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Dissolved carbon transport in a river-lake continuum: A case study in a subtropical watershed, USA

Xu, Zhen, Xu, Y. Jun
The Science of the total environment 2018 v.643 pp. 640-650
biogeochemistry, carbon dioxide, case studies, dissolved inorganic carbon, dissolved organic carbon, lakes, rivers, stable isotopes, watersheds, Louisiana
Rivers and lakes have been traditionally studied as separate entities for carbon transport. However, there is a gap in our knowledge of the connectivity of dissolved carbon in a river-lake continuum. In this study, we analyzed dissolved carbon along the Little River-Catahoula Lake in subtropical Louisiana, United States to assess carbon biogeochemistry in such a river-lake continuum. Monthly in-situ measurements and water sample collections were made at four locations during April 2015 to February 2016 to determine riverine carbon transport into and out of the lake. Results show that much of the dissolved inorganic carbon (DIC) in the river-lake continuum originated from 13C depleted sources with an average δ13CDIC of −18.5‰. Significant decreases in DIC were found after the river passed through the lake (from 482 to 399 μmol L−1), which was most prevalent when the lake was not affected by backwater flow from the downstream river. CO2 outgassing could be mainly responsible for the sink behavior of the lake for DIC. Dissolved organic carbon (DOC) in the studied watershed were mostly terrigenous with low δ13CDOC averaged at −29.2‰. Significant, consistent decreases in DOC concentrations were found from the river to the lake inflow and then to the lake outflow. During the majority of the year, the lake reduced DOC concentrations from the river inflow water, but switched to functioning as a source of DOC during warmer, dryer conditions in September and October due to increased water residence time. Therefore, the lake functioned both as a sink and as a source for DOC.