Coupled CH(4) production and oxidation support CO(2) supersaturation in a tropical flood pulse lake (Tonle Sap Lake, Cambodia)

Carbon dioxide (CO(2)) supersaturation in lakes and rivers worldwide is commonly attributed to terrestrial–aquatic transfers of organic and inorganic carbon (C) and subsequent, in situ aerobic respiration. Methane (CH(4)) production and oxidation also contribute CO(2) to freshwaters, yet this remains largely unquantified. Flood pulse lakes and rivers in the tropics are hypothesized to receive large inputs of dissolved CO(2) and CH(4) from floodplains characterized by hypoxia and reducing conditions. We measured stable C isotopes of CO(2) and CH(4), aerobic respiration, and CH(4) production and oxidation during two flood stages in Tonle Sap Lake (Cambodia) to determine whether dissolved CO(2) in this tropical flood pulse ecosystem has a methanogenic origin. Mean CO(2) supersaturation of 11,000 ± 9,000 [Formula: see text] atm could not be explained by aerobic respiration alone. (13)C depletion of dissolved CO(2) relative to other sources of organic and inorganic C, together with corresponding (13)C enrichment of CH(4), suggested extensive CH(4) oxidation. A stable isotope-mixing model shows that the oxidation of (13)C depleted CH(4) to CO(2) contributes between 47 and 67% of dissolved CO(2) in Tonle Sap Lake. (13)C depletion of dissolved CO(2) was correlated to independently measured rates of CH(4) production and oxidation within the water column and underlying lake sediments. However, mass balance indicates that most of this CH(4) production and oxidation occurs elsewhere, within inundated soils and other floodplain habitats. Seasonal inundation of floodplains is a common feature of tropical freshwaters, where high reported CO(2) supersaturation and atmospheric emissions may be explained in part by coupled CH(4) production and oxidation.