Cruise Report, RV ATAIR, Cruise 0671021-1, Bremerhaven - Thyboron —- Aberdeen, 18 Jul - 10 Aug 2023 | 25 
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 067/021-1 from the Niskin bottles closed at a depth of 5 m and filled 
into 3 | 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 0.4um) 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 
sonnected 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, R2023.2) for the pa- 
rameter chlorophyll-a in seawater during the measuring period in the BSH laboratory was suc- 
zessfully. The resulting z-scores are 0.5 / 0.4 / 0.1. The mean range for the duplicates (19 
samples) taken on cruise 067/021-1 was less than 0.02 ug/L CHLA. 
Required Volume [ml] for Filtration 
I 
2.6 
B_ 
11) 
10-1 
5 
up to 500, only for coastal stations 
up to 1000 
up to 1500 
up to 3000 
Table 5.5. Secchi depth and corresponding sample volume required for the filtration of chlorophyll-a 
samples.