| 12 Nov 2025
Assessing methane emissions from an offshore marine aggregate extraction site near Sylt, Eastern North Sea
Abstract. Coastal regions are estimated to contribute with up to 1 % to the global atmospheric methane (CH4) budget. However, these emissions remain highly uncertain due to strong spatial and temporal variability. Anthropogenic activities in coastal waters, such as sand mining and beach nourishment, have not yet been investigated or included in the calculation of the resulting greenhouse gas emissions. Since 1984, regular beach nourishment has been carried out every summer on the west coast of the island of Sylt (North Sea, Germany). During this process, dredging vessels extract a mixture of sand and water from the seabed of a spatially confined area (Westerland II), situated approximately 8 km off the coast and deposit the material on the western beach and foreshore of Sylt. High resolution measurements of CH4 in ambient air at the coastal atmospheric station Westerland (Sylt, Germany) show CH4 spikes of up to 400 ppb above background concentrations. These spikes occurred mainly during summer season, during low tide and under westerly wind conditions (from the sand dredging area).
To investigate the origin of the observed atmospheric CH4 spikes, combined in-situ measurements of dissolved and atmospheric CH4 together with water sampling, were performed on board the research vessel RV Mya II (AWI) along a coastal transect from Sylt to the Westerland II dredging site. In the vicinity of the dredging site, elevated CH4 concentrations were detected both in ambient air (400–500 ppb above background reaching up to 2450 ppb) and in surface and bottom waters (69–90 nmol L-1). This resulted in a mean diffusive flux of 45 ± 47 µmol m-2 d-1 with a maximum value of 340 µmol m-2 d-1 in the dredging area, in contrast to a diffusive flux of 3.0 ± 3.6 µmol m-2 d-1 during the transit. Our observations demonstrate significantly higher CH4 concentrations and fluxes in both the atmosphere and the water column above the sand dredging site, compared to the areas outside the dredging activities. Furthermore, the isotopic composition of dissolved CH4 within the dredging site characterized by more negative stable carbon and hydrogen isotope values, point to a microbial source of the excess CH4.
In "Assessing methane emissions from an offshore marine aggregate extraction site near Sylt, Eastern North Sea", the authors present the results of a one-day ship campaign aimed at confirming that the excess CH4 sometimes measured in summer at the ICOS site Westerland are coming from the sand extraction site Westerland II. To do so, they combine air and surface and bottom water sampling and measure CH4 but its isotopes as well for a few samples.
The manuscript is mostly clear and well written and should be published subject to minor revisions.
In general, the manuscript should be reread to make sure that people from both communities (atmosphere and ocean) fully understand each other parts and that all measurements are well presented. Below, I underline the points that I think would improve the manuscript.
p4 l143 "To convert the relative concentrations (ppm) reported by the GGA to absolute concentrations (nmol L-1), discrete water samples were collected at least once per hour (Bussmann et al., 2024)." can you please detail a bit this part? Why the temporality is needed to get the conversion from ppm to nmol/L?
p5 l149 what value is used for ceq?
p5 l159 Can you shortly define the Schmidt number?
p5 l165 please define CTD (also in Figure 1)
In 2.4, can you add the actual number of samples taken for the headspace measurements and for the isotopes? Listed only in 3.4, you say you have 4 stations but 11 samples. please clarify.
In Figure 5, can you add the location of the atmospheric isotopic samples ? Also, check the legend , 1a and 1b are only blue not black and blue (or clarify)
In Figure 7, can you add the location of the 11 isotopic samples (1 to 11)?
p12 3.6 you only describe the location of 4 bags, what about the others?
Minor corrections:
p1 l23 please define AWI
p2 l48 replace gromers by groomers
p4 l118 replace ICOS-Cal by ICOS-FCL
p14 l407 "Dissolved CH4 concentrations have been measured in the area south of our study area, but with the same method." Should it no read "but with a different method"?
p16 l480 replace non et all by none at all
p18 l523 a parenthesis is missing