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Background

There are CH4 sources and sinks, which can be both natural and anthropogenic, and the balance between them determines the growth rate of CH4. The CH4 levels have more than doubled since the industrialization and more than 60 % of the emissions come from anthropogenic sources the reaming 40 % comes from natural sources mainly wetlands.

Inland water systems (lakes and streams) are natural sources of CH4 emission, but until recent years they were often forgotten when estimating fluxes in the global carbon cycle. This is a problem because the estimated global terrestrial carbon sink is 2.6 (±1.7) Pg year-1, and it has been estimated that the open water flux of methane from lakes release 0.65 Pg CO2 equivalents year-1 globally. Together with CO2, the open water fluxes correspond to almost 80 % of the calculated global terrestrial sink of carbon. The main terrestrial sink is connected to the GHG emissions from the lakes since the runoff water add carbon to the lakes as organic matter, which is then partly degraded into CH4.

CH4 can be emitted to the atmosphere from lakes to the atmosphere three ways, diffusion, ebullition (bubbles) and through emergent aquatic plants. The CH4 are produced in a biological process (metanogenesis) in anoxic environments when organic matter is degraded, if oxygen is present it inhibits the process. In parts where there is oxygen the aerobic methane-oxidizing bacteria (MOB) will use the CH4 as energy and carbon sources, by using O2 as an electron acceptor, the final rest product will be CO2. A big part of the CH4 that diffuse from the sediment up to the oxic part of the water column will be oxidized. Besides diffusive flux, CH4 can reach the atmosphere through ebullition (bubbles) which ascends from the sediment without being oxidized. However, the amount of data about ebullition is limited and the data available are dominated by infrequent and short-term measurements.

The main causes for variations in open water CH4 emission from lakes is the amount of organic matter in the sediments, lakes area and morphometry. CH4 emission can also vary over a season for example if the lake is stratified during the summer. Total ebullition per lake increase with lake area due to more area of shallow sediments. The fact that ebullitions occur less in the deeper parts of the lakes can be explained by the higher hydrostatic pressure and that the sediments are more protected from turbulence.

Aims

The aim of this study is to

1) locate patterns in the spatial distribution of CH4 emissions in a lake;

2) link temporal changes in the lakes production, temperature, weather etc. with changes in CH4 emissions from early summer to fall (after the fall overturn);

3) evaluate the contribution of ebullition and diffusive components to the total flux and if this could be explained with temperature, weather, type of lake or water chemistry and

4) to see if and how two adjacent lakes differentiate in CH4 emissions over time.


Responsible for this page: Agneta Johansson
Last updated: 05/11/15