Methane production and ebullition in a shallow, artificially aerated, eutrophic temperate lake (Lake Elsinore, CA)

Abstract

Methane is an important component of the gases released from lakes. Understanding the factors influencing the release is important for mitigating this greenhouse gas. The volume of methane (CH4) and other gases in sediments, and the rate of CH4 ebullition, were determined for an artificially aerated, shallow, eutrophic freshwater lake in Southern California. Gas volume was measured at 28 sites in July 2010, followed by monthly sampling at 7 sites through December 2011. Gas volumes measured in July 2010 at the 28 sites exhibited a complex dependence on sediment properties; the volume of CH4 and other gases was negligible in very coarse-textured sediment with low water and organic carbon contents. Gas volumes increased strongly with increased silt content, and were highest in sediments with intermediate water contents (60 to 70%), organic carbon contents (2 to 3%) and depths (approximately 4m). Methane was the dominant gas collected from sediment (80 to 90%), while carbon dioxide comprised roughly 2 to 3% of sediment gas in the lake. Gas sampling during cool winter months revealed very low or undetectable volumes of gas present, while sediment gas volumes increased markedly during the spring and early summer months, and then declined in late summer and fall. The rate of CH4 ebullition, quantified with an echosounder, also varied markedly across the lake and seasonally. High rates of ebullition were measured at all 7 sites in July 2011 (up to 96mmolCH4m(-2)d(-1)), while the rates were >50% lower in September and negligible in December 2010. Ebullition rates were inversely correlated with depth and most other sediment properties, but strongly positively correlated with sand content. No simple relationship between ebullition rate and sediment gas volume across the set of sites was found, although ebullition rates at individual sites were strongly related to gas volume.