The southern North Sea (German Bight) is influenced by the inflowing water in the Tide-Elbe area and how it is further modified. For a better understanding of the different branches of relevant events it is necessary to investigate which are the main impacting factors for the terrestrial and coastal water quality. Therefore, during different cruises between 2019 and 2022 a group of parameters was continuously traced from the Elbe River basin (Czech/German border) to Tide-Elbe and Elbe estuary (Hamburg, Germany) until the German Bight. This dataset is about the third cruise of the project, Sternfahrt 3, conducted from the 10th to 11th of September 2019 in the south-east of Heligoland (North Sea). In this area a methane "plume" was located during the previous cruise in June 2019, thus the objective was to verify its horizontal and vertical extension as well as further investigations. To cover the whole area of the plume three research vessels (Littorina, Ludwig Prandtl and Uthörn) participated on the cruise. All ships started in Cuxhaven (northern Germany), on the first day Littorina and Uthörn followed a western course towards the south of Heligoland, while the Ludwig Prandtl crossed the plume on a more northern track. The second day the Ludwig Prandtl and Littorina took roughly the same course back to Cuxhaven. The vessel Uthörn however, first headed east, to explore the boundaries of the plume, and then crossed westwards back to Bremerhaven (northern Germany) (see map). In the beginning of each day all vessels met at one station (~ 100 – 600m distance) where measurements were conducted for 30 minutes to inter-calibrate the onboard sensor systems. During the cruises basic hydrographic parameters (salinity, temperature, oxygen saturation and pH), atmospheric and dissolved greenhouse gases were measured continuously. The inlets of the methane flow-through degasser (Los Gatos) and of the Ferry Box system (for physical and chemical oceanographic parameters) were submerged in a tank on deck which was connected with the underway water supply of the vessel feeding the container with surface water (~ 1m depth) with a flow rate of 100 L/minute. The Picarro air sampling system was set up on the outside recording permanently atmospheric partial carbon dioxide and methane. Additionally, vertical profiles and water samples were taken at specific stations along the tracks. The respective data can be found in the AWI O2A-database. The vertical data from the cruise are not included in this dataset. Further information about the whole campaign and the different cruises can be found in the article(s) cited in references.

The data were taken in situ on board of three research vessels continuously measuring surface water. Afterwards the datasets were reviewed and a standardized (Argo-Standards, R-Script written by Philipp Fischer) plausibility control (spike, gradient, range) was conducted for all parameters except for wind direction. To align the data between the ships the complete cruise data and the data from the inter-calibration intervals were compared. Based on those comparisons a correction factor was calculated and applied to all data. For more details see "Device descriptions from ship measurement systems".