Underwater Plants Set the Daily Rhythm of Nitrogen in a Karst Stream

Beneath the surface of a quiet lake in southern China, an invisible daily drama unfolds every twenty-four hours. During the day, dense carpets of submerged vegetation pull nitrogen out of the water to build their tissues while photosynthesis floods the stream with oxygen. After sunset, the plants switch to respiration, releasing carbon dioxide and gently reversing the chemical currents of the water. A new study has now captured this round-the-clock performance in unprecedented chemical detail, and its findings overturn a long-standing assumption about how nitrogen behaves in the calcium-rich waters that drain the world’s karst landscapes.

The research, published in Environmental Monitoring and Assessment, was led by Mingda Cao of Chizhou University together with colleagues from institutions across China, including Hefei University of Technology, the Institute of Karst Geology in Guilin, and China University of Geosciences in Wuhan. The team focused on the Dawang Cave subterranean stream and the downstream Dawang Lake in a classic karst region, where limestone bedrock dissolves slowly in carbon-dioxide-charged groundwater, loading the water with dissolved inorganic carbon and bicarbonate. Karst waters cover enormous areas of the planet, and understanding their chemistry matters for drinking water supplies, agricultural runoff management, and the global carbon budget.

To dissect the daily chemistry, the researchers established two monitoring sections. The first sat at the outlet of the Dawang Cave subterranean stream, a stretch devoid of subaquatic vegetation and dominated by upwelling groundwater. The second lay downstream in Dawang Lake, where abundant submerged plants thrive in the sunlit water column. Between noon on 6 September and noon on 7 September 2024, the team ran a continuous twenty-four-hour field campaign, sampling at high frequency and measuring an unusually complete suite of parameters: pH, dissolved oxygen, electrical conductivity, temperature, the major ions calcium and bicarbonate, the nitrogen species nitrate and ammonium, and, critically, the stable isotopic compositions of both nitrate and ammonium. From the oxygen data they also calculated net ecosystem production, a direct gauge of whether the ecosystem was, minute by minute, producing or consuming organic matter.

The contrast between the two sites was striking. At the unvegetated cave outlet, essentially every parameter barely moved over the full day. The team describes this as chemostatic behavior, a near-constant chemical signature imposed by the steady discharge of deep groundwater. In such systems, the subsurface reservoir acts like a giant chemical buffer: water spends long enough in the aquifer that it arrives at the surface with a composition set by slow rock-water reactions rather than by anything happening in the stream itself. For biogeochemists, this makes the cave outlet a natural control experiment, a baseline against which the biological pulse downstream can be measured.

Dawang Lake told a completely different story. There, the water chemistry swung on a clean diel cycle that tracked the sun. During daylight hours, photosynthesis dominated, net ecosystem production turned positive, and the concentrations of both nitrate and ammonium fell steadily as the submerged vegetation assimilated dissolved nitrogen into its biomass. At night, respiration took over, net ecosystem production flipped negative, dissolved oxygen sagged, and the nitrogen concentrations recovered. The isotopic records moved in lockstep. As nitrate and ammonium were consumed during the day, the remaining pools became progressively enriched in the heavy isotopes, with rising values of δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+. After dark the enrichment reversed. This pattern produced a significant negative correlation between nitrogen concentrations and their isotope ratios, the classic fingerprint of a preferential-removal process in which the lighter isotopes are taken up first.

The isotope data allowed the team to test the leading hypotheses about what removes nitrogen from such waters. Denitrification, the microbial conversion of nitrate to gaseous nitrogen forms, is often assumed to be the dominant sink in nitrate-laden streams, and it carries a diagnostic isotope signature: because the process preferentially strips light nitrogen and oxygen, the residual nitrate becomes enriched in both heavy isotopes at a characteristic ratio of roughly one to one between δ15N and δ18O. In Dawang Lake, the covariation of the two nitrate isotope ratios showed an enrichment ratio of approximately 0.79 to 1, and, crucially, all of the data points fell outside the typical denitrification field on the dual-isotope plot. The conclusion was unambiguous: denitrification was negligible over the monitoring period. The apparent loss of nitrate during the day was not being converted to gas and escaping the system; it was being built into living tissue.

Instead, the study identifies assimilation by subaquatic vegetation and the mineralization of organic nitrogen as the key processes governing the diel nitrogen cycle. By day, plants and attached algae take up nitrate and ammonium directly, and by night the microbial breakdown of organic matter returns nitrogen to the water. This coupling means that the carbon and nitrogen cycles in the lake are not merely parallel but genuinely intertwined: the same metabolic engine that draws down dissolved inorganic carbon and bicarbonate during photosynthesis simultaneously scrubs bioavailable nitrogen from the water, and the nocturnal release of carbon dioxide is accompanied by the remineralization of organic nitrogen. In high-dissolved-inorganic-carbon waters like those of karst regions, where researchers had often focused on carbonate chemistry and carbon sequestration, the new data show that the nitrogen side of the ledger is driven by the same biological heartbeat.

The technical achievement underlying these conclusions is the combination of high-resolution monitoring with paired nitrogen and oxygen isotopes measured around the clock. Traditional sampling, which might capture a single morning snapshot, would average out the diel cycle entirely and could easily misattribute the daytime nitrate decline to denitrification or dilution. The continuous approach reveals that what looks like net nitrogen removal on a daily average is actually a rapid exchange between the dissolved pool and living biomass, one that reverses every evening. The authors note that karst streams, with their naturally high alkalinity and stable groundwater inputs, are ideal natural laboratories for this kind of work, precisely because the strong chemostatic background makes any biological signal stand out clearly against it.

The broader implications reach into water quality management and climate science. Vegetated reaches downstream of groundwater springs may act as transient nitrogen filters, temporarily sequestering nutrients during the day, and the fate of that stored nitrogen, whether it is buried in sediments, exported downstream, or eventually mineralized, will shape nutrient loads far from the source. Because the study shows that assimilation, not denitrification, dominates at least during this monitoring period, the greenhouse gas consequences may differ from expectations in waters where denitrification would produce nitrous oxide, a potent greenhouse gas. At the same time, the photosynthetic drawdown of bicarbonate connects directly to the recognized carbon sink potential of aquatic phototrophs in carbonate terrain. More broadly, the work provides what the authors describe as high-resolution isotopic evidence that subaquatic vegetation metabolism drives coupled carbon-nitrogen diel cycles in karst streams, offering new insight into nitrogen biogeochemistry in high-DIC aquatic systems and a template for studying daily biogeochemical rhythms elsewhere.

For the scientists involved, the next step is extending these single-day campaigns across seasons and hydrological conditions, since a September snapshot under stable weather cannot capture storm pulses, seasonal plant growth, or temperature-driven shifts in microbial activity. But the central message of the Dawang study is already clear. In the quiet waters of a karst lake, the sun does more than light the surface: it switches an entire ecosystem’s chemistry on and off, twice a day, every day, and only by watching continuously can researchers hope to read the rhythm correctly.

Subject of Research: Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution monitoring

Article Title: Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution monitoring

Article References: Cao, M., Wang, Y., Zhang, J., Jiao, T., Huang, X., Zhao, S., Song, C., Yao, Z., & Zhang, X. (2026). Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution monitoring. Environmental Monitoring and Assessment, 198(10), Article 1088. https://doi.org/10.1007/s10661-026-15924-8

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15924-8

Keywords: karst stream, nitrogen cycling, stable isotopes, diel variation, subaquatic vegetation, net ecosystem production, denitrification, nitrate, ammonium, carbon-nitrogen coupling, high-resolution monitoring, biogeochemistry