3.1 Water sampling
Water samples were taken along transects (yellow dots in Fig. 1) in June 2019 (lower density of sampling points), March 2020, and July 2020. They were collected close to shores with a pipette from ca. 40-60 cm below water surface and directly transferred into a GC-vial. The vials were instantly closed and stored in a cooling box, before being placed in a fridge until further procession.
At selected spots, time-series were sampled 2-4 weekly (A: Schlei, A. P. Møller Skolen Schleswig; E: Zingster Strom, Biological Station; I: Vitter Bodden, Kloster/Hiddensee) or monthly (B-D, F-G: Zingster Bodden) from March 2020 to March 2021. Samples from A and I were taken accordingly to shore samples, while samples B-H were taken in deep parts of the boddens, close to buoys marking long-term monitoring locations. For the latter, a Limnos water sampler was used to obtain 2L samples from 0.5-1.0 m depth below water surface, of which 1.5 mL were transferred into GC vials.