Sunlight and Dissolved Organic Matter Decide the Fate of Pollutants in the Baltic Sea

Sunlight may be one of the Baltic Sea’s most underappreciated cleanup crews. A new laboratory study of coastal waters off Rostock, Germany, shows that the degradation of pharmaceuticals, hormones, a sunscreen filter, and caffeine in sunlit surface water depends less on how much dissolved organic matter is present than on where that organic matter comes from. The finding, published in Environmental Advances, carries a sobering implication for a warming world: as climate change drives more terrestrial organic material into coastal seas, some of the most ecotoxic contaminants in the water may linger longer than anyone expected.

The research team, led by Jann-Gerrit T. Müller of the Leibniz Institute for Baltic Sea Research in Warnemünde, set out to answer a deceptively simple question. Contaminants of emerging concern, a category that includes pharmaceuticals, personal care products, and industrial chemicals, are routinely detected in coastal waters at concentrations ranging from nanograms to micrograms per liter. Yet their fate is not governed by inputs alone. It depends on the chemistry of the receiving water, and chief among those variables is the vast, poorly characterized pool of dissolved organic matter, or DOM, that pervades every aquatic ecosystem.

DOM is far more than background noise. This complex mixture of carbon compounds can shield pollutants from light by absorbing photons before they reach their targets, or it can quench the reactive intermediates that break contaminants down. At the same time, DOM can act as a photosensitizer, generating an arsenal of photoproduced reactive intermediates, including excited triplet states of DOM itself, hydroxyl radicals, and singlet oxygen, that attack pollutants indirectly. Which of these competing roles wins out depends on the composition of the DOM, and that composition shifts dramatically with the seasons. Spring runoff delivers humic, terrestrial compounds to the Baltic coast, while summer phytoplankton blooms flood the water with protein- and carbohydrate-like material.

To disentangle these effects, the researchers designed a factorial photodegradation experiment using water from three regionally relevant sources. North Sea water from the German Bight supplied marine DOM, a forest stream near Rostock supplied terrestrial DOM, and brackish water from the Heiligendamm pier on the southwestern Baltic coast served as the unamended background matrix. After filtering all waters through 0.22-micrometer membranes and extracting DOM by solid-phase extraction, the team prepared four matrices: ultrapure water, background water alone, and background water spiked with either terrestrial or marine DOM at roughly three times the background dissolved organic carbon concentration.

Into each matrix went a cocktail of eight contaminants selected for their high detection frequency and risk potential in a recent year-long survey of the Rostock estuary: the pharmaceuticals sulfamethoxazole, carbamazepine, and diclofenac; the natural estrogens estrone and 17β-estradiol; the synthetic estrogen 17α-ethinylestradiol; the sunscreen UV filter benzophenone-4; and caffeine. Each compound was spiked to 2 micrograms per liter, a level ten to one hundred times above typical coastal concentrations but far closer to environmental reality than the concentrations used in many comparable studies. Twenty-eight bottles were incubated for 68 days under UVA-340 lamps emitting at 300 to 450 nanometers with an integrated irradiance of 17 watts per square meter, closely matching the mean diurnal UV irradiance measured at the nearby Warnemünde coast. Identical bottles wrapped in opaque boxes served as dark controls.

Optical monitoring revealed that the two DOM sources were chemically distinct from the start. Terrestrial DOM showed the highest aromaticity and average molecular weight, indicated by elevated carbon-specific UV absorbance at 254 nanometers and lower spectral slopes. Parallel factor analysis of fluorescence spectra resolved seven components, dominated by humic-like fluorophores that accounted for 90 percent of total fluorescence in the terrestrial matrix, 80 percent in the marine matrix, and 70 percent in the unamended Baltic water. Over the irradiation period, humic-like fluorescence declined by roughly a third while protein-like fluorescence rose by more than half, and the two DOM-spiked matrices converged optically, suggesting that terrestrial DOM contained a larger pool of photodegradable, optically active material.

