Environmental Problems, Their Causes, and Sustainability 1

EcologicalServicesClimatemoderationNutrient cyclingWaste treatmentand dilutionFlood controlGroundwaterrechargeHabitats foraquatic andterrestrialspeciesGeneticresources andbiodiversityScientificinformationNatural CapitalFreshwater SystemsEconomicServicesFoodDrinking waterIrrigation waterHydroelectricityTransportationcorridorsRecreationEmploymentFigure 7-15 Natural capital: major ecological and economicservices provided by freshwater systems.Lakes normally consist of four distinct zones thatare defined by their depth and distance from shore asshown in Figure 7-16. The top layer is the littoral zone(“LIT-tore-el”). It consists of the shallow sunlit watersnear the shore to the depth at which rooted plants stopgrowing, and it has a high biological diversity. It hasadequate nutrients from bottom sediments. Bulrushesand cattails are plentiful near the shore and water liliesand entirely submerged plants flourish at the deepestdepths of the littoral zone.Next is the limnetic zone (“lim-NET-ic”): the open,sunlit water surface layer away from the shore that extendsto the depth penetrated by sunlight. As the mainphotosynthetic body of the lake, its producers supplythe food and oxygen that support most of the lake’sconsumers.Next is the profundal zone (“pro-FUN-dahl”): thedeep, open water where it is too dark for photosynthesis.Without sunlight and plants, oxygen levels are low.Fish adapted to its cooler and darker water are foundin this zone.Finally, at the bottom of the lake we find the benthiczone (“BEN-thic”). Mostly decomposers and detritusfeeders and fish that swim from one zone to theother inhabit it. It is nourished mainly by detritus thatfalls from the littoral and limnetic zones and by sedimentwashing into the lake.During the summer and winter, the water in deeptemperate zone lakes becomes stratified into differenttemperature layers, which do not mix. Twice a year, inthe fall and spring, the waters at all layers of theselakes mix in overturns that equalize the temperature atall depths. These overturns bring oxygen from the surfacewater to the lake bottom and nutrients from thelake bottom to the surface waters.The volume of fresh water that we use provides uswith free services worth about $3 trillion a year—equalto about 7% the value of all goods and services providedannually by the entire global economy.What Life Zones Are Found in FreshwaterLakes? Life in LayersLakes consist of sunlit surface layers near and awayfrom the shore, and at deeper levels a dark layer and abottom zone.Lakes are large natural bodies of standing fresh waterformed when precipitation, runoff, or groundwaterseepage fill depressions in the earth’s surface. Causesof such depressions include glaciation (the Great Lakesof North America), crustal displacement (Lake Nyasa inEast Africa) and volcanic activity (Crater Lake inOregon; see photo on title page). Lakes are suppliedwith water from rainfall, melting snow, and streamsthat drain the surrounding watershed.How Do Plant Nutrients Affect Lakes?Too Much of a Good Thing Is Not GoodAlake’s supply of plant nutrients from its environmentaffect its physical and chemical conditions andthe types and numbers of organisms it can support.Ecologists classify lakes according to their nutrientcontent and primary productivity. A newly formedlake generally has a small supply of plant nutrientsand is called an oligotrophic (poorly nourished) lake(Figure 7-17, left, p. 140). This type of lake is oftendeep, with steep banks. Glacier melt and mountainstreams carrying little sediment supply water to suchlakes. Because there is little sediment or microscopiclife to cloud the water, such a lake usually has crystalclearblue or green water and has small populations ofphytoplankton and fish (such as smallmouth bass andtrout). Because of their low levels of nutrients, theselakes have a low net primary productivity.Over time, sediment, organic material, and inorganicnutrients wash into an oligotrophic lake, and138 CHAPTER 7 Aquatic Biodiversity