Methane dynamics in seasonally euxinic coastal systems

This thesis explores the dynamics of methane in the water column and sediment of seasonally euxinic coastal systems. The focus lies on how eutrophication and deoxygenation affect methane removal processes. Field studies were conducted to examine how variations in redox conditions influence aerobic and anaerobic methane removal pathways, and to quantify methane emissions to the atmosphere via diffusion and estimate how much methane escapes in the form of bubbles. The research was carried out in two study areas: the seasonally euxinic marine Lake Grevelingen in the Netherlands and the brackish, eutrophic Stockholm Archipelago. A range of techniques was employed, including high-resolution water and sediment sampling, in-situ methane flux measurements, microbial community analysis, and reactive transport modeling. Results showed that methane removal in Lake Grevelingen occurs through aerobic and possibly also anaerobic pathways, with much of the methane being removed near the oxycline during stratified periods. Aerobic methanotrophic bacteria (Methylomonadaceae) adapted to low-oxygen conditions played a crucial role in removing methane during summer stratification, A significant portion of the methane still escaped to the atmosphere both during periods of water column mixing and summer stratification, likely mostly in the form of bubbles, highlighting the role of eutrophic coastal systems in marine methane emissions. In the Stockholm Archipelago, methane emissions were found to be highest in areas suffering from long-term anoxia and shallow sulfate-methane transition zones (SMTZ), indicating that eutrophication and deoxygenation create hotspots of methane emissions. Additionally, in Lake Grevelingen, sedimentary molybdenum enrichments and isotopic signatures were evaluated as redox proxies. The results showed that sediments of seasonally euxinic systems can record isotopic signatures similar to those of permanently euxinic environments. Overall, as coastal systems are becoming more eutrophic and deprived of oxygen, the efficiency of the microbial filter in the water column is expected to decrease, leading to enhanced methane emissions to the atmosphere, both via diffusion and in the form of bubbles.