Environmental Problems, Their Causes, and Sustainability 1

Figure 22-6 Biological magnificationof PCBs (polychlorinated biphenyls)in an aquatic food chain in theGreat Lakes. Most of the 209 differentPCBs are insoluble in water,soluble in fats, and resistant to biologicaland chemical degradation—properties that result in their accumulationin the tissues of organismsand their biological amplification infood chains and webs. Although thelong-term health effects on peopleexposed to low levels of PCBs areunknown, high doses of PCBs in laboratoryanimals produce liver andkidney damage, gastric disorders,birth defects, skin lesions, hormonalchanges, smaller penis size, andtumors. Boys in Taiwan exposed toPCBs while in their mothers’ wombsdeveloped abnormally smallpenises. In the United States, manufactureand use of PCBs have beenbanned since 1976. Before then,millions of metric tons of these longlivedchemicals were released intothe environment. Many of them stillexist in bottom sediments of lakes,streams, and oceans.Phytoplankton0.0025 ppmWater0.000002 ppmHerring gull eggs124 ppmHerring gull124 ppmZooplankton0.123 ppmLake trout4.83 ppmRainbow smelt1.04 ppmWhat Is Cultural Eutrophication andHow Can It Be Reduced? Too Muchof a Good ThingVarious human activities can overload lakes withplant nutrients, which decrease dissolved oxygenand kill some aquatic species.Eutrophication is the name given to the natural nutrientenrichment of lakes, mostly from runoff of plantnutrients such as nitrates and phosphates from surroundingland. Over time, some lakes become moreeutrophic (Figure 7-17, right, p. 139), but others do notbecause of differences in the surrounding drainagebasins.An increase in plant nutrients can be beneficial topopulations of floating phytoplankton that feedaquatic organisms. In turn, this can increase thegrowth rate and abundance of some fish and other desirablespecies.But excessive inputs of nutrients can upsetaquatic ecosystems. Near urban or agricultural areas,human activities can greatly accelerate the input ofplant nutrients to a lake. This process is called culturaleutrophication. It is mostly nitrate- and phosphatecontainingeffluents from various sources that causesuch a change (Figure 22-7).During hot weather or drought, this nutrient overloadproduces dense growths or “blooms” of organismssuch as algae and cyanobacteria and thickgrowths of water hyacinths, duckweed, and otheraquatic plants. These dense colonies of plant life can reducelake productivity and fish growth by decreasingthe input of solar energy needed for photosynthesis.In addition, when the algae die, their decompositionby swelling populations of aerobic bacteria depletesdissolved oxygen in the surface layer of waternear the shore and in the bottom layer. This oxygen depletioncan kill fish and other aerobic aquatic animals. Ifexcess nutrients continue to flow into a lake, anaerobicbacteria take over and produce gaseous decompositionproducts such as smelly, highly toxic hydrogen sulfideand flammable methane.According to the U.S. Environmental ProtectionAgency (EPA), about one-third of the 100,000 mediumto large lakes and about 85% of the large lakes near majorpopulation centers in the United States have some498 CHAPTER 22 Water Pollution