Methane and Carbon Dioxide Dynamics in the Paraguay River Floodplain (Pantanal) in Episodic Anoxia Events

Abstract

Worldwide wetlands contribute to the global carbon cycle by emitting about a third of the global methane (CH₄) emissions. However, CH₄ and carbon dioxide (CO₂) dynamics remain poorly understood in the largest tropical wetland on Earth, the Pantanal. In this chapter, we aim to characterize the CH₄ and CO₂ biogeochemistry in the floodplain of the Paraguay River, near Corumbá, during the course of annual anoxia phenomena locally known as dequada. The strong anoxia is associated to the flooding of terrestrial habitats that enhances respiration, dissolved oxygen (DO) consumption, and methanogenesis. The extremely low DO also leads to high fish mortality in the region. CH₄ and CO₂ concentration in surface waters and diffusive water–air fluxes were measured in the oxbow Tuiuiú Lake and in the Paraguay River main stem in order to identify temporal and spatial patterns. The whole dataset shows that, for instance, dissolved CH₄ and diffusive CH₄ fluxes increased dramatically during the dequada. In the study area, CH₄ emissions can reach 9–85 mg CH₄ m⁻² h⁻¹ during dequada climax. Riverine anoxic waters steadily penetrate the oxbow Tuiuiú Lake, indicating water inflow from the river main stem, whereas small reminiscent patches of oxbow waters not mixing with anoxic river waters may function as survival refuges to the aquatic wildlife. Clearly, the DO distribution during several dequadas in major rivers of the Pantanal highlights the importance of geomorphology on the biogeochemistry in the riverine floodplains of the Pantanal wetland.