Because of human activity (such as the agricultural use of fertilisers), nitrates and phosphates leach into rivers and ultimately arrive in the sea. These nutrients can significantly modify the coastal ecosystem by causing a proliferation of undesirable phytoplankton (microscopic algae). Scientists call this phenomenon “eutrophication”.
According to the international agreements such as the OSPAR convention for the prevention of marine pollution, the countries responsible for eutrophication must take measures to curb this problem, for example by limiting the use of fertilisers and constructing waste water treatment plants. There is also an international obligation to monitor the evolution of this problem.

The quality of Belgian coastal waters is monitored regularly using samples taken by the Research Vessel Belgica. However, the spatial and temporal coverage of seaborne measurements is too limited to adequately monitor the rapid four-dimensional variation of water quality parameters, and they are also very expensive.

Under cloud-free conditions, satellite observations can complement these measurements by providing daily snapshots of the entire region for two key parameters that determine eutrophication: chlorophyll and suspended particulate matter.

SATELLITE DATA

Traditionally, this kind of research has been carried out by seaborne measurement campaigns. Such campaigns are expensive and miss much of the real picture, since measurements can be taken at only one location at a time. Satellites can complement such campaigns by providing images of the entire region at a single time, and can fill in the gaps between seaborne campaigns with additional imagery.
Dredging operations are necessary to keep the approach to ports deep enough for shipping, but their cost is high. Research into the study of sediment movements near the coast can help optimise such operations. Such research, carried out using computer simulations and seaborne measurements, is facilitated by satellite imagery of suspended sediments.

Satellite data for this project comes from the NOAA-AVHRR series of satellites (from which suspended sediments and sea surface temperature can be deduced) and from the SeaStar-SeaWiFS sensor (suspended sediments and chlorophyll). The SeaWiFS

sensor measures the intensity of light at a number of distinct wavelengths. These images show measurements made for blue (left) and red (right) wavelengths. The images are shown with a grey scale for intensity: the stronger the signal, the whiter the image. This image shows a false colour SeaWIFS image obtained by combining measurements at blue, green and red wavelengths.

This basis is being extended to a large range of current and future optical remote sensors (MOS, LANDSAT, SPOT, MODIS, MERIS) using a generic methodology.

Some fine tuning
Auxiliary data for atmospheric and whitecap corrections comes from meteorological observations, and calibration/validation data is obtained by measurements made at sea using the Research Vessel Belgica. Measurement of water colour is made at sea using a hand-held spectroradiometer. Such measurements are used to validate the mathematical model describing ocean colour as a function of chlorophyll and suspended sediment content.