Dissolved Organic Matter Optical Properties Offer New Window Into Deep Ocean Circulation, Study Finds

A recent study published in Deep Sea Research Part I: Oceanographic Research Papers, led by Celine Guéguen of the Université de Sherbrooke, demonstrates that the optical properties of dissolved organic matter (DOM) can serve as reliable tracers for distinct deep-water masses in the South Atlantic Ocean.¹ The research also documents clear seasonal shifts in DOM composition at subsurface depths, offering a potential new tool for assessing frontal dynamics and biological productivity.

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What did the researcher do in their study?

The research team deployed excitation–emission matrix fluorescence combined with parallel factor analysis (EEM–PARAFAC) alongside UV–visible (UV-vis) absorbance spectroscopy to characterize DOM across major oceanic fronts, deep water masses, and mixing zones.¹ The samples that were collected during winter expeditions in July–August 2019 and spring cruises in September–November 2019 and 2023 along a latitudinal transect crossing the Subtropical and Subantarctic Fronts.¹

What were the distinct signatures found in deep water masses?

Five fluorescent DOM components were identified in the data set—one humic-like, two microbial humic-like, and two protein-like. Of these, the humic-like and microbial humic-like components produced measurably distinct optical signatures for three major water masses. These major water masses are Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW), and Lower Circumpolar Deep Water (LCDW).¹

One aspect to this study that researchers had to account for was making sure the biogeochemical transformation of DOM did not mix with physical mixing. To make sure this did not happen, the researchers applied an optimum multiparameter (OMP) analysis using hydrological and DOM parameters.¹ The analysis revealed that physical mixing was the dominant control on the distribution of most optical properties across the three water masses.¹ However, the authors noted that one microbial humic-like component and one absorbance parameter exhibited non-conservative behavior at the boundary between AAIW and NADW.¹ This observation suggests localized biogeochemical processing at that frontal interface.

What seasonal variability was detected in the subsurface layer?

In the upper mesopelagic layer, which is roughly 25 to 100 meters depth, DOM optical properties shifted markedly between seasons. During winter sampling, humic-like components allowed for clear differentiation of the Subtropical Front and the Subantarctic Front, with measurable optical gradients recorded across both boundaries.¹ Those gradients were absent in spring profiles, which the authors attribute to elevated biological production at the surface diluting or transforming the DOM signal.¹

The finding indicates that DOM optical properties may function as indirect indicators of frontal position and seasonal productivity cycles.

Why does this study matter?

The South Atlantic plays a central role in global thermohaline circulation, serving as a conduit between water masses originating in the North Atlantic and the Southern Ocean.² Tracking how these water masses mix and evolve has historically relied on temperature, salinity, and dissolved inorganic tracers.^1,2 The identification of DOM optical parameters as complementary proxies adds a biochemical dimension to that toolkit.

The research is also relevant to ocean carbon accounting. DOM constitutes one of the largest reservoirs of organic carbon on Earth, and understanding its origin, transport, and transformation in the deep ocean is a prerequisite for accurately modeling the biological carbon pump.

References

1. Fériot, C.; Guéguen, C. Optical Tracing of Dissolved Organic Matter in the South Atlantic Ocean: Linking Water Masses and Biogeochemical Processes. Deep Sea Res. Part I: Ocean. Res. Pap. 2026, 229, 104676. DOI: 10.1016/j.dsr.2026.104676
2. Piola, A. R.; Campos, E. J. D.; Donners, J.; et al. The South Atlantic Contribution to the Global Thermohaline Circulation. NOAA. Available at: https://www.aoml.noaa.gov/phod/SAMOC/Piola_SACOSreport.pdf (accessed 2026-04-28).