solids, and fecal coliform bacteria as mandated by the Clean Water Act (Kealy et al., 1993). The results were disturbing: just 2.2 percent of the rivers changed their use support status—meaning that nonswimmable waters became swimmable, nonfishable waters became fishable, and so on—as a result of additional point-source controls. These figures may be low because they do not reflect the substantial improvements in water quality that have occurred even where use of the water remains unchanged, nor do they indicate reductions in types of contaminants other than organic matter, fecal coliforms, and suspended solids. Nonetheless, they indicate that controlling point sources of contamination eliminates only a part of the water quality problem. In contrast, the researchers estimated that reducing the nonpoint-source load by half would achieve much more significant reductions—for example, increasing the percentage of swimmable waters from 33 percent to 47 percent. Other researchers have estimated that 99.9 percent of sediment and more than 80 percent of nitrogen and phosphorus enter waterways via nonpoint sources (Shaw and Raucher, 1993).
Limnologists have made substantial contributions toward understanding and partially correcting damage to freshwater ecosystems. Notable contributions by limnologists include the following (see Chapter 3 for more details):
Understanding the effects of excess nutrients and organic matter: By the mid-twentieth century, limnologists were conducting research that eventually quantified how human discharges of excess nutrients (primarily nitrogen and phosphorus) and organic matter (such as that contained in sewage) cause water quality to deteriorate rapidly through growth of excess algae and loss of dissolved oxygen (Hasler, 1947; Sawyer, 1947; Vollenweider, 1968). This discovery eventually led to programs and laws to reduce nutrient and organic matter discharges, and as a result the quality of many important water bodies has improved significantly.
Identifying damage resulting from acid rain: Limnologists have shown that acid rain—caused by fossil fuel combustion and metal smelting—can lead to complete loss of important species, such as trout, in affected water bodies (Schindler et al., 1985). Such discoveries have been important catalysts in national and global agreements to control acid rain.
Contributing to wetland restoration: The world's wetlands have been subjected to extensive drainage and destruction, particularly for agriculture and forestry in developed countries (Kivinen, 1980). Limnologists are playing a key role in developing the science needed to restore and protect wetlands.