PhD defence: Methane dynamics in seasonally euxinic coastal systems

This thesis shows how nutrient pollution (eutrophication) and low oxygen levels (deoxygenation), increase methane emissions from coastal waters to the atmosphere. Coastal systems are not generally considered to act as major sources of methane; most methane produced in the sediments of such systems was thought to be removed through the activity of methanotrophic (methane-eating) micro-organisms. We show that methane emissions from such systems can instead be substantial.

This study, which focused on two coastal areas, Lake Grevelingen in the Netherlands, and the Stockholm Archipelago in Sweden, shows that eutrophication and deoxygenation increase the production of methane and reduce the efficiency of the microbial methane filter. While in Lake Grevelingen methane was partially removed by microbes, substantial amounts of methane still escaped from the lake during summer and at times when the waters were mixed in spring and fall. Most methane escaped in the form of methane bubbles. In the Stockholm Archipelago, methane emissions were highest in areas with shallow methane-producing zones in the sediments and persistent low oxygen levels in the water.

Coastal areas worldwide are increasingly suffering from eutrophication and deoxygenation. This research shows that these environmental changes are weakening the natural ability of coastal systems to remove methane. Methane emissions from coastal waters will likely increase in future and should be considered in climate and environmental assessments.