Astrid Bracher

The Fram Strait is the primary gateway between the North Atlantic and the Arctic Ocean, where ongoing Atlantification is reshaping hydrographic conditions and marine ecosystems. We applied ML-PhyTAO, a machine learning based retrieval model for Arctic phytoplankton functional types (PFTs), to generate a consistent 28-year (1998–2025) satellite record of total chlorophyll-a (TChla) and major PFT chlorophyll-a concentrations (Chla) in the Fram Strait. Trained with an extensive Arctic phytoplankton pigment dataset and ocean color and environmental products, the model provides robust performance for long-term analyses. Using a dynamic water mass classification based on sea surface temperature and sea ice fraction, the Fram Strait was partitioned into Atlantic Water (AW), Mixed Water (MW), and Polar Water (PW). Distinct seasonal cycles were observed among water masses, with later but more intense phytoplankton blooms in AW. Peak biomass timing advanced by 2–13 days decade⁻¹ for TChla, diatoms, and haptophytes in both AW and MW. Trend analyses revealed declining biomass primarily in MW and the AW–MW transition zone, while haptophytes and dinoflagellates increased within AW. Sea surface salinity, temperature, and sea ice fraction showed stronger and more spatially coherent relationships with PFT variability than modeled nutrient fields, indicating that Atlantification related hydrographic changes are driving shifts in phytoplankton phenology and community composition. This study highlights the unique value of long-term, consistent PFT observations for detecting climate-driven ecosystem change, linking hydrographic variability to community shifts, and supporting future ecosystem and biogeochemical assessments and modelling studies in the rapidly changing Arctic Ocean.

A global in situ dataset for validation of satellite products from the ESA Ocean Colour Climate Change Initiative (OC-CCI) is presented. This version of the compilation, with data starting in 1996, extends to 2025, which is important for the validation of recent algorithms and satellite products. The dataset comprises in situ observations of the following variables: spectral remote-sensing reflectance (rrs), chlorophyll-a concentration ("chla"; HPLC and fluorometric), spectral inherent optical properties (IOPs: algal pigment absorption "aph", detritus plus gelbstoff absorption "adg", and particle backscattering "bbp"), spectral diffuse attenuation coefficient (kd) and total suspended matter (tsm). Data were obtained from multiple archives acquired via open internet services, or from individual projects, acquired directly from data providers. Compared to the previous version (Valente et al., 2022) this release incorporates updates from existing sources and introduces data from 13 additional sources. Notably, AERONET-OC coverage has been significantly expanded (39 sites compared to 11 in v2022) and MOBY Platinum data have been included from 2024. Methodologies were implemented for homogenisation, quality control and merging of all data. Minimal changes were made to the original data, other than conversion to a standard format, elimination of some points after quality control and averaging of observations that were close in time and space. The harmonisation procedures also include the application of a bidirectional reflectance distribution function (BRDF) correction to "rrs". The result is a merged table available in text format. Overall, the dataset increased by ~115 %, reaching 319,183 rows, with each row representing a unique station in space and time (cf 148,432 rows in Valente et al., 2022). Records of chlaovera increased by ~31 % (82,543 to 107,922); "rrs" observations nearly doubled +94 % (68,641 → 133,325); IOPs showed substantial expansion, with aph increasing by ~150 % (4,265 to 10,655) and adg by 78 % (1,654 to 2,948). The most pronounced change was for bbp, which increased by ~10,258 % (792 to 82,033), while moderate increases were observed for kd (+30 %; 2,454 to 3,197) and "tsm" (+51 %; 1,546 to 2,334). Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) are included in the final table. By making the metadata available, provenance is better documented, and it is also possible to analyse each set of data separately. The compiled data are available at (Salem et al., 2026).

