Browsing by Author "Holmes, RM"
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Item Open Access Inorganic carbon speciation and fluxes in the Congo River(Geophysical Research Letters, 2013-02-16) Wang, ZA; Bienvenu, DJ; Mann, PJ; Hoering, KA; Poulsen, JR; Spencer, RGM; Holmes, RMSeasonal variations in inorganic carbon chemistry and associated fluxes from the Congo River were investigated at Brazzaville-Kinshasa. Small seasonal variation in dissolved inorganic carbon (DIC) was found in contrast with discharge-correlated changes in pH, total alkalinity (TA), carbonate species, and dissolved organic carbon (DOC). DIC was almost always greater than TA due to the importance of CO2*, the sum of dissolved CO2 and carbonic acid, as a result of low pH. Organic acids in DOC contributed 11-61% of TA and had a strong titration effect on water pH and carbonate speciation. The CO2* and bicarbonate fluxes accounted for ~57% and 43% of the DIC flux, respectively. Congo River surface water released CO2 at a rate of ~109 mol m-2 yr-1. The basin-wide DIC yield was ~8.84 × 104 mol km-2 yr-1. The discharge normalized DIC flux to the ocean amounted to 3.11 × 1011 mol yr-1. The DOC titration effect on the inorganic carbon system may also be important on a global scale for regulating carbon fluxes in rivers. Key Points The carbonate chemistry near the Congo River mouth is comprehensively studied Organic acids have a titration effect on the inorganic carbon system Surface CO2 and inorganic carbon fluxes to the ocean are characterized ©2013. American Geophysical Union. All Rights Reserved.Item Open Access The biogeochemistry of carbon across a gradient of streams and rivers within the Congo Basin(Journal of Geophysical Research: Biogeosciences, 2014-04) Mann, PJ; Spencer, RGM; Dinga, BJ; Poulsen, JR; Hernes, PJ; Fiske, G; Salter, ME; Wang, ZA; Hoering, KA; Six, J; Holmes, RMDissolved organic carbon (DOC) and inorganic carbon (DIC, pCO2), lignin biomarkers, and theoptical properties of dissolved organic matter (DOM) were measured in a gradient of streams and rivers within the Congo Basin, with the aim of examining how vegetation cover and hydrology influences the composition and concentration of fluvial carbon (C). Three sampling campaigns (February 2010, November 2010, and August 2011) spanning 56 sites are compared by subbasin watershed land cover type (savannah, tropical forest, and swamp) and hydrologic regime (high, intermediate, and low). Land cover properties predominately controlled the amount and quality of DOC, chromophoric DOM (CDOM) and lignin phenol concentrations (8) exported in streams and rivers throughout the Congo Basin. Higher DIC concentrations and changing DOM composition (lower molecular weight, less aromatic C) during periods of low hydrologic flow indicated shifting rapid overland supply pathways in wet conditions to deeper groundwater inputs during drier periods. Lower DOC concentrations in forest and swamp subbasins were apparent with increasing catchment area, indicating enhanced DOC loss with extended water residence time. Surface water pCO2in savannah and tropical forest catchments ranged between 2,600 and 11,922 μatm, with swamp regions exhibiting extremely high pCO2(10,598-15,802 μatm), highlighting their potential as significant pathways for water-air efflux. Our data suggest that the quantity and quality of DOM exported to streams and rivers are largely driven by terrestrial ecosystem structure and that anthropogenic land use or climate change may impact fluvial C composition and reactivity, with ramifications for regional C budgets and future climate scenarios. Key Points Vegetation cover predominately controls fluvial C concentration and composition Small streams (20 m wide) and wetlands are significant sources of aquatic CO2Changing vegetation cover, or hydrologic conditions impact regional carbon budgets ©2014. American Geophysical Union. All Rights Reserved.