Abstract
Transport and transformation of riverine dissolved carbon is an important component of global carbon cycling. The Atchafalaya River (AR) flows 189 kilometers through the largest bottomland swamp in North America and discharges ~25% of the flow of the Mississippi River into the Gulf of Mexico annually, providing a unique opportunity to study the floodplain/wetland impacts on dissolved carbon. The aim of this study is to determine how dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the AR change spatially and seasonally, and to elucidate which processes control the carbon cycling in this intricate swamp-river system. From May 2015 to May 2016, we conducted monthly river sampling from the river’s inflow to its outflow, analyzing samples for concentrations and δ13C stable isotope composition of DOC and DIC. We found that DIC concentrations in the AR were three times higher than the DOC concentrations on average, and showed more pronounced downstream changes than the DOC. During the study period, the river discharged a total of 5.35 Tg DIC and a total of 2.34 Tg DOC into the Gulf of Mexico. Based on the mass inflow–outflow balance, approximately 0.53 Tg (~10%) of the total DIC exported was produced within the floodplain/wetland system, while 0.24 Tg (~10%) of the DOC entering the basin was removed. The AR’s water was consistently oversaturated with CO2 partial pressure (pCO2) above the atmospheric pCO2 (with pCO2 varying from 551 µatm to 6922 µatm), indicating a large source of DIC from river waters to the atmosphere as well as to the coastal margins. Largest changes in carbon constituents occurred during periods of greatest inundation of the swamp-river basin and corresponded with shifts in isotopic composition. This effect was particularly pronounced during the initial flood stages, supporting the hypothesis that subtropical floodplains can act as effective enhancers of the biogeochemical cycling of dissolved carbon.
Highlights
Large rivers play an important role in the global carbon cycling, acting as conduits of terrestrial carbon from land to sea, and as active systems for carbon transformation and processing [1,2]
This study demonstrates that a large river with extensive river corridor and floodplain wetlands in its coastal margin can act as a significant source of dissolved inorganic carbon both to coastal systems and to the atmosphere, and may be important sinks for dissolved organic carbon
The influence of floodplain interactions on biogeochemical cycling of carbon in this system may be due to a net heterotrophic response to high productivity within this large subtropical river swamp system, with high CO2 production occurring in floodplain soils, vascular plant root zones, and in the water column
Summary
Large rivers play an important role in the global carbon cycling, acting as conduits of terrestrial carbon from land to sea, and as active systems for carbon transformation and processing [1,2]. Water 2019, 11, 1395 floodplains of the central Amazon [4] and Zambezi River basin [5] contributed large inputs of inorganic carbon into the river waters, while other studies have demonstrated varying effects of floodplain interactions on DOC [6,7,8]. It is, not yet clear to what degree extensive floodplains in temperate and subtropical regions affect riverine dissolved carbon fluxes. Riverine DOC is often partly mineralized in-stream to produce DIC. DIC can be taken up through photosynthesis to produce new organic matter, or can be lost or gained through atmospheric exchange
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