Climate change has dramatically altered the Arctic seas with significant decrease in sea ice extent and thickness and warming water temperature. The ecological impacts of such change have been described for many parts of the Arctic Ocean, but long-term records of biological indicators are still missing. Among those, photosynthetic and accessory pigments are one of the key tools that aid quantification of phytoplankton and sea–ice algae biomass and characterisation of community composition. To address this gap, we present the first pan-Arctic compilation of in situ algal pigment data obtained exclusively by High-Performance Liquid Chromatography (HPLC), containing 10 798 samples collected across 77 Arctic research cruises between 2000 and 2024. As a result of large-scale collaborative effort, this database covers both open water and sea–ice environments across coastal, shelf and open domains. The database (https://doi.org/10.11583/DTU.29445104, Heidemann et al., 2026) includes measures of up to 26 pigments, with 8 major marker/accessory pigments being considered in this study, namely Alloxanthin (Allo), 19'-Butanoyloxyfucoxanthin (But-fuco), Chlorophyll a (Chl a ), Chlorophyll b (Chl b ), Fucoxanthin (Fuco), 19'-Hexanoyloxyfucoxanthin (Hex-fuco), Peridinin (Peri), and Zeaxanthin (Zea). This publicly available database provides crucial data that can be used to assess phytoplankton dynamics, validating remote sensing observations and can serve as a resource for future Arctic ecological- and modelling studies.

Marine combined carbohydrates in aerosol particles (CCHOaer) have the potential to influence cloud formation and properties, but it remains unclear to what extent they reach altitudes relevant for cloud processes. Balloon-borne measurements of major sea spray aerosol (SSA) constituents, including sodium (Naaer+) and CCHOaer, were conducted in autumn 2021 and spring 2022 in Ny-Ålesund (Svalbard). Total suspended particles were collected at 321–1112 m, covering both the marine boundary layer and the free troposphere, with Naaer+ ranging 23–850 ng m−3 and CCHOaer 3.8–274 ng m−3. The chemical composition of balloon-borne aerosol samples was compared with synchronized ground level measurements at the balloon's winch (Naaer+: 35–3710 ng m−3; CCHOaer: 1.9–194 ng m−3), and at the Old Pier (Naaer+: 140–1470 ng m−3; CCHOaer: 1.6–10.0 ng m−3), where freshly emitted SSA particles were sampled. Surface seawater from the Kongsfjorden was analyzed to evaluate the sea-air transfer of marine CCHO. Air mass histories, atmospheric mixing, and cloud conditions were evaluated for three selected cases to explain vertical concentration patterns. A strong correlation (R=0.78, p<0.001) between combined xylose (< 0.2–14.1 ng m−3) in CCHOaer and oxalateaer (< 1–67 ng m−3) across all altitudes, suggests either coproduction or a connection through atmospheric processing. These results provide a first comprehensive picture of how local primary sea-air transfer of marine combined carbohydrates, long-range transport, in-situ formation, and atmospheric processing together shape their distribution.

Understanding phytoplankton composition and distribution is essential for assessing marine ecosystem processes, yet satellite and in situ datasets each have substantial limitations. This study develops a fusion framework that combines gap-filled satellite-derived total chlorophyll a (TChla) and phytoplankton functional type (PFT) chlorophyll a (Chla) concentrations with in situ data points extracted from a hyperspectral flow-through spectrophotometric system (ACS). The objective is to generate spatially continuous and accurate Chla fields. The in situ hyperspectral absorption data obtained during the RV Polarstern PS113 expedition were used to derive TChla and PFT Chla concentrations with associated uncertainties, using an empirical orthogonal function based regression model (EOF-GLM). The ACS-derived estimates were then merged with the gap-filled satellite products through optimal interpolation to produce a consistent spatiotemporal dataset. The ACS-derived TChla and PFT Chla products showed strong agreement with the ground truth Chla concentrations (HPLC-Chla), with the coefficient of determination (R²) values above 0.66 for all groups and median percentage-deviation (MedPD) values between 27% and 46%. The fusion dataset further improved performance, reaching R² values higher than 0.69 and MedPD values between 21% and 44%. The combined product provides spatially and temporally continuous Chla fields and resolves regions with extensive satellite data gaps more effectively than either dataset alone. This fusion approach supports more reliable analyses of phytoplankton dynamics and can guide sampling strategies in areas with limited satellite coverage. Future work should focus on refining uncertainty estimates and testing the framework across different oceanic regions to improve its stability and broader applicability.

