Stability of Dissolved Organic Carbon in Antarctica and Its Impacts on Climate Models

As currents on the Antarctic continental shelf resume after seasonal melting and phytoplankton blooms intensify, dissolved organic carbon (DOC) measured in surface waters shows surprising constancy. Two scientific expeditions aboard the icebreaker Polarstern allowed for comparison of samples taken over a thousand kilometers apart, and results published in Nature Communications indicate that neither the total amount nor the chemical composition of DOC varies significantly, even when water masses undergo large displacements.

What the study reveals: DOC stability despite massive changes

Researchers found that DOC concentrations remain practically identical between the two campaigns, conducted a few years apart. Despite increased biological activity, revealed by chlorophyll a peaks, and major rearrangements of water masses caused by polar winds, average DOC content does not exceed natural expected variations, on the order of a few percent only. This observation contradicts models that anticipated a rise in DOC proportional to the intensification of phytoplankton productivity.

Moreover, analysis of DOC molecular signatures shows that the proportion of aromatic compounds, polysaccharides, and proteins remains stable. In other words, not only does the total amount not change, but the 'bouquet' of dissolved carbon chemistry also does not change, even when surface waters are mixed with deeper, colder water masses. This chemical resilience suggests that microbial degradation processes and vertical transport operate at equilibrium rates that compensate for additional carbon inputs.

How the study was conducted: Sampling, chemical analyses, and comparison with models

Teams aboard the Polarstern collected hundreds of water samples at different depths, from the surface layer to 500m, along transects covering the continental shelf. Each sample was filtered immediately to retain the fractionation of dissolved organic carbon, then stored at -20°C to preserve its chemical integrity. In the laboratory, scientists used high-resolution mass spectrometry and liquid chromatography to quantify DOC and identify its molecular constituents.

Meanwhile, obtained data was compared with ocean circulation simulations produced by the ECMWF model and Copernicus satellite products, to evaluate the influence of temperature, salinity, and nutrient flux variations on DOC. No major adjustments were necessary to align observations with model outputs, which reinforces confidence in the study's findings.