Carbon fluxes in subtropical shallow lakes: contrasting regimes differ in CH4 emissions

Abstract

Fluxes of carbon dioxide (CO2) and methane (CH4) in shallow lakes are strongly affected by dominant primary producers which mostly has been studied in temperate and boreal regions. We compared summer CO2 and CH4 fluxes (diffusion and ebullition) in littoral and pelagic zones of three subtropical shallow lakes with contrasting regimes: clear-vegetated, phytoplankton-turbid, and sediment-turbid, and assessed fluxes in different seasons in the clear-vegetated system. Significant differences among the lakes occurred only for CH4 fluxes. In the sediment-turbid lake we found undersaturated CH4 concentrations were below atmospheric equilibrium, implying CH4 uptake (< 0 mg m−2 day−1), likely due to low availability of organic matter. Differences between zones occurred in the clear-vegetated and phytoplankton-turbid lakes, with higher total CH4 emissions in the littoral than in the pelagic zones (mean: 4342 ± 895 and 983 ± 801 mg m−2 day−1, respectively). CO2 uptake (< < 0 mg m−2 day−1) occurred in the littoral of the phytoplankton-turbid lake (in summer), and in the pelagic of the clear-vegetated lake even in winter, likely associated with submerged macrophytes dominance. Our work highlights the key role of different primary producers regulating carbon fluxes in shallow lakes and points out that, also in the subtropics, submerged macrophyte dominance may decrease carbon emissions to the atmosphere.