Anthropogenic Carbon Pump in an Urbanized Estuary

Abstract:

The importance of estuaries as a carbon source has been increasingly recognized over the recent decades. However, constraining sources of CO₂ evasion from urbanized estuaries remains incomplete, particularly in densely populated river systems receiving high loads of organic carbon from anthropogenic sources. To account for major factors regulating carbon fluxes the tidal reach of the Han River estuary along the metropolitan Seoul, characterization of organic carbon in the main stem and major urban tributaries were combined with continuous, submersible sensor measurements of pCO₂ at a mid-channel location over a year and continuous underway measurements using a submersible sensor and two equilibrator sytems across the estuarine section receiving urban streams. Single-site continuous measurements exhibited large seasonal and diurnal variations in pCO₂, ranging from sub-ambient air levels to exceptionally high values approaching 10,000 ppm. Diurnal variations of pCO₂ were pronounced in summer and had an inverse relationship with dissolved oxygen, pointing to a potential role of day-time algal consumption of CO₂. Cruise measurements displayed sharp pCO₂ pulses along the confluences of urban streams as compared with relatively low values along the upper estuary receiving low-CO₂ outflows from upstream dams. Large downstream increases in pCO₂, concurrent with increases in DOC concentrations and fluorescence intensities indicative of microbially processed organic components, imply a translocation and subsequent dilution of CO₂ carried by urban streams and/or fast transformations of labile C during transit along downstream reaches. The unique combination of spatial and temporal continuous measurements of pCO₂ provide insights on estuarine CO₂ pulses that might have resulted from the interplay between high loads of CO₂ and organic C of anthropogenic origin and their priming effects on estuarine microbial processing of terrigenous and algal organic matter.