Freshwater ecosystems (lakes and rivers) cover about 2.5 X 106 lan2, less than 2\% of the Earth’s surface (Wetzel 1983). Only 0.014\% of Earth’s water occurs in the soils, rivers and lakes of the biosphere (la Riviere 1989): freshwater lakes contain 1.25 X 105 km3 of water (Wetzel 1983), while rivers contain about 1.2 X 103 km3 (Wetzel 1983). Combined, lakes and rivers contain about 0.009\% of Earth’s water (Wetze11983). Fresh water is a crucial resource for terrestrial ecosystems and human life. In addition, extensive riparian areas associated with rivers and lakes include some of the most productive ecosystems of the landscape: The small pool of water available for life is distributed irregularly, and water often limits terrestrial productivity and economic development (Gleick 1993). Climate and geomorpho.logy determine the global distribution and diversity of fresh waters. Diversity of fresh waters across landscapes depends in part on regional geochemistry and land use. At the species level, diversity is best known for fishes (8400 species, 40\% of the world’s total fish diversity; WCMC 1992) and aquatic vascular plants (1 022 species, ScultllOrpe 1967). Other groups are less well known. For example, phytoplankton diversity may be in the order of 105 species, but only 10\% or so of these are known (WCMC 1992).