Our coastal waters are heavily over-fertilised. The nutrients are not poisonous but they disturb the balance between different organisms. The most important nutrients in this connection are nitrogen and phosphorus.
At this point sewage treatment etc become meaningless and stands revealed as a gigantically expensive device for postponing leakage by a few years or decades. This could be called the bathtub syndrome: a bath with a plug in (the ideally efficient water treatment plant), but with the tap (phosphate import) turned on full.
The only sustainable solution requires a different structure to society, so that phosphates and other nutrients imported into conurbations are returned to the food-growing process. But this will be extremely difficult and expensive to achieve with the current size of cities. In the long run it seems we will have to restructure society into small towns and villages, with most of our food being produced locally - a development which will have other advantages too. Research into the necessary social structures is being carried out in Sweden.
Folke Guenther, Department of Systems Ecology, University of Stockholm. Home: Godelovsvagen 5, S-240 13 Genarp, Sweden (tel 46 40480059).