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

>" 7N

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.