22 Cruise Report, RV ATAIR, Cruise 067I033-1, Bremerhaven – Thybor?n – Aberdeen, 23 Jul – 15 Aug 2024 Chlorophyll-a Sampling and Filtration To determine the chlorophyll-a content in the surface waters of the North Sea, water samples were taken during cruise 067I033-1 from the Niskin bottles closed at a depth of 5 m and filled into 3 L PE bottles. After sampling, the water sample was filtered using a low-pressure filtration system. Filtration was done through a glass fibre filter (? = 47 mm and pore size 0.4µm) on a glass filter plate. For this purpose, the PE bottle was shaken at first to allow for homogensosa- tion of the water sample. Then, a measured volume was filled into a filtration tulip, which was connected to a suction bottle. The volume to be measured was determined by the previously recorded Secchi depth, see Table 5.5. The exact sampling depth, the Secchi depth, the filtered volume and the room temperature were recorded. After filtration, the glass fibre filter with the sample residue was removed from the filtration tulip, folded twice and transferred to a PE cen- trifuge tube with a screw cap. This tube was wrapped in aluminium foil to avoid exposure to light. The sample was labelled by assigning a unique sample identification number similar to Bedford numbers, which was applied directly to the tube and the aluminium foil. The sample was then frozen and stored at -80°C to allow for later analysis in the home laboratory. One duplicate sample was analysed per day. On-board filtration was performed as described in BSH (2022). Figure 5.8 shows the location of stations with water sampling for subsequent chlorophyll-a determination. Corresponding water sampling was carried out on daylight stations with Secchi depth measurement. If there was a delay between collecting the water from the Niskin bottle and the filtration, the water sample was stored in the dark and cooled in the refridgerator. Spectrophotometric analysis was carried out in the home laboratory after the cruise to deter- mine the total chlorophyll-a content in seawater. The method follows Jeffrey and Humphrey (1975), and the phaeopigment content was determined according to Lorenzen (1967) and Ami- not & Rey (2001). The interlaboratory performance study (QUASIMEME, R2024.2) for the pa- rameter chlorophyll-a in seawater during the measuring period in the BSH laboratory was suc- cessfully. The resulting z-scores are 0.2 / 1.3. The mean range for the duplicates (12 samples) taken on cruise 067I033-1 was less than 0.05 µg/L CHLA. Secchi Depth [m] Required Volume [ml] for Filtration 0-2 up to 500, only for coastal stations 2-5 up to 1000 5-10 up to 1500 10-15 up to 3000 Table 5.5. Secchi depth and corresponding sample volume required for the filtration of chlorophyll-a samples.