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State of the WorldsFreshwater Ecosystems:Physical, Chemical, andBiological ChangesStephen R. Carpenter, Emily H. Stanley,and M. Jake Vander ZandenCenter for Limnology, University of Wisconsin, Madison, Wisconsin 53706;email: [email protected], [email protected], [email protected]

Annu. Rev. Environ. Resour. 2011. 36:7599

First published online as a Review in Advance on July 29, 2011

The Annual Review of Environment and Resources is online at environ.annualreviews.org

This articles doi:10.1146/annurev-environ-021810-094524

Copyright c 2011 by Annual Reviews. All rights reserved

1543-5938/11/1121-0075$20.00

Keywordsaquatic invasive species, climate change, ecosystem services,freshwater biogeochemistry, land-use change, natural capital

Abstract Surface freshwaterslakes, reservoirs, and riversare among thextensively altered ecosystems on Earth. Transformations inchanges in the morphology of rivers and lakes, hydrology, bchemistry of nutrients and toxic substances, ecosystem metabolisthe storage of carbon (C), loss of native species, expansion of ispecies, and disease emergence. Drivers are climate change, hydrow modication, land-use change, chemical inputs, aquatic inspecies, and harvest. Drivers and responses interact, and their relships must be disentangled to understand the causes and consequof change as well as the correctives for adverse change in any gitershed. Beyond its importance in terms of drinking water, freshsupports human well-being in many ways related to food and beduction, hydration of other ecosystems used by humans, dilutiodegradation of pollutants, and cultural values. A natural capital work can be used to assess freshwater ecosystem services, comuses for freshwaters, and the processes that underpin the longmaintenance of freshwaters. Upper limits for human consumptfreshwaters have been proposed, and consumptive use may appthese limits by the mid-century.

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Forest Dryland Cultivated Polar Urban

Renewable or accessible water

Ecosystem

P r o p o r t

i o n o

f p r e c

i p i t a

t i o n

0 . 0

0 . 1

0 . 2

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RenewableAccessible

Figure 1Renewable and accessible freshwater as a proportion of precipitation for selected ecosystem types (Reference 16, chapter 7, ta

Water withdrawals by humans are used foragriculture (76% or 3.81 103 km3 year 1),in various industries (15% or 0.77 103 km3

year 1), and within domestic households (9%or 0.38 103 km3 year 1) (9). However, most of the water withdrawn for agriculture is con-sumed (transferred to the atmosphere as water vapor), whereas most of the water withdrawnfor industrial and domestic use is returned tosurface ows.

Surface freshwaters supply approximately three-quarters of the water withdrawn for hu-man use. Ecosystems differ in their capacitiesto generate renewable water [i.e., water that is not lost immediately to evapotranspiration(ET)] and accessible water that is available forhuman use (Figure 1 ). Forests receive roughly half of the global precipitation over land andcontribute approximately half of the global re-newable freshwater. Drylands are larger in areathan are forests and receive appoximately thesame annual volume of precipitation, but they generate far less runoff.

WHY ARE FRESHWATERSCHANGING?Freshwaters are always in ux, but current changes are novel and exceptionally large.

Major drivers of change are climate, humanalteration of water ows, land-use and coveralterations, chemical inputs, aquatic inva-sive species, and human harvest includingaquaculture (Figure 2 ).

Climate ChangeClimate change directly affects freshwaters as aresult of increased temperatures, greater vari-ability of precipitation among locations andover time, andrising sealevels.Severalobservedclimate trends are signicant for freshwaters:increasedprecipitationover land north of 30 Nsince 1901, decreased precipitation over landbetween 10 S and 30 N after the 1970s, andincreasing intensity of precipitation in someregions and droughts in others (12). In turn,

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Figure 2 Many interacting drivers, and interacting responses, affect the condition of freshwaters on any particular landscape. Reprinted fromCarpenter & Biggs (5) and used by kind permission from Springer Science+ Business Media B.V.

changing precipitation and temperature are as-sociated with decreased extent of glaciers andduration of ice cover and snowpack; increasedthawing of permafrost; and regional changes inET, lake levels, and streamow (12). Modeledfuture trends relevant to freshwaters are warm-ing temperatures, more variable precipitation,and rising sea level (12).

