For the inhabitants of many of the world's major cities and towns,
estuaries provide their nearest glimpse of a natural habitat; a habitat
which, despite the attempts of man to pollute it or reclaim it, has
remained a fascinating insight into a natural world where energy is
transformed from sunlight into plant material, and then through the
steps of a food chain is converted into a rich food supply for birds and
fish. The biologist has become interested in estuaries as areas in which
to study the responses of animals and plants to severe environmental
gradients. Gradients of salinity for example, and the problems of living
in turbid water or a muddy substrate, prevent most animal species from
the adjacent sea or rivers from entering estuaries. In spite of these
problems, life in estuaries can be very abundant because estuarine mud
is a rich food supply which can support a large number of animals with a
large total weight and a high annual production. Indeed estuaries have
been claimed to be among the most productive natural habitats in the
world. When the first edition of this book appeared, biologists were
beginning to realise that the estuarine ecosystem was an ideal habitat
in which to observe the processes controlling biological productivity.