available in SeaBASS and NetCDF formats via the following links: https://seabass.gsfc.nasa.gov/archive/PML/AMT (Jordan et al., 2025 a) and https://doi.org/10.5281/zenodo.12527954 (Jordan et al., 2024).

Satellite-derived observations of ocean colour provide continuous data on chlorophyll-a concentration (Chl-a) at global scales but are limited to the ocean’s surface. So far, biogeochemical models have been the only means of generating continuous vertically resolved Chl-a profiles on a regular grid. MULTIOBS is a multi-observations oceanographic dataset that provides depth-resolved biological data based on merged satellite- and Argo-derived in situ hydrological data. This product is distributed by the European Union’s Copernicus Marine Service and offers global multiyear, gridded Chl-a profiles within the ocean’s productive zone at a weekly temporal resolution. MULTIOBS addresses the scarcity of observation-based vertically resolved Chl-a datasets, particularly in less sampled regions like the Eastern Mediterranean Sea (EMS). Here, we conduct an independent evaluation of the MULTIOBS dataset in the oligotrophic waters of the EMS using in situ Chl-a profiles. Our analysis shows that this product accurately and precisely retrieves Chl-a across depths, with a slight 1% overestimation and an observed 1.5-fold average deviation between in situ data and MULTIOBS estimates. The deep chlorophyll maximum (DCM) is adequately estimated by MULTIOBS both in terms of positioning (root mean square error, RMSE = 13 m) and in terms of Chl-a (RMSE = 0.09 mg m⁻³). The product accurately reproduces the seasonal variability of Chl-a and it performs reasonably well in reflecting its interannual variability across various depths within the productive layer (0–120 m) of the EMS. We conclude that MULTIOBS is a valuable dataset providing vertically resolved Chl-a data, enabling a holistic understanding of euphotic zone-integrated Chl-a with an unprecedented spatiotemporal resolution spanning 25 years, which is essential for elucidating long-term trends and variability in oceanic primary productivity.

In situ measurements of particulate inherent optical properties (IOPs) – absorption (a_p(λ)), scattering (b_p(λ)), and beam attenuation (c_p(λ)) – are crucial for development of optical algorithms that retrieve biogeochemical quantities such as Chlorophyll a, particulate organic carbon (POC) and total suspended matter (TSM). Here we present a compilation of particulate absorption-attenuation spectrophotometric data measured underway on nine Atlantic Meridional Transect (AMT) cruises between 50° north to 50° south from 2009–2019. The compilation includes co-incident high performance liquid chromatography (HPLC) phytoplankton pigment concentrations, which are used to calibrate transects of Total Chlorophyll a (Tot_Chl_a) concentrations derived from the a_p(λ) line-height method. The IOP data are processed using a consistent methodology, and include propagated uncertainties for each IOP variable, uncertainty quantification for the Tot_Chl_a concentrations based on HPLC match-ups, application of consistent set of quality-control filters, and standardisation of output data fields and formats. The total IOP dataset consists of ~310,000 measurements at a 1 minute binning (~270,000 hyper-spectral) and > 700 co-incident HPLC pigment surface samples (~600 of which are coincident with hyper-spectral IOPs). We present geographic variation in the IOPs, HPLC phytoplankton pigments, and the a_p-derived Tot_Chl_a concentrations, which are shown to have uncertainties between 8–20 %. Additionally, to stimulate further investigation of accessory pigment extraction from a_p(λ), we quantify pigment correlation matrices and identify spectral characteristics of end-member a_p(λ) spectra where accessory pigment groupings are present in higher concentrations relative to Tot_Chl_a. All data are made publicly available in SeaBASS and NetCDF formats via the following links: https://seabass.gsfc.nasa.gov/archive/PML/AMT, and https://doi.org/10.5281/zenodo.12527954.