Phosphorus is the master nutrient of freshwater systems. In modest quantities it sustains food webs, but when excess amounts flood into a lake from farms, cities, and industry, it triggers algal blooms that can choke waterways, kill fish, and render water undrinkable. What makes phosphorus management so maddening is that even after external inputs are curtailed, lake sediments can continue releasing the nutrient they have stored over decades—a phenomenon known as internal loading. The new study tackles this problem head-on by asking a deceptively simple question: do sediments in different kinds of lakes behave differently, and if so, why?

The answer, according to the research team led by Wei Huang of Donghua University and Feifei Che of the Chinese Research Academy of Environmental Sciences, is a resounding yes. Using standard sorption experiments, the team measured how much phosphorus each lake’s sediments could absorb from the water above. The plateau lake’s sediments proved to be powerful sponges, with a mean phosphorus sorption capacity of 2.24 milligrams per gram, and every sampled region of YamdrokTso behaved as a phosphorus sink, pulling the nutrient out of the water column and locking it into the bed. Taihu Lake told the opposite story: most of its sediments functioned as phosphorus sources, actively leaking the nutrient back into the overlying water, with only a few zones still acting as sinks.

To quantify this contrast, the researchers calculated a phosphorus release risk index, or ERI, for sediments in both lakes. The plateau sediments averaged an ERI of just 30.10 percent, signaling a comparatively low risk of phosphorus escaping into the water. The plain lake sediments, by contrast, reached a maximum ERI of 86.80 percent—a figure that approaches the theoretical ceiling for release risk. In practical terms, this means that Taihu’s bed is teetering on the edge of becoming a major internal phosphorus factory, capable of sustaining algal blooms even if every external source of the nutrient were eliminated tomorrow.

The study went beyond simple source-sink accounting by dissecting the chemical forms of phosphorus in the sediments and tracing which forms actually feed the water column. Sedimentary phosphorus exists in several distinct fractions, each with its own mobility. NaOH-extractable phosphorus, typically bound to iron and aluminum oxides, is notoriously labile and can be released when oxygen levels drop at the sediment surface. HCl-extractable phosphorus, bound to calcium carbonates and apatite-like minerals, is generally more stable. Organic phosphorus, embedded in decaying biomass and microbial cells, must be mineralized by enzymes before it becomes available to algae.

By examining both surface sediments, which represent short-term exchanges with the water, and deeper bottom sediments, which represent long-term legacies, the team uncovered a striking asymmetry. In Taihu Lake, every phosphorus fraction in both the surface and bottom sediments contributed significantly to the phosphorus concentrations in the overlying water, with NaOH-extractable phosphorus emerging as the dominant contributor. This suggests that the plain lake’s sediments are thoroughly mobilized, with even historically buried phosphorus being recycled upward. In YamdrokTso, the picture was more restrained: organic phosphorus and HCl-extractable phosphorus from the surface layer contributed only moderately to water-column concentrations, while over the long-term gradient HCl-extractable phosphorus was the primary contributor—indicating a slow, mineral-bound release rather than an aggressive biological pump.

Perhaps the most novel dimension of the study is its exploration of the microbial machinery behind these differences. The researchers used functional prediction analysis to estimate the activity of key biochemical pathways involved in phosphorus activation within the sediment microbial communities. Two pathways stood out: inositol phosphate metabolism, which governs the breakdown of a major store of organic phosphorus in soils and sediments, and the phospholipase D signaling pathway, which is involved in cleaving phosphorus from membrane lipids. In the plateau lake sediments, both pathways were suppressed, consistent with the sediments’ role as phosphorus sinks. In the plain lake sediments, the same pathways were strongly enriched, providing a mechanistic explanation for why Taihu’s bed so readily relinquishes its phosphorus.

This link between geochemical behavior and microbial functional traits is what elevates the study beyond a conventional monitoring exercise. It suggests that the source-sink status of a lake sediment is not simply a matter of how much phosphorus has accumulated, but of whether the resident microbial community possesses—and expresses—the enzymatic toolkit needed to unlock it. Cold, high-altitude, oligotrophic conditions at YamdrokTso appear to keep that toolkit switched off, while the warm, nutrient-rich, disturbance-prone environment of Taihu keeps it running at full throttle. The implication is sobering: as climate change warms lakes and human activity continues to intensify around lowland waters, the microbial engines of phosphorus release may only grow stronger.

The comparative design also carries a warning for restoration efforts. Many eutrophic lakes worldwide, from Chesapeake Bay to Lake Erie to countless European waters, have undergone decades of external nutrient reduction with disappointing recoveries, precisely because internal loading keeps the algae fed. The Taihu results illustrate why: when sediments across most of a lake act as sources, and when microbial pathways for phosphorus activation are broadly enriched, the internal reserve can sustain blooms for years or even decades after external inputs decline. Conversely, the YamdrokTso findings offer a rare piece of good news, showing that under the right conditions sediments can remain reliable phosphorus sinks, buffering the water column against eutrophication.

For policymakers, the study argues that endogenous pollution control strategies must be tailored to regional lake types rather than copied wholesale from one system to another. Interventions that work by enhancing sediment phosphorus binding—such as lanthanum-modified clays or other sorbent amendments—may find more durable success in systems where microbial activation pathways are weak, whereas in lakes like Taihu, suppressing the microbial and chemical drivers of release may be equally important as capping the phosphorus itself. The authors frame their work as theoretical support for regional endogenous lake pollution control, and the message is clear: the battle against eutrophication will not be won at the water’s edge alone. It will be won, or lost, in the mud below, where geography, chemistry, and an invisible community of microbes decide the fate of every phosphorus atom a lake has ever absorbed.

Subject of Research: Endogenous phosphorus release from lake sediments and associated microbial functional pathways in plateau versus plain lakes

Article Title: Endogenous phosphorus release risks and functional pathway responses: a comparative study on plateau and plain representative lakes

Article References: Huang, W., Zhang, L., Wang, X., Guo, Y., Deng, F., Chen, H., & Che, F. (2026). Endogenous phosphorus release risks and functional pathway responses: a comparative study on plateau and plain representative lakes. Environmental Monitoring and Assessment, 198(10), Article 1075. https://doi.org/10.1007/s10661-026-15902-0

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15902-0

Keywords: phosphorus, lake sediments, internal loading, eutrophication, YamdrokTso Lake, Taihu Lake, plateau lakes, phosphorus sorption, microbial communities, inositol phosphate metabolism, phospholipase D, water quality