Carbon Fertilization in Karst Waters: A Nature-Based Solution for Carbon Sinks and Eutrophication Control

A sweeping review published in Frontiers in Geochemistry argues that one of the most overlooked allies in the fight against climate change and water pollution may be hiding in plain sight: the limestone-rich rivers, lakes, and reservoirs that cover roughly 15 percent of Earth’s land surface. Led by Mingyu Shao and Zaihua Liu of the Institute of Geochemistry at the Chinese Academy of Sciences, the study synthesizes nearly two decades of research into how dissolved carbon dioxide in karst waters can simultaneously boost carbon sequestration and suppress the algal blooms that plague freshwater systems worldwide.

The concept at the heart of the review is called the carbon sink by carbonate weathering coupled with aquatic photosynthesis, or CCW. When rainwater percolates through limestone and other carbonate rocks, it dissolves them, pulling atmospheric carbon dioxide into the water as dissolved inorganic carbon. In karst regions, this process produces dissolved inorganic carbon concentrations that are six to ten times higher than in non-karst watersheds. Aquatic plants, algae, and microorganisms then convert a portion of this dissolved inorganic carbon into autochthonous organic carbon through photosynthesis, a process the researchers term autochthonous production. That organic carbon, once buried in sediments or transported downstream to the ocean, becomes a long-term stable carbon sink rather than a transient one.

What makes the CCW theory particularly compelling as a nature-based solution is its dual benefit. The review details how elevated dissolved carbon dioxide acts as a fertilizer for aquatic photosynthesis, alleviating what the authors identify as a chronic carbon limitation in high-pH karst waters. In these alkaline environments, the carbonate equilibrium system is dominated by bicarbonate and carbonate ions, leaving free carbon dioxide at often less than one percent of the total dissolved inorganic carbon pool. This scarcity constrains photosynthetic organisms that preferentially use carbon dioxide over bicarbonate, which requires energy-intensive carbon concentrating mechanisms to assimilate.

The fertilization effect ripples through the entire aquatic food web in ways that suppress eutrophication rather than exacerbate it. When carbon dioxide is scarce, cyanobacteria—blue-green algae equipped with highly efficient carbon concentrating mechanisms—dominate because they can strip bicarbonate from the water when other organisms cannot. These blooms produce toxins, deplete oxygen, and degrade water quality. But when dissolved carbon dioxide is abundant, species with weaker carbon concentrating mechanisms, such as green algae and diatoms, gain a competitive edge. The review cites field data from Erhai Lake in Yunnan and Caohai Lake in Guizhou showing that cyanobacteria decline relative to other phytoplankton when the carbon-to-nitrogen and carbon-to-phosphorus molar ratios in the water exceed specific thresholds, suggesting that carbon fertilization can tip the ecological balance away from harmful bloom-forming species.

Submerged aquatic plants, the foundation of healthy shallow lake ecosystems, also benefit disproportionately from carbon fertilization. Unlike phytoplankton, these rooted plants have limited capacity to use bicarbonate and typically begin switching to it only when carbon dioxide concentrations exceed 50 micromoles per liter, far above the 10 to 15 micromole threshold for many algae. Their higher carbon-to-nitrogen and carbon-to-phosphorus stoichiometric ratios mean they are better adapted to carbon-rich environments. Dense beds of submerged plants improve water clarity, stabilize sediments, provide habitat for aquatic fauna, and release chemical compounds that inhibit algal growth, creating a self-reinforcing cycle of water quality improvement.

The review also illuminates a second, purely geochemical mechanism by which autochthonous production removes phosphorus, the nutrient most responsible for eutrophication. As aquatic organisms photosynthesize, they raise the pH of the water, shifting the carbonate equilibrium toward carbonate ions and triggering the precipitation of calcium carbonate. This newly formed calcite has an enormous specific surface area and abundant adsorption sites. Dissolved phosphate co-precipitates with the calcite crystals or adsorbs onto their surfaces, converting soluble reactive phosphorus into an insoluble form that settles into the sediment. The authors report that when pH rises above 9.5, more than 74 percent of phosphorus can co-precipitate with calcium carbonate, a self-purification mechanism they call the AOCP-CP removal hypothesis, linking the organic carbon pump to the carbonate pump.

Field evidence for this mechanism comes from multiple sources. In Sicilian reservoirs, managers successfully controlled cyanobacterial outbreaks by spreading lime powder to induce calcium carbonate precipitation and phosphorus removal. Enclosure experiments in nutrient-poor lakes showed that added phosphorus stimulated productivity and calcium carbonate precipitation, but much of the phosphorus was ultimately buried in sediment through co-precipitation, creating a negative feedback on eutrophication. X-ray diffraction analyses of calcium carbonate products exposed to phosphate solutions revealed the formation of hydroxyapatite, the most thermodynamically stable calcium phosphate mineral, confirming the long-term sequestration of phosphorus in these systems.

The scale of the potential carbon sink is substantial. The review estimates that enhanced autochthonous production in inland waters could sequester between 0.38 and 1.8 billion tons of carbon per year, a range that positions these aquatic ecosystems as significant contributors to closing the global carbon budget. Data from the Shawan Karst Test Site in Guizhou Province showed that autochthonous organic carbon fixed in surface waters ranged from 156 to 493 tons of carbon per square kilometer per year, increasing with dissolved inorganic carbon concentration. In the Pearl River, a typical karst river, phytoplankton biomass and total organic carbon both rose with dissolved inorganic carbon, and autochthonous organic carbon accounted for 65 percent of the total organic carbon pool.

The authors acknowledge that significant research gaps remain. Most existing studies have focused on single regions and short time periods, lacking the global coverage and high-frequency monitoring needed to fully quantify carbon fluxes. The role of methane and nitrous oxide emissions from these waters in offsetting carbon gains remains poorly constrained. The mechanisms linking carbon fertilization to community structure shifts and phosphorus removal need further elucidation across different climatic zones, land uses, and lithologies. To address these gaps, the review proposes establishing a global monitoring network for carbon fluxes in karst surface waters, employing hydrological modeling, radioisotopes, stable carbon isotopes, and machine learning to trace carbon sources and quantify lateral transport and vertical burial.

Looking forward, the authors propose coupling enhanced carbonate rock weathering with aquatic photosynthesis as a next-generation carbon removal strategy. By deliberately managing aquatic productivity to convert dissolved inorganic carbon from enhanced weathering into stable organic carbon, the approach could approach near-complete carbon sequestration efficiency. As climate targets slip further out of reach and eutrophication intensifies across the globe, the humble chemistry of limestone and sunlight may prove to be one of the most elegant and scalable nature-based solutions available.

Subject of Research: Carbon fertilization of aquatic photosynthesis in karst surface waters as a nature-based solution for carbon sequestration and eutrophication mitigation

Article Title: Carbon fertilization of autochthonous production in karst surface waters and its role in carbon reduction and eutrophication mitigation—a nature-based solution (NbS)

Article References: Shao, M., Liu, Z., Zeng, S., Sun, H., & He, H. (2025). Carbon fertilization of autochthonous production in karst surface waters and its role in carbon reduction and eutrophication mitigation—a nature-based solution (NbS). Frontiers in Geochemistry, 3, Article 1622714. https://doi.org/10.3389/fgeoc.2025.1622714

Image Credits: AI Generated

DOI: 10.3389/fgeoc.2025.1622714

Keywords: karst, carbon cycle, eutrophication, nature-based solutions, carbonate weathering, aquatic photosynthesis, carbon sink, water quality, Carbon, fertilization, autochthonous, production