Little streams make big rivers, as the popular saying goes. This also signifies that any pollution at a precise place in the watercourse will have repercussions downstream with more or less amplitude and speed. Researchers at the University of Liege have developed a mathematical and computer model which enables them to predict the evolution of pollution in the Escaut river estuary. A tool which could be used, for example, in the framework of environmental planning measures with a view to reducing the concentrations of nitrates and phosphates in our rivers and estuaries.
One day when I was walking through the Batte open market in Liege, my daughter dropped her teddy bear in the Meuse. Drama! I tried to comfort her by explaining that her electric blue teddy (teddies are often ugly) was going to float happily in the water until the mouth of the river in The Netherlands and that, once in the sea, it would be adopted by a little fish who didn’t have a teddy – bingo! Deep down my child found this outcome rather happy and we could go and fetch the Sunday chicken. My daughter, without realising, had just learnt a hydrodynamic law: that what is thrown in to the watercourse will end up in the sea. Not only the teddies, but the pollution as well : plastic bags abandoned by the travelling Sunday market traders; dirty water filled with organic waste from the towns and villages that do not yet have a sewage works; industrial pollution from factories on the river banks; agricultural fertilizers washed away by the rain etc.
The pollution measured in the estuaries of the big European rivers is worrying. It has a non negligible impact on the flora and fauna. In certain fish, for example, we record very high levels of organic pollutants (notably PCB’s) to such an extent that their reproduction systems could be altered. (see the article – Organic pollutants : the sea is under attack.) Filled with nutriments of human origins (nitrates, phosphates, etc.) estuaries and coastal waters can be the scene of phytoplankton blooms, a sudden explosion in the mass of the photosynthetic micro organisms that live suspended in the water. In certain cases, the growth is so fast that it consumes all the oxygen in the water and asphyxiates the other organisms. In other cases, the species of phytoplankton which develop this way produce toxins which are harmful to the aquatic fauna, but also to mankind via the consumption of fish or shell fish. North Sea fishermen well know the brown hue which invades the sea in the spring. Their nets are messy with a scum that they call ‘tobacco juice’. In fact it is a phytoplankton mass that is sometimes found on the beaches in the form of unsavory white foam, to the great displeasure of the swimmers.