How do small seafloor animals influence greenhouse gas exchange in the Baltic Sea?

Greenhouse gas emissions are increasing to unprecedented levels, but the point sources for these emissions are still not yet fully understood, especially in environments that are not under our nose. Marine sediments, also known as the seafloor, have been shown to be important contributors of the two strong greenhouse gases methane and nitrous oxide to the water column. While the majority of this sediment-to-water methane is oxidized (or consumed) in the water column, in shallow coastal zones a part of it can be exchanged with the atmosphere. To date, however, there is currently very scarce information on how animals living in the sediment, and their interaction with microbes, affect greenhouse gas emissions.

Our study suggests that small seafloor animals play an important role in the regulation of greenhouse gas exchange, especially on methane emissions. The study is based on four different experiments including trace gas, stable isotopes and molecular analyses. We were able to demonstrate that seafloor worms and clams, which are often classified as macrofauna, enhance the flux of methane of up to eight times compared to bare sediments (see figure).

Although this was a surprising result, other studies have already showed that invertebrates have the ability to increase methane fluxes. Methanogenesis, the biological production of methane, can indeed happen in the guts of termites, widespread terrestrial invertebrates. Clams and worms inhabiting the seafloor also have this ability, our study demonstrates, due to the methanogenic microorganisms associated with their digestive tracts. Our estimates indicate that ~10% of the total methane emission from the Baltic Sea is due to macrofauna-inhabited sediments. These small animals may thus play an important, but so far neglected, role in regulating emissions of greenhouse gases from coastal sediment ecosystems.

The Baltic Sea is one of the most studied waterbodies in the world and methane emissions from the Baltic are reasonably well quantified. However, our study helps understand and itemize where these emissions come from and account for spatial variability in the emissions. Our results indicate that the spatial variability in methane fluxes is also related to macrofauna community composition.

Reference: Methane fluxes from coastal sediments are enhanced by macrofaunaScientific Reports (2017).

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