Abstract
We report sea-air fluxes of methane in physically and biologically distinct inshore habitats of the Baltic Sea with the goal to establish empirical relationships that allow upscaling of local site-specific flux measurements. Flux measurements were conducted using floating chambers with and without bubble shields, and by using a boundary layer gas transfer model before, during, and after an annually occurring algal bloom from June to October 2019. Water and air temperature, salinity, wind, sediment organic content, and organic content of floating algal biomass were found to successfully discriminate the different habitats in terms of methane flux, both over periods of days and over a season. Multivariate statistical analysis was used to establish the relative environmental forcing of methane emissions over one growth season for each flux method. Floating algal biomass carbon and sediment organic content were identified as the most important controlling factors for methane emissions based on flux chamber measurements over a period of days to weeks, whereas water and air temperature and wind velocity were the most important factors based on the gas transfer model on these time scales. Over the season, water and air temperature were the most important controlling factors with both methods. We present a first attempt how our observations can be extrapolated to determine the coastal methane emission along the coastline.
Highlights
The trace gas methane contributes about 23% to the radiative balance of Earth’s atmosphere and its increasing emissions contribute significantly to ongoing global warming (Mende et al, 2019)
We categorized five sampling areas of the island shoreline based on salinity, wind velocity, water temperature, air temperature, water depth, organic content of floating algal biomass, shore vegetation, and sediment organic carbon content into four different habitats (Table 1)
There were different forcing constellations depending on habitat. These results suggest that the calculated hierarchy of forcing parameters for methane fluxes was dependent on the field method and habitat
Summary
The trace gas methane contributes about 23% to the radiative balance of Earth’s atmosphere and its increasing emissions contribute significantly to ongoing global warming (Mende et al, 2019). Aquatic ecosystems are responsible for about 53% of the total global methane emissions from anthropogenic and natural sources (Rosentreter et al, 2021). Methane has anthropogenic sources from fertilizer application, animal food stocks, industrial and agricultural emissions, and fossil fuel combustion (Seitzinger et al, 2006; Saunois et al, 2016), and natural sources such as thawing permafrost, wetlands, natural gas seeps, and emissions from lakes and the coastal ocean that react sensitively to climate change and human activities (Nisbet et al, 2019). In particular the inshore habitats, are of interest for global methane emissions, because they are most directly affected by anthropogenic disturbances of the land-ocean interface (Smith and Hollibaugh, 1993; Gattuso et al, 1998; Battin et al, 2008; Torres-Pulliza et al, 2020). Inshore coastal habitats have been identified as potentially important blue carbon repositories to mitigate CO2 accumulation in the atmosphere (Macreadie et al, 2019), but the efficiency of this carbon sink is partially offset by methane emissions (Rosentreter et al, 2018)
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