The inherent colour of natural waters is determined by the absorption and scattering of light and by the concentrations of dissolved and suspended substances in the water. The colour is mainly determined by coloured dissolved organic matter (CDOM), suspended sediment particles (total suspended material, TSM) and the chlorophyll contained in algae. The water often contains several substances at the same time, which are superimposed to produce an overall colour impression of the water.
In reverse, the colour of the water allows conclusions to be drawn about its substances and the water quality. Determining the colour of water has been used for over 130 years and goes back to the scientists François-Alphonse Forel and Wilhelm Ule. The Forel-Ule scale they developed is a historical standard that has since been calibrated with modern spectrometers. The scale contains 21 different colour tones, from the blue of a clear ocean to the green and brown of waters rich in algae and humic substances. The colour comparison is an easy-to-use method and is therefore particularly suitable for the ‘Dagmar Aaen’, on which there is no room for extensive analysis.
Use of a Secchi disc and Forel-Ule scale on board the ‘Dagmar Aaen’.
The results of the colour measurements from a comparison with the colour scale or using a smartphone app can be entered digitally into a global EyeOnWater map, where individual measurements can be found.
On the Ocean Change Expedition 2024, an approach for the automated recording of colour values was developed for the first time for these observations. For this purpose, a camera was attached to the jib boom’s nock, which records images of the water surface at 15-minute intervals.
As a sponsor of the Ocean Change Expedition for several years, SubCtech is supporting the crew throughout the expeditions and has equipped the ‘Dagmar Aaen’ with scientific equipment, such as an OceanPack®, continuously measuring pCO2, sea surface salinity, temperature and oxygen while sailing. For the 2024 Expedition the camera system for autonomous detection of ocean colour was developed and added.
Camera mounted at the very front of the ‘Dagmar Aaen’
The images are sent to the GEOMAR Helmholtz Centre for Ocean Research in Kiel via Starlink for further processing. An automated routine determines the corresponding positions and an average colour value for each photo providing this information for the BELUGA website (https://beluga.geomar.de/ocean-change-2024) and the EyeOnWater project. In addition to the visualisation of the photos on the BELUGA website as coloured markers on the course of the ‘Dagmar Aaen’, all images are automatically imported into the EyeOnWater system once a day. During import, the colour value of each photo is calculated using a special algorithm (WACODI) according to the Forel-Ule scale. In the official map of the EyeOnWater project (https://www.eyeonwater.org/observations/map/color), the photos transferred from the ‘Dagmar Aaen’ can then be viewed with the corresponding Forel-Ule colour values.
Example of colour mean values on the BELUGA page of the Ocean Change Expedition (GEOMAR)
Example of Forel-Ule colour values on EyeOnWater.org
As the environmental conditions (such as the degree of cloud cover) are important as so-called ‘metadata’ for correct recording of the colour values, the process is to be further developed to extract these conditions, for example, from the logbook entries of the ‘Dagmar Aaen’ (every two hours), from the webcam attached to the mast or from satellite data. Even if the process is still to be improved, it can already be stated that the automated recording of colour values from the water surface represents an important step towards efficient and more comprehensive observation.
This data, obtained automatically for the first time in high density and away from busy coastal areas, is an important contribution to environmental monitoring and a technical model for other installations on ships.
The project is a co-operation between the Ocean Change Expedition of Arved Fuchs, the Leibniz Institute for Baltic Sea Research, the company SubCtech (camera technology), the GEOMAR (BELUGA platform and infrastructure), the Dutch company Maris (EyeOnWater website) and the GMT – German Association of Marine Technology. (idea and inspiration) and is an example of the opportunities offered by close co-operation between research, industry and civil society.
