Methane sources and sinks in karst systems: the Nerja cave and its vadose environment (Spain)

Abstract Karst caves are considered a natural sink of atmospheric methane (CH 4 ). Studies generally focus on measuring CH 4 within the cave environment, and little information is available about the factors influencing CH 4 abundance, such as the surrounding vadose and saturated zones of the aquifer, inputs of carbon dioxide (CO 2 ) – a potential precursor of CH 4 – and methanotrophic bacteria, which is likely the main cause of CH 4 consumption. In this paper we report the first study of CH 4 budget in Nerja, one of the main karst caves in Spain, based on seasonal monitoring of CH 4 and CO 2 concentration and stable C isotopic ratio in the cave system, integrated with analogue data from the underlying and overlying saturated/vadose zone, and methanotrophic bacteria activity (rDNA and Polymerase Chain Reaction analyses) in cave sediments and water. The results show that the cave environment consumes, via methanotrophic activity and through γ- and α–Proteobacteria, CH 4 coming from both the atmosphere and the vadose/saturated zones of the aquifer, where it is produced microbially. However, vadose zone methanogenesis may seasonally exceed the methanotrophy capacity of the cave, resulting in a net CH 4 increase in it. This competition process may be an important factor limiting the sink potential of karst caves.