Climate change has dramatically altered the Arctic seas with significant decrease in sea ice extent and thickness and warming water temperature. The ecological impacts of such change have been described for many parts of the Arctic Ocean, but long-term records of biological indicators are still missing. Among those, photosynthetic and accessory pigments are one of the key tools that aid quantification of phytoplankton and sea-ice algae biomass and characterisation of community composition. To address this gap, we present the first pan-Arctic compilation of in situ algal pigment data obtained exclusively by High-Performance Liquid Chromatography (HPLC), containing 10.798 samples collected across 77 Arctic research cruises between 2000 and 2024. As a result of large-scale collaborative effort, this database covers both open water and sea-ice environments across coastal, shelf and open domains. The database (10.11583/DTU.29445104) includes measures of up to 26 pigments, with 8 major marker/accessory pigments being considered in this study, namely Alloxanthin (Allo), 19’-Butanoyloxyfucoxanthin (Butfuco), Chlorophyll a (Chl-a), Chlorophyll b (Chl-b), Fucoxanthin (Fuco), 19’-Hexanoyloxyfucoxanthin (Hexfuco), Peridinin (Peri), and Zeaxanthin (Zea). This publicly available database provides crucial data that can be used to assess phytoplankton dynamics, validating remote sensing observations and can serve as a resource for future Arctic ecological- and modelling studies.

Marine carbohydrates are produced by a wide range of micro- and macroorganisms in seawater and are transferred to the atmosphere via sea spray aerosol (SSA). Recent laboratory and modelling studies suggest that these compounds can influence fog and cloud microphysics as ice nucleating particles. However, observational evidence from the atmosphere remains limited, as most field studies have relied on ship- or land-based filter samples, leaving their relevance for cloud processes at cloud-relevant altitudes largely unconstrained.Here, we present new measurements of marine carbohydrates and other SSA components at altitudes between 300 and 1200 m, obtained using a tethered helium balloon in Ny-Ålesund (Svalbard) during 2021-2022. These observations are compared with fresh SSA directly collected at the Kongsfjorden coast and with surface seawater samples to assess contributions beyond local ocean emissions. Our results highlight the key role of meteorological conditions in lifting and redistributing SSA constituents, including marine carbohydrates, to higher atmospheric layers. The study further examines potential additional sources and formation pathways, providing new insights into the atmospheric behaviour of marine carbohydrates and their implications for cloud microphysics.We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project Number 268020496—TRR 172, within the framework of the Transregional Collaborative Research Center “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3”

Primary marine organic aerosol (PMOA) constitutes an important fraction of the aerosol population over remote oceanic regions and plays a relevant role in aerosol–cloud–climate interactions. In the Arctic, ongoing sea-ice retreat and intensified summer ice loss are expected to enhance marine aerosol emissions. Here, we employ an extended version of the aerosol–climate model ECHAM6.3-HAM2.3 to examine the spatial distribution and long-term temporal evolution of PMOA emissions and transport in the Arctic for the period 1990–2019, accounting for changing climatic and sea-ice conditions. Marine biogeochemical fields are provided by the offline model FESOM2.1-REcoM3, from which three aerosol-relevant biomolecular species groups - polysaccharides (PCHO), amino acids (DCAA), and polar lipids (PL) - are represented. Their transfer from the ocean to the atmosphere is parameterized using OCEANFILMS, recently implemented in ECHAM6.3-HAM2.3 to enhance the marine emission scheme.The model results indicate that PMOA emission fluxes are primarily controlled by marine biological activity and sea-salt production, the latter mainly depending on near-surface winds. Biomolecular concentrations show limited variability in equatorial regions but pronounced seasonal cycles toward high latitudes. In seawater, PCHO dominates the simulated organic pool, followed by DCAA and PL. In contrast, PL contributes the largest fraction to aerosol-phase organic matter due to the comparatively strong air-seawater affinity of lipids. Arctic PMOA emissions and atmospheric transport peak between May and September, coinciding with the phytoplankton bloom and the seasonal sea-ice minimum. Substantial regional differences are evident in the timing of biomolecule production and aerosol emissions across the Arctic. Simulated PMOA seasonality agrees reasonably well with available ground-based observations, given the uncertainties in both measurements and model assumptions.Over the 30-year period, accumulated Arctic aerosol emissions and burdens increased by at least 7% and 4%, respectively, when comparing the first and second halves of the study period. Summer (June–August) trend analyses reveal a pronounced decline in sea ice that is associated with increasing concentrations of organic biomolecules in inner Arctic waters. Positive PMOA emission anomalies have become more frequent over the past 15 years, indicating a sustained upward trend. On average, PMOA production has increased by 0.8% per year since 1990, with changes varying among biomolecular groups and Arctic subregions.