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The color of the Pacific

Bestimmung der Meeresfarbe: Farb-Palette der Forel-Ule Skala
Determining ocean color: Color scale of Forel-Ule.

On this research expedition we are using two classical methods to characterize the surface water of the ocean. These methods were developed in the 19th century, so they do not require advanced technology or power supplies.

Daniela Voss und Daniela Meier
Daniela Voss and Daniela Meier

Today’s contribution: Daniela Voss und Daniela Meier from the working group Marine Sensor Systems at the ICBM in Wilhelmshaven. On board they form part of the „bio-optics” team, that is mainly interested in the optical properties of seawater.

The depth to which visible light penetrates in the ocean is measured using a Secchi-disk. Our Secchi disk is round and 90 cm in diameter, divided into quarters like a pizza, with alternate quarters painted black and white. The Secchi disk is lowered over the stern (the back) of the ship, and we look to see how deep we can lower it and still recognize its distinctive pattern. At our first station in the Pacific this disk was still visible at a depth of 46 meters. In the North Sea, where the water close to the coast is quite cloudy, we usually can see the disk only at a depth of a few meters. And in the Ems River, often we can only see it at a depth of 20 cm.

In the 1890’s, the Forel-Ule scale was developed. It is a scale consisting of 21 colors, from Indigo-blue to cola-brown. Every water sample can be matched with one of the colors on this scale. The color gives an indication of the material that is found in the water. The water from a depth of 46 meters at our first station belonged to Index 3 (Indigo-Blue region). This water is extremely clear, but it is possible to have water that is even clearer than this. We are very curious to see if we find water belonging to the Index 1 category, the clearest water that can be described.

Everyone who wants to determine the color of ocean water (during the ferry trip to Spiekeroog or at the beach in Mallorca) can take a look at this

website: With a smartphone app, anyone can determine the color of the ocean at any location anywhere in the world, and can upload the result to a public databank. Everyone can thereby contribute to the development of a large and comprehensive data set about our ocean, lakes, and rivers. This databank has been developed on the basis of initial investigations that took place starting all the way back in 1890.