A sea is not always blue

The sea is generally blue or green, but in certain circumstances it can take on a white, brown or yellow hue. This variation is because it is not only the interaction of light with the water itself that determines the sea colour, but also its interaction with the components that are present in the sea, via processes of refl ection, absorption and diffusion on the sea surface, within the water column or on the seabed. As a result, the optical properties of algae, of dissolved organic substances and of suspended non-organic particles – such as sand, mud and clay – can have a considerable effect on the colour of the sea. This means that by taking precise measurements of sea water colour (or of spectral refl ectance to be more precise) – for example using a satellite-based measuring instrument – and using the appropriate mathematical models, scientists are able to estimate the concentration of these components.

The colour of deep oceanic waters is determined almost solely by the chlorophyll concentration which is in turn linked to the presence of suspended phytoplankton. Spectral refl ectance is now a well-established method of estimating chlorophyll concentration in these waters. The colour of coastal waters, however, is not determined by these phytoplankton alone as these waters very often contain dissolved organic matter and non-organic particles, such as suspended sediment originating from river outfl ows and the seabed. These particles affect interactions of the light with the water and therefore also the signal received by the satellite. As a result, estimating concentrations of this suspended matter as well as the chlorophyll is rendered much more diffi cult. The growing interest in mapping parameters such as chlorophyll and suspended matter concentrations in coastal waters is due to the number and variety of applications that can benefi t from such data. Examples include the detection of algal bloom, the evaluation and monitoring of eutrophication levels and the validation of sediment transport models. A number of multidisciplinary research teams worked on this project that, although concentrated on the Belgian coast, developed a methodology that can be exported to any other coastal region in the world.