Wave-induced release of methane: Littoral zones as a source of methane in lakes

This study investigates the role of surface waves and the associated disturbance of littoral sediments for the release and later distribution of dissolved methane in lakes. Surface wave field, wave-induced currents, acoustic backscatter strength, and the concentration and distribution of dissolved methane were measured simultaneously in Lake Constance, Germany. The data indicate that surface waves enhance the release of dissolved methane in the shallow littoral zone via burst-like releases of methane during the passage of wave groups. The amount of released methane depends on the surface wave field and the water temperature that controls the methane production in the sediments. The dissolved methane concentrations in the shallow littoral zone were always higher than concentrations in the deep water and open water, while methane concentrations in the epilimnion were typically higher than methane concentrations in the metalimnion and upper hypolimnion. The relatively high epilimnetic methane concentrations in the pelagial can be explained by lateral transport of methane from the littoral zone to the pelagic zone. Littoral zones can thus be an important source of methane in lakes. Methane released by surface waves from littoral sediments may cause elevated near-surface methane concentrations in large areas that enhance the flux of methane at the air–water interface and, thus, the overall methane emissions from lakes to the atmosphere. Methane (CH4) is a climate-relevant trace gas that accounts for , 20% of the Greenhouse Effect (Wuebbles and Hayhoe 2002; IPCC 2007). Lakes, reservoirs, and wetlands provide 6–16% of the total natural methane emissions (more than oceanic emission), indicating that lakes are a significant source in global methane budgets (St. Louis et al. 2000; Bastviken et al. 2004). Methane is a major product of carbon metabolism in lakes. Anaerobic carbon mineralization, in terms of methanogenesis in anaerobic sediments, can account for up to 50% of the overall carbon mineralized in freshwater lakes (Bastviken et al. 2008). A large proportion of the produced methane is oxidized by methanotrophic bacteria at oxic water and sediment interfaces (Frenzel et al. 1990; Bastviken et al. 2002; Liikanen et al. 2002). The main emission pathways from the waterbody to the atmosphere are summarized by Bastviken et al. (2004), as follows: ebullition from anaerobic sediments (Mattson and Likens 1990; Fendinger et al. 1992; Ramos et al. 2006), diffusive flux across the air–water interface (Stumm and Morgen 1996), plant-mediated flux from the littoral sediments (Segers 1998), and the flux of methane stored in the anoxic waterbody during the stratification period that is rapidly released during overturning and mixing (Michmerhuizen et al. 1996; Riera et al. 1999). The proportion of each individual pathway to the overall lake emission is highly dependent on lake characteristics (e.g., lake size, stratification pattern, nutrient load, and plant cover; Bastviken et al. 2004). In the past, many investigations were focused on profundal sediments as source of methane (Casper 1992), internal cycling (Rudd and Taylor 1980; Bastviken et al. 2003; Kankaala et al. 2006), and later oxidation in the water column (Utsumi et al. 1998; Bastviken et al. 2002). Methane produced in epilimnetic sediments is considered an important source for ebullition (Walter et al. 2006, 2008) and the plant-mediated flux (Bastviken et al. 2004; Wang et al. 2006) that causes direct fluxes to the atmosphere. On the other hand, diffusive fluxes from shallow littoral zones were considered to be less important than fluxes from the anoxic profundal sediments (Frenzel et al. 1990; Liikanen et al. 2002). However, other studies (Michmerhuizen et al. 1996; Murase et al. 2005; Bastviken et al. 2008) indicated that especially during summer the methane fluxes from littoral sediments can be higher than those from profundal sediments, because high sediment temperatures at shallow depths support high methane production rates. In addition, it has been observed (Schmidt and Conrad 1993; Schulz et al. 2001; Murase et al. 2005) that dissolved methane concentrations in the epilimnion can be higher than those in the hypolimnion, which indicates that methane release from profundal sediments cannot be the sole source of epilimnetic methane degassing to the atmosphere. The main differences between littoral and profundal sediments are not only the warmer sediment temperatures in the littoral during summer (which favor higher methane production rates; Thebrath et al. 1993; Casper 1996) but also the exposure to surface waves (Hofmann et al. 2008), which may enhance the release of methane from the sediments. In the absence of waves, the exchange of dissolved methane above the sediment–water interface is dominated by molecular diffusion that limits the flux of methane to the water column and that is accompanied by high methane oxidation rates at the sediment–water interface (Huttunen et al. 2006). Waves cause intense oscillating currents (Hofmann et al. 2008) that may enhance the flux of methane above the sediment–water interface by advective sediment pore-water exchange (wave pumping) (Precht and Huettel 2003) and by resuspension (Nielson 1994; Hofmann 2007) that breaks up the upper * Corresponding author: [email protected] Limnol. Oceanogr., 55(5), 2010, 1990–2000 E 2010, by the American Society of Limnology and Oceanography, Inc. doi:10.4319/lo.2010.55.5.1990