Freshwater is a most precious natural resource. To the developed world, refreshing, untainted water is presumed from the taps of millions of householders. The many rivers, streams, ponds and lakes are for the pleasure and enjoyment of the leisure hours of urban dweller and rural inhabitant alike-boating, fishing, sailing and swimming come readily to mind. To the agriculturalist and industrialist it is often the cornerstone of their enterprises. To the environmentalist and naturalist it is the basis of the wetland and open water communities which provide the habitats for a wealth of flora and...