title: In-stream metabolism and atmospheric carbon sequestration in a groundwater-fed karst stream.
abstract: Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of dissolved carbonate minerals emits CO2back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2+, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3km downstream (LY site). Diel variations in pH, DO, and δ13CDICwere inversely related to diel changes in SpC, DIC, Ca2+and pCO2.This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2+and δ13CDIC,with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2degassing from stream to the atmosphere, which is estimated to be 3-5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73tCkm-2a-1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2evasion.
authors.affiliation: Key Laboratory of Karst Dynamics, MLR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China. Electronic address: junbingpu@karst.ac.cn.
authors.forename: Junbing
authors.initials: J
authors.lastname: Pu
authors.affiliation: Key Laboratory of Karst Dynamics, MLR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China.
authors.forename: Jianhong
authors.initials: J
authors.lastname: Li
authors.affiliation: Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA.
authors.forename: Mitra B
authors.initials: MB
authors.lastname: Khadka
authors.affiliation: Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA.
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Martin
authors.affiliation: Key Laboratory of Karst Dynamics, MLR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Karst Dynamics, MLR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China.
authors.forename: Shi
authors.initials: S
authors.lastname: Yu
authors.affiliation: Key Laboratory of Karst Dynamics, MLR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China.
authors.forename: Daoxian
authors.initials: D
authors.lastname: Yuan
keywords: Aquatic phototrophs; Carbon sequestration; Diel cycle; Dissolved inorganic carbon; In-stream metabolism; Karst system