The color of Oceans

© N.Pansiot/Tara Expéditions

Watching the sea, its waves, its color variations – there’s nothing more poetic. Aboard Tara, we observe it, live with its pace, but when it comes to describe and understand it, poetry often gives way to pragmatism and complicated words like calibration, adsorption, backscatter.

Marie Barbieux, scientific coordinator between Malta and Marseilles, is doing a thesis on ocean color and explains the origins of these color variations.

What are the factors that determine ocean color?

At first glance, it is easy to observe changes in the sea’s color from deep blue in clear weather to gray on cloudy days, passing through the pink-orange at sunset and sunrise. This takes into account the reflection of sunlight and the appearance of the sky on the surface of the ocean. But the color of the ocean also depends on the organic and mineral particles, and other dissolved substances in the water. In situ, we can get an idea of the concentration, the nature and size of these particles by studying their backscattering and/or absorption of light. Recently, satellites can also provide global access to this information; from space we are now able to study the infinitely small.

The waters Tara passes through do not all have the same color: near the coast we find many different types of particles and dissolved substances which come from land drainage. Water thus appears brown or brownish. But when sailing offshore in pelagic waters, it’s generally assumed that the sea’s color depends only on the amount of phytoplankton it  contains.

This color can vary depending on the phytoplankton species present in marine areas. There are, for example, dinoflagellate species that cause real “red tides,” coccolithophores that turn the sea milky, and diatoms that make it look green. On the contrary, when the waters are low in phytoplankton (called “oligotrophic”), the color of the sea tends to indigo or violet. This is the case in the Pacific near Easter Island, where the clearest water in the world is observed; light penetrates to a depth of more than 100 meters, compared to 30 meters in the North Atlantic.

How is the expedition helping to increase knowledge about ocean color?

Aboard Tara, we have different instruments with some rather strange names as HTSRB, Alpha or BB3 to record data on ocean color in situ. Our measurements can then be compared with those made by satellites. Until now, scientists did not have much optical data on the schooner’s route, between Cyprus and Malta (east-west transect) or between Algiers and Marseilles (north-south transect). The Tara Mediterranean expedition is a boon for such studies!

You’re doing a thesis on ocean color, can you explain your work?

The title of my thesis is very long, but actually quite clear: “Study of bio-optical anomalies and impact on biogeochemistry in the Mediterranean Sea and Southern Ocean using a synergistic multi-tool approach.” As we have seen, the color of the sea is different in different places. In the Mediterranean, we realized that the satellite measurements showed abnormalities. It seems that the amount of phytoplankton in the water was overestimated  compared to data collected in the field. There are various explanations for these anomalies: the presence of Saharan dust, or the large amount of CDOM (Colored Dissolved Organic Matter) which could influence the color of the water. My work will be to analyze, prove or disprove these hypotheses.

Interview by Noëlie Pansiot

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