Scientific context and objectives

Introduction

The sea is not always grey, green or blue. Its colour depends on its composition and the way in which its main components absorb and diffuse light. These components include suspended sediments (sand, mud, clay), vegetable plankton (phytoplankton) through the chlorophyll it contains, dissolved matter from organic degradation, etc.

One of the aims of remote sensing as applied to oceanography is to develop and use knowledge of the optic properties of the components of seawater in order to obtain algorithms (by inverting these properties). These can be used to draw charts expressing the chlorophyll and suspended sediment concentrations in the water on the basis of measurements taken from satellites.

Why this research?

Substantial quantities of chemical compounds such as nitrates and phosphates used in numerous human activities (for instance the fertilisers used in agriculture) reach the sea via rivers and the atmosphere. If these products are found in the ocean in excessive quantities, they can have an adverse effect on the marine environment by affecting the ecosystem and altering its balance. In fact, they are nutrients, which can be absorbed by the lower level of the food chain, phytoplankton. Certain unwanted species then proliferate uncontrollably. One of the visible consequences of this in our waters is the increased presence on beaches of foam from the degradation of supernumerary colonies of plankton.

International conventions such as that protecting the marine environment of the north-eastern Atlantic (OSPAR) have been drawn up and ratified by countries bordering the ocean, including Belgium. These conventions oblige the signatory countries whose waters are affected by eutrophication to reduce the quantities of nutrients in the sea, that is to limit the use of fertilisers and to construct waste water treatment plants. This is also the subject of several European directives. (Nitrates: 91/676/EC; waste water: 91/271/EC). Countries also have an obligation to monitor the development of the problem. This is traditionally done by means of measurement campaigns at sea. These campaigns, which prove rather costly, provide 'the truth about the sea', but cannot be used to obtain a comprehensive view of the situation on a spatial level, as the measurements are only taken at a limited number of points and each measurement is taken at a different time. The images taken by satellite can therefore complement these measurements taken at sea by providing an overall and more frequent view of the region in question. This phenomenon is also studies by means of ecological models.

Remote sensing is also used to assist with the study of the movement of suspended sediment. In fact, the access routes to Belgian ports have to be dredged constantly to deal with a problem of silting. The economic impact of these dredging operations is so great that it justifies research projects aimed at improving them.

 Data and methods

 Examples of derived products