Eutrophication and Deoxygenation Drive High Methane Emissions from a Brackish Coastal System

Area of Science:

• Environmental Science
• Marine Chemistry
• Geochemistry

Background:

• Coastal environments are significant sources of atmospheric methane.
• Eutrophication and deoxygenation can increase methane release from coastal areas.

Purpose of the Study:

• Investigate methane dynamics in the eutrophic Stockholm Archipelago.
• Identify factors controlling methane emissions in coastal marine environments.

Main Methods:

• Studied methane release across sites with varying water column redox conditions.
• Assessed organic matter degradation rates via the sulfate-methane transition zone (SMTZ) depth.
• Measured methane concentrations and isotopic composition in sediment and water.

Main Results:

• Highest benthic methane release (2.2-8.6 mmol m^-2 d^-1) observed where SMTZ is shallow (2-10 cm).
• Significant methane removal occurs in the water column through microbial processes.
• High ¹³C depletion in methane suggests substantial bubble dissolution.
• Atmospheric methane fluxes ranged from 0.03 to 1.7 mmol m^-2 d^-1, highest with shallow SMTZ and anoxic bottom waters.

Conclusions:

• Eutrophication and deoxygenation create hotspots for coastal methane emissions.
• Shallow SMTZ and anoxic, sulfidic bottom waters are critical factors driving high methane release.
• Understanding these dynamics is crucial for predicting future climate impacts.