Warmer water temperatures decrease theamount of oxygen that can be dissolved in sur-face waters and thereby decrease the availabil-ity of oxygen for respiration by organisms andprocessing of organic matter and pollutants.

Warming also decreases thermal niche ahabitat availability for aquatic organisms trequire cold water, such as galaxid or salmoshes (13, 14). At higher latitudes and eletions, warming decreases the duration of ice season and melts glaciers and permafrthereby changing the seasonality, magnituand sources of hydrologic ows. Greater vability of precipitation, over time and amolocations, adds to the variability of hydroloinputs to lakes and streams.

Rising sea levels increase the extent of water penetration into tidal rivers. The effe

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of rising sea levels on lowland rivers are exac-erbated by decreased buildup of sediment dueto siltation in impoundments upstream and de-creased runoff due to irrigation (15).

Hydrologic Change

Freshwater is distributed unevenly in spaceand time. To counter this heterogeneity, wa-ter resource development and management ac-tivities attempt to make water accessible forall human needs at all times. The tendency to store or divert water when it is present in excess or procure it when it is lacking isglobal in scope and is accomplished by irri-gation, drainage, groundwater pumping, im-poundment, levee construction, and interbasintransfer. Because of these manipulations andclimate change, the worlds hydrologic cycle ischanging with notable, widespread alterationsin the hydrologic regimes of the worlds freshwater ecosystems (16).

Water scarcity is the single largest globalchallenge facing water resource management.Over large continental areas, human demandfor freshwater approaches or exceeds current supply, and approximately 2.4 billion peoplelive in water-stressed environments (9). Provi-sioning of water to water-stressed regions forhuman needs is routinely done at the expenseof ecosystems, both aquatic and terrestrial (1618). Conventional responses to water scarcity are withdrawals of deep groundwater and sur-face water storage. In many regions in North Africa, the Middle East, South andCentral Asia, western North America, and Australia, groundwater withdrawals exceed recharge, so groundwater pools are declining (7) and groundwater-dependent ecosystems are threatened (19).

Surface water storage in ponds and reservoirs is the most common strategy for ensuringa reliable water supply, especially becausedam building can also provide other economicbenets such as electricity generationand oodcontrol (20). Although reservoirs may providea reliable source of water to meet humandemand, they have transformative effects onrivers, including fragmentation, ow regime

modication, enhanced evaporative loss, andincreased water residence times, in addition to wide-ranging consequences impacting aquaticcommunities. Dam construction has shiftedfrom developed to developing regions, espe-cially China, India, and South America in thepast 20 years (21). Thus hydrologic changes to

rivers are continuing to increase both in spatialextent and degree of alteration. Our current understanding of the effects of dams on freshwater environments is strongly biased towardthe impacts of large structures, yet small dams(less than 15 m) outnumber large dams by per-haps tenfold. The number of small ponds andimpoundments has increased over the past twodecades, particularly in arid regions. Over thepast 50 years, increases have been 60% in India(10)and900%insomeareasofAfrica(22).Thistrendwill likely continue,andthepoorlyappre-ciated but signicant roles of these diminutivebut numerous systems will also expand (10).

Transfer of water among different drainagebasins is often expensive, yet it is common inplaces where water can be moved from snowy mountainous areas to low-lying arid regions(23). Large metropolitan areas in the westernUnited States (e.g., Los Angeles, California,and Phoenix, Arizona) rely on such transfers,delivering water from the Sierra Nevada andRocky Mountains via an extensive series of canals and aqueducts. More elaborate planshave been proposed, ranging from towingicebergs to large-scale desalinization oper-ations (24). Such unconventional strategies will likely continue and expand over the next several decades, and they will affect freshwaterenvironments in ways that are only beginningto be recognized and quantied.

