Sampling Plankton from Kattegat, Baltic Proper, and Gulf of Bothnia

Twenty-one water samples were collected in the Kattegat, the Baltic Proper and the Gulf of Bothnia using a FerryBox system installed in the ship TransPaper during 13th–19th of July 2013. The ship followed the route: Gothenburg (Sweden)—Kemi (Finland)—Oulu (Finland)—Lübeck (Germany)—Gothenburg. The FerryBox system consists of a pump with a water inlet at 3 m depth, a circuit of multiple sensors for temperature, conductivity, chlorophyll and phycocyanin fluorescence, turbidity, and oxygen as well as automated water sampling devices. A detailed description of the FerryBox system is found in Karlson et al. (in press ). Manual water sampling for DNA analysis was carried out both on the Northward and Southward legs. Approximately, 10 L of seawater were collected in a polycarbonate carboy. Subsamples of 200–500 mL were filtered onto 0.22 μm pore-size mixed cellulose ester membrane filters (Merck Millipore co., Cat. No. GSWP04700) to capture plankton. The filters were frozen in liquid nitrogen on board and kept at −20 to −80°C until DNA extraction.