A novel sediment gas sampler and a subsurface gas collector used for measurement of the ebullition of methane and carbon dioxide from a eutrophied lake

A novel sediment bubble gas sampler and a subsurface bubble gas collector w ere designed to measure the ebullition of gases from profundal sediments of aquatic ecosystems. The sediment gas sampler was constructed to collect bu bble gas samples directly from the uppermost sediment layers for gas compos ition analysis. The floating subsurface gas collector, designed to trap the bubbles released naturally from sediments, permitted the measurement of bo th the volume and the composition of the bubble gas. Due to its low cost, l ight weight and rapid sampling capability, the gas collector is ideal for s tudies requiring many replicate collectors. These devices were used for mea surement of the ebullition of methane (CH4) and carbon dioxide (CO2) during an open water period from hypereutrophic Lake Postilampi, situated within the midboreal zone in Finland. The bubble gas obtained from the sediment wi th the sediment gas sampler had higher concentrations of CH4 and CO2 than t he bubbles trapped in the gas collectors. This indicated that the bubble ga s composition changed, either naturally during the migration of the bubbles from the sediment through the water column to the gas collectors, and/or d uring their storage in the collectors prior to sampling. The mean CH4 ebull ition from Lake Postilampi was estimated to be in the range from 36 to 46 m g m(-2) d(-1), based on the bubble gas CH4 concentrations measured from the gas collectors and sediment, respectively. The bubbles contained only 0.02 -0.57% of CO2 and thus, the ebullition had no significance in the release o f CO2 from the lake. (C) 2001 Elsevier Science B.V. All rights reserved.