Benjamin Lloyd Miller, Gordon William Holtgrieve, Mauricio Eduardo Arias, Sophorn Uy, and Phen Chheng

PNAS February 22, 2022 119 (8) e2107667119

Significance

Freshwaters inextricably link flows of carbon between the land, oceans, and atmosphere. Resulting carbon dioxide supersaturation relative to the atmosphere in most of the world’s lakes and rivers has long been assumed to come from aerobic respiration. Although carbon dioxide also comes from the oxidation of anaerobically produced methane, this has been largely ignored within freshwaters. Here, we use stable carbon isotopes of carbon dioxide and methane to show that a nontrivial proportion of the total dissolved carbon dioxide in a tropical flood pulse lake comes from methane oxidation. Seasonal pulses of flooding are common in the tropics, suggesting that coupled methane production and oxidation likely contribute more broadly to flows of carbon between the land, understudied tropical freshwaters, and atmosphere.

Abstract

Carbon dioxide (CO₂) 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 (CH₄) production and oxidation also contribute CO₂ to freshwaters, yet this remains largely unquantified. Flood pulse lakes and rivers in the tropics are hypothesized to receive large inputs of dissolved CO₂ and CH₄ from floodplains characterized by hypoxia and reducing conditions. We measured stable C isotopes of CO₂ and CH₄, aerobic respiration, and CH₄ production and oxidation during two flood stages in Tonle Sap Lake (Cambodia) to determine whether dissolved CO₂ in this tropical flood pulse ecosystem has a methanogenic origin. Mean CO₂ supersaturation of 11,000 ± 9,000 μatm could not be explained by aerobic respiration alone. ¹³C depletion of dissolved CO₂ relative to other sources of organic and inorganic C, together with corresponding ¹³C enrichment of CH₄, suggested extensive CH₄ oxidation. A stable isotope-mixing model shows that the oxidation of ¹³C depleted CH₄ to CO₂ contributes between 47 and 67% of dissolved CO₂ in Tonle Sap Lake. ¹³C depletion of dissolved CO₂ was correlated to independently measured rates of CH₄ production and oxidation within the water column and underlying lake sediments. However, mass balance indicates that most of this CH₄ 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 CO₂ supersaturation and atmospheric emissions may be explained in part by coupled CH₄ production and oxidation.

See: https://www.pnas.org/content/119/8/e2107667119

Fig. 1: Dissolved O₂ deficit and CO₂ supersaturation, relative to atmospheric equilibrium in open water, edge, and floodplain environments of TSL (A) during the high-water and falling-water stages of the flood pulse (B). Dissolved O₂ deficit and CO₂ supersaturation are calculated as the difference between atmospheric equilibrium, according to Henry’s Law. Orange lines show atmospheric equilibrium at a dissolved O₂ deficit and CO₂ supersaturation of 0 μ mol ⋅ L⁻¹. A slope (m) of −1.0 represents the equimolar consumption of dissolved O₂ and production of dissolved CO₂ expected during aerobic respiration (black dashed line). Instead, a slope of −0.1 was observed during both the high-water and falling-water stages. O₂ deficits were strongly correlated to CO₂ supersaturation during the high-water stage, but there was no such correlation during the falling-water stage.