Influence of different carbon sources on methane fluxes in sediments of eelgrass beds and unvegetated areas: a case study on the Swedish west coast

Abstract: One of the best ways to mitigate climate change is through natural carbon sequestration. An efficient way to mitigate climate change is through carbon sequestration from coastal habitats. Seagrass meadows, for example, store up to ten times more carbon per square meter than terrestrial forests and are therefore of great importance as so called blue carbon sinks. However, the anoxic marine sediments can contain methanogenetic archaea that emits methane. Methane is a greenhouse gas that is 28-34 times as potent as carbon dioxide. These emissions threaten to undo part of the carbon sink potential of these ecosystems. It is yet not known how different sources of carbon from different types of vegetation influence these emissions. This study aims to investigate how methane emissions vary between different types of carbon sources by using teabags filled with different organic materials and measuring the methane emissions during decomposition using static gas chambers. This was tested in situ in “Potätabukten” in the Gullmar Fjord on the Swedish west coast. It was tested both in an eelgrass (Zostera marina) bed and in bare sediment in order to find any differences between the two habitats. Methane emissions were relatively low in the area (0.39 ± 0.67 CH4 µg m-2 h-1 ). The amount of sedimentary carbon around the teabags was also measured and found to be relatively high (0.70 ± 0.20%). Sedimentary carbon content was significantly higher in the eelgrass bed (0.82 ± 0.14%) than the bare sediments (0.56 ± 0.17%). This in combination with the low methane emissions indicates that the seagrass meadow is a potential blue carbon hotspot. Decomposition rate was measured for the different carbon sources and filamentous macro algae were found to decompose significantly faster than non filamentous macro algae, indicating that filamentous algae are a more labile source of carbon. No significant difference in methane production was found between carbon sources nor habitats. The low levels of methane and non-significant differences between the carbon substrates are theorized to be due to the relatively short time of exposure (5 weeks) in combination with low water temperatures during this spring which slows the degradation processes. This conforms to earlier studies where methane emissions have been found to be lower during colder periods of the year.