Methane concentration profiles in a lake with a permanently anoxic hypolimnion (Lake Lugano, Switzerland-Italy)

Abstract

Methane concentrations were measured in the water column of the northern basin of Lake Lugano in 1993 and 1994. Methane profiles show three distinct zones: (1) low concentrations generally below 0.1 mmol m −3 from the surface to 80–90-m depth; (2) a. sharp rise in concentration up to 50 mmol m −3 from 80–90- to ∼ 150-m depth; and (3) from ∼ 150-m depth to the bottom with a lower concentration gradient and with the highest methane concentrations in near-bottom water was ∼ 80 mmol m −3. These profiles result from a combination of several factors such as fluxes from sediments, spatially variable vertical mixing, and aerobic and anaerobic bacterial oxidation in the water column. δ 13C values indicate a biogenic origin for the methane. The methane inventory in the anoxic hypolimnion is ∼ 2000 metric tonnes, yet its transfer to the surface water is almost completely cancelled by oxidation at the permanent redox interface situated at ∼ 90-m water depth. Increased methane concentrations in near-surface water are not related to the deep-water methane reservoir but probably result from bacterial production of methane in the photic zone. Surface water is over saturated with methane in respect to the atmosphere. Anthropogenic eutrophication in temperate zone lakes has affected the carbon budget and methane production and storage. The potential impact of these changes on global emissions to the atmosphere should be evaluated.