20 | Cruise Report, RV ATAIR, Cruise 0671013-1, Bremerhaven - Skagen - Aberdeen, 19 Jul - 11 Aug 2022 
5.8 Secchi Disc and Chlorophyll-a Sampling 
61°N 
59° 
37° 
Secchi 
"anth 
20 
19 
18 
17 
16 
:5 
+ 
3 
“2 
1 
10 
5 
55° 
533° — 
Al 
Al 
ZV Atair 
Cruise 0671013-1 
19 Jul - 11 Aug 2022 
Zremerhaven-Skacen-Aberdeer 
51° + — . 
8°W 6° 
A 
F 
MC 
2° 
5° 
a 
L—F-—— r 
B° 10° 12°E 
Figure 5.5. Secchi depths observed during cruise 0671013-1, summer 2022. 
Secchi Depth 
The Secchi depth is a measure for the transparency or clarity of the oceanic water column. 
Values typically range from < 1 m near estuaries to > 12 m in the open North Sea. Secchi 
depth measurements followed instructions described in BSH (2007). During cruise 067/013-1. 
the Secchi depth was measured using a Secchi disc, a white-painted metal plate with an ad- 
ditional weight, both connected to a rope. The rope in turn was connected to a mobile winch 
installed on the port side of the vessel. Mostly at daylight stations, the Secchi disc was lowered 
into the surface layer of the ocean until it was no longer visible. Coloured tapes at 0.5 m inter- 
vals along the rope marked every 0.5 m, 1 m, and 5 m. The mark on the rope closest to the 
sea surface represents the Secchi depth, ie. the depth of transparency at that location. The 
accuracy of the Secchi depth observation is +0.5 m. Figure 5.5 highlights the station locations 
and corresponding Secchi depths observed mostly observed under daylight conditions during 
ecruise 0671013-1. 
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 067/013-1 from the Niskin bottles closed at a depth of 5 m and filled