Cruise Report, RV ATAIR, Cruise 067I033-1, Bremerhaven – Thybor?n – Aberdeen, 23 Jul – 15 Aug 2024 21 5.7 Secchi Disc, Chlorophyll-a & Phytoplankton Sampling Figure 5.7. Secchi depths observed during cruise 067I033-1, summer 2024. 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 every 0.5 m, 1 m, and 5 m. The mark on the rope closest to the sea surface represents the Secchi depth, i.e. 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 067I033-1.