The pollutant results were strikingly compound-specific. Diclofenac vanished fastest, degrading below detection limits within hours in every matrix, confirming direct photolysis as its dominant removal pathway. Sulfamethoxazole degraded fastest in ultrapure water, though the authors caution that pH-dependent speciation makes that result a poor guide to natural conditions. Caffeine and carbamazepine, by contrast, degraded more slowly in the unamended background water than in the DOM-spiked matrices, indicating that higher organic matter concentrations promoted their indirect photodegradation, likely through excited triplet states of DOM. The UV filter benzophenone-4 proved stubbornly persistent, degrading by no more than 82 percent even in DOM-containing matrices, a finding consistent with its design as a stable sunscreen agent and raising concerns about its accumulation in recreational coastal waters.

The estrogens told the most consequential story. All three degraded equally fast under irradiation, confirming photodegradation as an effective sink, but their breakdown was consistently and distinctly slower in the terrestrial DOM matrix than in the marine or brackish matrices, an effect that appeared independent of dissolved organic carbon concentration. The researchers attribute this inhibition to the higher aromaticity and humic-like fluorescence of terrestrial DOM, which may preferentially quench the reactive oxygen species known to drive estrogen degradation. In the dark controls, 17β-estradiol disappeared while estrone accumulated, consistent with microbial interconversion between the two compounds, whereas the synthetic estrogen 17α-ethinylestradiol showed no dark degradation at all, suggesting it could accumulate under light-limited conditions.

Estimated half-lives in unamended coastal water, doubled to approximate the natural day-night cycle, ranged from about four hours for diclofenac to fifteen days for sulfamethoxazole, with estrone and the synthetic estrogen at roughly three days each. Translated into environmental terms, diclofenac would fall below a risk quotient of 0.1 within a day, while the synthetic estrogen would need about thirty days and sulfamethoxazole about forty-three. Caffeine, with an estimated photodegradation half-life of 158 days, and carbamazepine at 28 days, may owe their high detection frequencies in regional surveys partly to this photochemical sluggishness. The authors emphasize that these estimates assume favorable irradiation conditions in clear, shallow surface waters and that the study’s small sample size limits generalization.

The climate connection is what elevates these findings from a laboratory curiosity to an environmental warning. Projected increases in terrestrial runoff, flooding, and permafrost thaw are expected to deliver ever more terrestrial DOM to coastal waters, and the Baltic Sea is already showing signs of browning. If terrestrial organic matter systematically retards estrogen photodegradation, as this study indicates, rising riverine inputs could prolong environmental exposure to compounds that disrupt endocrine systems at sub-nanogram-per-liter concentrations. The effect appears small in absolute terms, but given the extreme ecotoxicity of estrogens, even modest delays in degradation may matter. The authors call for factorial experiments with multiple independent DOM sources at varying carbon concentrations, combined with measurements of microbial community composition and metabolic activity, to separate photochemical from biological degradation and to build more reliable predictions of how coastal seas will process the pollutant mixtures of the Anthropocene.

Subject of Research: Photodegradation of emerging organic pollutants as a function of dissolved organic matter quality in coastal Baltic Sea waters

Article Title: Dissolved organic matter quality and emerging pollutants in coastal Baltic Sea waters: Insights into photodegradation

Article References: Müller, J.-G. T., Osterholz, H., & Kanwischer, M. (2026). Dissolved organic matter quality and emerging pollutants in coastal Baltic Sea waters: Insights into photodegradation. Environmental Advances, 26, Article 100755. https://doi.org/10.1016/j.envadv.2026.100755

Image Credits: AI Generated

DOI: 10.1016/j.envadv.2026.100755

Keywords: Baltic Sea, dissolved organic matter, photodegradation, contaminants of emerging concern, estrogens, pharmaceuticals, caffeine, benzophenone-4, PARAFAC fluorescence, coastal biogeochemistry, climate change, water quality