Sign 7 - EUTROPHICATION
 

      Eutrophication is a major problem in lakes and marshes today. The undesirable overgrowth of over-fertilized algae and waterweeds often result from increased storm runoff from farms and cities. The ditches and storm sewers carry lawn and farm fertilizer, dog excrement, farm manure, fertile eroded topsoil and sometimes septic tank overflow, including phosphate detergents. Sometimes, too, there are canning and dairy product wastes and municipal sewage which, even when treated, still contains abundant fertilizers. Phosphorus, the essential mineral that is the hardest for plants to get in sufficient quantity, is present in all these sources.

       The overgrowth of plants in itself is not bad; in fact it could increase wetland productivity, but two results offset this benefit. (1) Plant diversity declines. For example, one nuisance waterweed, European milfoil, has almost completely replaced some 50 kinds of waterweeds in University Bay since 1910, with a corresponding reduction in animal variety. On the microscopic level, a few nuisance blue-green algae often replace a wide variety of greens, in Lake Mendota and other over-fertilized waters, and serve as indicators of this condition. (2) Overgrowth overtaxes the oxygen supply. Water plants begin to clog boat lanes; algae-filled pea soup water or green or brown scums become unappealing to swimmers; and summer fish kills eventually result from anoxia in warm water (which stores less oxygen) wherein bacteria become especially active. Bacterial decay of this accumulating waste of dead plants and animals steals more and more oxygen, so that bad odors, oil slicks, and even poisons are produced as bacteria shift over to anaerobic respiration (see diagram below).

       What to do? Chemical weed control does not get at the cause (does not remove minerals from the system), and may harm desirable life. The first step is to prevent further fertilization from outside. The surrounding playing fields should not receive lawn fertilizer, nor even the herbicides that can, in very low concentrations, actually stimulate plant growth in the marsh. Second, once the sources of phosphates and other fertilizers are cut off, the waters and muds must be made less fertile since the minerals are stored and recycled there as plants grow and decay, Repeated harvest of water plants would help; an organic fertilizer might be made from dried waterweeds and algae. Direct chemical extraction from water might be quicker but at present is expensive. The third method, natural filtration and extraction of mineral salts by soil, sod and upland vegetation, could be attempted here experimentally. This marsh's fertile water could be pumped out to irrigate and fertilize the grass of the playing fields in the summer. The somewhat less fertile lake water could be brought in to replace what does not run back overland or seep back underground.

       Anoxia can occur in winter, too, when heavy snow cuts off light from the plants beneath the ice for too long. Fish will also winterkill in shallow water that freezes to the bottom. The mud settling here has made the water shallow and killed even the hardier frogs, turtles and dragonflies. Hence fish stocking will not be attempted at present. If fish appear here, we can study the human and natural causes of fish dispersal!