Ebullition dominates methane emissions in stratified coastal waters

Area of Science:

• Environmental Science
• Geochemistry
• Marine Biology

Background:

• Coastal areas are significant sources of atmospheric methane (CH4).
• Understanding CH4 sources and regulation in coastal waters is crucial for climate change feedback assessments.
• Existing knowledge on CH4 origins in surface coastal waters and subsequent sea-air emissions is limited.

Purpose of the Study:

• To investigate coastal methane (CH4) dynamics in the Tvärminne Archipelago, Baltic Sea.
• To identify the primary sources and processes regulating CH4 formation and oxidation in brackish coastal systems.
• To compare CH4 biogeochemistry and microbiology between nearshore and offshore stations with varying environmental conditions.

Main Methods:

• Water column data collection from six stations in the Tvärminne Archipelago.
• In-depth sediment biogeochemistry and microbiology analysis at nearshore and offshore sites.
• Analysis of isotopic signatures (δ13C-CH4) in sediment and water columns.
• Evaluation of CH4 production in the water column and benthic diffusion rates.

Main Results:

• Methane (CH4) diffusion from sediment to water was negligible at the nearshore station due to high oxidation rates.
• Significant benthic CH4 diffusion occurred at the offshore station, but oxidation limited flux to the water column.
• Water column CH4 production was found to be negligible.
• Isotopic signatures (δ13C-CH4) in stratified surface waters suggest ebullition, rather than diffusion, as the primary CH4 source.

Conclusions:

• Methane (CH4) dynamics in brackish coastal systems are complex, influenced by stratification and redox conditions.
• Oxidation processes significantly reduce CH4 flux from sediments to the water column.
• Ebullition is hypothesized as the dominant pathway for CH4 transfer to surface waters in stratified brackish coastal environments, with implications for sea-air emissions.