24 | 
3 
sn 
Cruise Report, RV ATAIR, Cruise 0671021-1, Bremerhaven - Thyboron —- Aberdeen, 18 Jul - 10 Aug 2023 
Secchi Disc, Chlorophyll-a and Phytoplankton Sampling 
>" 7MN 
59° 
7° 
35° 
‘842 15 
‚4 495 «9 
A E 
14.5 
Secchi 
Depth 
nn 
20 
19 
18 
17 
16 
15 
‘4 
3 
2 
1 
10 
7 
33° - 
ZV Atair 
Druise 0671021-1 
'8 Jul - 10 Aug 2023 
3remerhaven-Thvyboran-Aberdeer: 
51° 4 — 
8W 6° A 
7° 
0° 
7° 
a° 
R° 
FF 
8° 10° 
12°E 
Figure 5.7. Secchi depths observed during cruise 0671021-1, summer 2023 
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 follow instructions described in BSH (2007). The Secchi depth was 
measured using a Secchi disc, a white-painted metal plate with an additional weight, both 
connected to a rope. The rope in turn was connected to a mobile winch installed on the port 
side of the vessel. 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 intervals along the rope mark 
avery 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 5.7 highlights the station locations and corresponding Secchi 
depths observed under daylight conditions during cruise 067/021-1.