Land-Use ChangeLand-use change, the conversion of naturallands to human use or the alteration of manage-ment practices on human-dominated lands, isa major driver of ecosystem change (25). Land-use changeaffects freshwater ows by changingthefates ofprecipitation amongET,runoff, andgroundwater recharge. Conversion of natural


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ecosystems to cropland or urban uses generally increases runoff as well as ood frequency andintensity, and it decreases ET and groundwaterrecharge (16). Invasion by terrestrial plant species (such as Tamarix in the western UnitedStates) with high ET rates has the oppositeeffect of decreasing runoff to surface waters

(26). Agricultural land use for pasture or crop-land occupies 37% of terrestrial land area (27). Among human activities, agriculture uses thegreatest amount of freshwater (28), emits thehighest levels of greenhouse gases into the at-mosphere (2931), and contributes the most tosoil erosion and runoff of nutrients to freshwa-ters (32, 33). Agriculture accounts for 52% and84% of global human emissions of the green-house gases CH4 and N2O, respectively (31).Intensive agriculture employs high fossil fuelinputs and reduces soil storage of carbon (C),although the C balance of agriculture variesconsiderably among tillage practices (30, 31,34). Croplands, high-density livestock opera-tions, and urban lands add silt, nutrients, andtoxic pollutants to surface waters (32).

Substantial expansion of agricultural pro-duction and water use will be needed in com-ing decades to meet the demand for food andperhaps biofuels (28, 35). Alternative agricul-tural practices have different implications forintensication or extensication of land use,magnitude of water use, nutrient emission,and greenhouse gas emission (31, 34, 36, 37).Future trends in agricultural production willaffect freshwater sources directly through wa-ter withdrawals, nutrient emissions, and effectson riparian ecosystems as well as indirectly through climate change.

Although much smaller in extent thanagriculture, Earths urban areas are growingrapidly. Food and ber production that sup-ports urban populations requires substantialows of water (38). Although groundwater ac-counts for only one-quarter of the worlds wa-ter withdrawals, it makes up approximately half theworlds potable water andprovides drinking water for many of the worlds cities (7).

Changing Chemical Inputs The history of pollution of freshwater perhaps as long as that of water resoudevelopment. Both are consequences of humpopulation expansion. Hydrology and biogechemistry interact in many ways (39), andwamanagement and water quality impacts sh

strong spatial correlation (40). The increain chemical inputs to freshwaters is a reof diffuse inputs from landscapes dominaby human use (agricultural and urban areand atmospheric sources as well as from didischarges of waste waters from sources sas mining, industry, or municipal sewage. Tdiverse array of anthropogenic chemicals addto freshwaters includes organic compounheavy metals, acids, and alkalis, some of ware toxic to aquatic organisms or humans.

The role of agriculture in providing diff(nonpoint-source) inputs of nutrients (nitrgen, N, and phosphorus, P) and other chemicals to aquatic systems is well establi(32). Urban areas are also becoming signcant sources of chemicals to freshwater enronments. Limited investment in water supand treatment infrastructure and rapid urbpopulation growthparticularly in developicountriesinteract to create water-quantand -quality problems (41). When prese

wastewater treatment mitigates nutrient- aorganic-matter loading to freshwaters. Treament of point-source efuents is far more comon in economically developed nations, as than 20% of the population was connectedsewer systems in Africa and southern Asi2000(16). Even inEurope, estimates of the pcent of the population connected to wastewter treatment facilities range from 35% to 80among different countries (42). Furthermomuch of the water infrastructure in develop

countries such as the United States is now aand prone to leakage and failure (43). Urbareas also contribute surface runoff enrichin metals or organic compounds as well asmospheric inputs of automobile and industremissions that caneventually move into aquasystems. Southeast Asia, a region of r


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economic development with limited environ-mental regulations, is now a major source of these pollutants.

