Nutrient Limitation of Biological Carbon Pump in Karst Rivers and Implications for Eutrophication Mitigation:A Case Study of the Lijiang River Basin
The biological carbon pump(BCP)is a key mechanism in forming long-term stable carbonate weathering carbon sinks.Eutrophication represents a distinct phase of the BCP,characterized by poor water quality and dominance of harmful cyanobacteria.In karst waters,limiting nutrients for the BCP extend beyond nitrogen and phosphorus to include carbon.Preliminary studies have been made in karst simulation experiments,and lakes/reservoirs on carbon fertilization(carbon limitation)of the BCP and its mitigation of cyanobacterial eutrophication,yet case studies in karst rivers remain scarce.In this study,water samples were systematically collected in a typical karst river,the Li River basin,from 2022 to 2023.The results revealed that spatial heterogeneity in hydrochemistry among monitoring sites contribute to lithology of basin.Seasonal dynamics in hydrochemistry reflected the combined influence of the dilution effect and the BCP effect.The primary productivity of water in Lijiang River was limited by Phosphorus firstly,then Carbon.Contrary to previous studies,a positive correlation was found between the ratio of cyanobacteria to(green algae+diatoms)and dissolved CO2,which may contribute to the cyanobacterial genetic diversity of CO2 Concentrating Mechanism(CCM),physiological flexibility,and hydrodynamic conditions.Consequently,interventions such as artificial CO2 enrichment to enhance BCP and alleviate eutrophication in karst waters should be approached with caution,especially in rivers that have not reached eutrophic conditions.
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