Spatial distribution and molecular transformation pathways of dissolved organic matter along the Huai River basin (Anhui section) during the wet season

Water is never just water. Even in a clear river, it carries a hidden cargo of dissolved organic matter, a complex soup of tiny carbon-based molecules that have washed off the land or been released by living things in the water itself. Scientists call this mixture dissolved organic matter, or DOM. It is a vital part of the planet's carbon cycle, acting as a bridge between the soil, the atmosphere, and the life within the river. Because this material comes from so many different sources—decaying leaves, sewage, soil erosion, or the waste of fish and plants—its exact chemical makeup changes constantly. Understanding what is in this soup, where it comes from, and how it changes as it flows downstream is essential for managing water quality and protecting ecosystems. When heavy rains fall, the story of this water changes dramatically, as the surge of runoff flushes new materials into the river, mixing and transforming them in ways that are difficult to see with the naked eye.

In the Huai River Basin, a major waterway in central China, researchers set out to trace these invisible changes during the wet season. This region sits in a transition zone between northern and southern climates, where the monsoon rains bring intense periods of precipitation. During these wet months, the river swells, and the landscape washes into the channel, carrying a fresh load of organic material. A team of scientists from Anhui University of Science and Technology collected water samples from the upper reaches of the river in Fuyang, down through the middle sections, and to the lower reaches near Bengbu. Their goal was to map out how the character of this dissolved organic matter shifts from the source to the mouth of the river, and to uncover the chemical processes that reshape it along the way. They did not just look at how much organic matter was there; they used advanced tools to identify the specific types of molecules present and to watch how they might be changing as the water moved.

The researchers first looked at the water through the lens of fluorescence, a technique that makes certain organic molecules glow when hit with specific light. By analyzing these glowing signals, they identified three main types of components in the river. One type resembled substances found in soil, known as humic acids, which are complex and stable. Another type looked like tryptophan, a building block of proteins, which suggests it came from living organisms or recent biological activity. The third was a mix of fulvic acids, which are smaller and more reactive versions of soil humus. What they found was a clear journey from one end of the river to the other. In the upstream areas, the water was dominated by the protein-like signals, indicating that life within the river itself—perhaps algae or bacteria—was producing a significant amount of this material. However, as the water flowed downstream, the signal shifted. The protein-like glow faded, and the humic, soil-like signals grew stronger. This pattern suggested that as the river moved, it was picking up more material from the land, such as decaying plant matter and soil, which diluted the local biological production and added a heavier load of terrestrial organic matter.

To understand the size and nature of these molecules, the team used a method that separates organic carbon based on how big the molecules are. They found that the small streams and tributaries feeding into the main river carried significantly higher concentrations of organic carbon than the main channel itself. These tributaries were rich in biopolymers, which are large, complex molecules often linked to living cells, and humic substances from the soil. This pointed to a heavy influence from agricultural runoff and domestic sewage in the areas surrounding these smaller waterways. In contrast, the main river channel showed a different signature. While it carried less of the large biological molecules, it contained higher levels of hydrophobic organic carbon, a type of material that does not mix well with water and tends to stick to particles. The researchers noted that one specific site on the main river, where ship traffic was dense, had particularly high levels of this hydrophobic material, suggesting that the activities of vessels and the fuel they use were contributing a distinct type of pollution to the river's chemical mix.

The most detailed look came from a high-resolution mass spectrometer, a machine capable of identifying the exact chemical formula of thousands of individual molecules in a single drop of water. This analysis revealed that the dissolved organic matter was primarily made of two types of compounds: those containing only carbon, hydrogen, and oxygen, and those that also included nitrogen. The molecules were mostly shaped like lignin, the tough material that gives wood its rigidity, or like lipids, which are fats. As the water traveled from the upstream to the downstream sections, the chemical makeup shifted again. The downstream samples contained a higher proportion of molecules that included nitrogen and sulfur. Since these elements are common in human waste and agricultural fertilizers, their increased presence downstream served as a chemical fingerprint of human influence, confirming that urban wastewater and farm runoff were altering the river's chemistry as it flowed toward the sea.

Finally, the researchers used the data to map out how these molecules might be transforming as they traveled. By looking at the differences between the molecules found at different points, they could infer the chemical reactions taking place. The most common changes appeared to be the loss of methyl groups, a process called demethylation, and the addition of hydrogen atoms, known as hydrogenation. These reactions suggest that the molecules were becoming more stable and less reactive as they moved downstream. The demethylation process, in particular, seemed to be breaking down the complex structures of the soil-derived lignin, while hydrogenation was saturating the chemical bonds, making the molecules less likely to break apart. This transformation was driven by the combination of the river's physical movement and the biological activity within it. The study concludes that the Huai River during the wet season is a dynamic system where the balance between life within the water and inputs from the land constantly shifts, creating a complex and changing chemical landscape that reflects both natural processes and human activity.