Why are some seas blue, others black and others translucent?

Answer

The water of the sea on the Belgian coast is sometimes green, brown or grey. This is not to say that our sea is dirty.

The color of the sea can differ depending on the composition of the water. The water may contain components such as sand, mud, clay (this is called ‘sediments in suspension’), small plants (phytoplankton), the dissolved material from decaying material, and so on. All of these components will absorb and diffuse the light in a different way and that gives a different color to the water. Simply put, you can say that if there is a lot of plant material in the water, the water will be greener. If there is a lot of mud and sand in it, the water will be browner. When the water of a sea is really translucent, there won’t be many plants or sand in it.

The color of the sea is closely monitored by the scientists. This is mainly done through research campaigns at sea: scientists board an oceanographic research vessel such as the Belgica, take water samples and examine them. But this is a very time consuming activity. That is why the use of satellite images is so interesting. Every time a satellite flies over our sea, it takes a picture of the sea. Researchers can then deduce from the color of the sea in the photo how much plant material or mud is in the water.

The scientists want to know this for several reasons. They want to find out where the sand, clay, and mud on the bottom go fromgoes. And the amount of plant material gives an idea about the health of the sea. When too many nitrates and phosphates enter the sea via rivers, they can have a bad influence on the sea. They are a kind of fertilizer and some plankton species then grow much too fast. When these plankton colonies die, foam builds up on our beaches and we want to avoid that. We can therefore study the growth of these plankton species with these satellite images.

The pictures of the color of the sea were taken above the Belgian part of the North Sea!