Environmental Problems, Their Causes, and Sustainability 1

Nitrogen compoundsproduced by carsand factoriesDischarge of untreatedmunicipal sewage(nitrates and phosphates)Discharge ofdetergents(phosphates)Natural runoff(nitrates andphosphates)Inorganic fertilizer runoff(nitrates and phosphates)Discharge of treatedmunicipal sewage(primary and secondarytreatment: nitratesand phosphates)Dissolving ofnitrogen oxides(from internal combustionengines and furnaces)Lake ecosystemnutrient overloadand breakdown ofchemical cyclingManure runofffrom feedlots(nitrates, phosphates,ammonia)Runoff from streets,lawns, and constructionlots (nitrates andphosphates)Runoff and erosion(from cultivation,mining, construction,and poor land use)Figure 22-7 Natural capital degradation: principal sources of nutrient overload causing cultural eutrophicationin lakes and coastal areas. The amount of nutrients from each source varies according to the types andamounts of human activities occurring in each airshed and watershed. The enlarged populations of algae andplants (stimulated by increased nutrient input) die. Then their decomposition by aerobic bacteria lowers levelsof dissolved oxygen. This can kill fish and other aquatic life and reduce biodiversity and the aesthetic andrecreational value of the lake.degree of cultural eutrophication. One-fourth of thelakes in China also suffer from cultural eutrophication.Cultural eutrophication also occurs in marineecosystems, especially in coastal waters and partiallyenclosed estuaries and bays. It also affects enclosedseas, such as the Mediterranean, Baltic, and Black Seas.There are several ways to prevent or reduce culturaleutrophication. They include using advanced (but expensive)waste treatment systems to remove nitratesand phosphates before wastewater enters lakes, banningor limiting the use of phosphates in householddetergents and other cleaning agents, and using soilconservation and land-use control to reduce nutrientrunoff.There are also several ways to clean up lakes sufferingfrom cultural eutrophication. Examples are mechanicallyremoving excess weeds, controlling undesirableplant growth with herbicides and algicides,and pumping air through lakes and reservoirs to avoidoxygen depletion (an expensive and energy-intensivemethod).As usual, pollution prevention is more effectiveand usually is cheaper in the long run than cleanup.Good news. If excessive inputs of plant nutrients arestopped, a lake can usually return to its previous state(see the two case studies that follow).Case Study: Lake Washington—A SuccessStoryLake Washington near Seattle has recovered from severecultural eutrophication.Lake Washington in the metropolitan area of Seattle isa success story of recovery from severe cultural eutrophicationcaused by decades of sewage and otherinputs.Recovery took place within about 4 years after thesewage was diverted into the nearby Puget Sound.This worked for three reasons. First, a large body ofwater (Puget Sound) with a rapid rate of exchange involvingPacific Ocean waters was available to receiveand dilute the sewage wastes. Second, the lake had notyet filled with weeds and sediment, because of itslarge size and depth. Third, preventive action wastaken before the lake had become a shallow, highly eutrophiclake. Today, the lake’s water quality is good.http://biology.brookscole.com/miller14499