Arctic seawater was collected at two sites close to Utqiagvik, North Slope, Alaska on 7, 9 and 15 April 2016 in the Beaufort Sea. Site "Elson Lagoon" (EL) was located North of Utqiagvik (2016-04-07, 71.334° N, -156.363° W), covered with 1.5 m thick sea ice and shallow with approximately 1.5 m depth. Water was collected from a depth of 1.5 m in a narrow layer of water between the sea ice and the sediment. Site "Ice Mass Balance Buoy" (IMB) was located 1 km offshore of Utqiagvik (2016-04-07, 71.373° N, -156.548° W), and (2016-04-09, 71.372° N, -156.540° W). This site was characterized by 1 m thick fast ice cover and a water depth of approximately 7 m. Water was collected at five depths from 1 to 6.5 m.
Seawater was additionally sampled in Narragansett Bay, Rhode Island, from the pier of the Graduate School of Oceanography, University of Rhode Island (41.492° N, -71.419° W) on 15 July 2016 to test our method for applicability under different environmental conditions. Sampling depth was 2-3 m at a water depth of approx. 6 m.
Seawater was amended with different methane concentrations to determine methane oxidation rates in a headspace approach. Methane concentration and isotope ratios were determined with a Picarro G2201-i cavity ring-down spectrometer (Picarro, Santa Clara, California, USA) coupled to a Small Sample Isotope Module (SSIM) (Picarro, Santa Clara, California, USA). Several treatments were amended with sodium hydroxide (NaOH) to serve as killed controls or with 2-bromoethanesulphonate (BES) an inhibitor for methane production.
