Approximately 1.4 pentillion tons (1,400,000,000, 000,000,000) of water cover the surface of the earth—466 billion tons for each of the six billion people on the planet. Amazing as it may seem, most of this water has been affected by human activity. Numerous aquatic species are in decline because of degraded water quality, development or alteration of aquatic habitats, and overhunting or overfishing. Figure 5.1 shows the number of aquatic and wetlands species at risk by watershed.

Water Pollution—Many, Many Causes

Humans burn fuels, produce wastes, and use large amounts of fertilizers, pesticides, and other chemicals. These by-products of industrialization end up in the environment and are often harmful to living organisms. The condition of water-dwelling animals is in fact often a good measure of the condition of the environment; their demise suggests that something may be wrong in their habitat. Figure 5.2 illustrates the...