Aquatic Invasive SpeciesHumans intentionally and accidentally trans-

port vast numbers of live organisms outside of their native range. Only a fraction of intro-duced species establish self-sustaining popula-tions, and a fraction of those become a nui-sance, spread, and cause harm (44). Never-theless, species invasion is now recognized asan important driver of global environmentalchange (45, 46), and freshwater ecosystems areespecially vulnerable to species invasion and itseffects (47).

The scope of biological invasions in freshwater ecosystems is enormous. Biomass andspecies diversity of many freshwater ecosys-tems (e.g., the Laurentian Great Lakes) andeven entire faunas (Mediterranean shes) arecurrently dominated by nonnative species (48,49). There are > 180 nonnative species inthe Laurentian Great Lakes, and the rate of new invasions is accelerating (48). The Non-indigenous Aquatic Species Database (Bio-logical Resources Division, U.S. GeologicalSurvey, http://nas.er.usgs.gov/ ) reports 1,147freshwater-introduced species (and 77 addi-tional species inhabiting brackish water) in theUnited States. Approximately 40% of these arenative to the United States but now occur beyond their native range. Although many aquaticinvasive species were accidentally introduced,others have been introduced deliberatelystocked for aquaculture, pest control, aestheticreasons, and recreational sheries.

Aquatic invasive species have altered thephysical, chemical, andecological conditionsof freshwater ecosystems, and they are one of thetwo most important factors threatening aquaticecosystems and biodiversity (50). Ross (51) re-ported that in 77% of cases the introduction of nonnativeshcorrespondedwith the reductionor elimination of native shes. Introductionsof predatory sport shes are the major threat to endemic shes in the western United States

(52). Introducing the Nile perch to Lake Vic-toria caused the extinction of perhaps hundredsof native haplochromine cichlids (53). Invasivespecies were a central factor in the food-webdisruption of the Laurentian Great Lakes (see The Laurentian Great Lakes, sidebar below).Introduction of opossum shrimp ( Mysis relicta)

to Flathead Lake, Montana, disrupted the food web and affected the plankton, sh, bear, andeagle communities (54). Invasion by zebramussels is accelerating extinctionrates of nativeUnionid mussels (55). The spread of nonnativefreshwater species is a key contributor to theongoing biotic homogenization of freshwaterecosystems occurring at the global level (56).

The implications of aquatic invasive speciesfor ecosystem services are not well known.

THE LAURENTIAN GREAT LAKES

The history of the Laurentian Great Lakes is that of a progressiof successive anthropogenic impacts spanning nearly twoturies. The nineteenth century saw the wholesale loss of forlake-shoreline alteration, wetland destruction, and dam buildon tributaries. It was also a period of intense shery exploitaBy the mid-1800s, sheries had already overexploited thepopulations, and declines and collapses of sh stocks continto themid-twentiethcentury.Invasivesealamprey invasionther decimated sheries from the 1940s onward. Concern cultural eutrophication as well as the bioaccumulation of substances peaked in the 1960s and 1970s. Zebra mussel invin the 1980s focused attention on the multitude of nonnaspecies (> 180 species) in the Laurentian Great Lakes, bothtentional and accidental in origin. More recent concerns inclchanginglake levels andthe system-wideeffectsof climate ch(158).

Over this period, some impacts have abated. Nutrient loaand levels of some contaminants have declined, and sea lampopulations have been successfully controlled. Neverthelessecosystems have been fundamentally transformed. Much onative biota has been lost, including a suite of endemic d water coregonids. These have been replaced by a suite of native shes such as rainbow smelt, alewife, and stocked Psalmonids. Food webs composed of nonnative species nowport valuable recreational sheries and pose some managemchallengesowingtocompetinghumangoalsfortheseecosyste


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Awareness of the need for valuation studies isgrowing (57). Examples of economic impactsinclude water hyacinth effects on navigation inLake Victoria and other tropical African lakes(58), zebra mussel fouling of water systems(59), and Tamarix reduction of riparian wateravailability in the southwestern United States

(26). Annual economic damages from each of the above examples are in the US$107 to 108

range.Perhaps the most troubling feature of

aquatic invasive species is their irreversibility once established. In contrast withchemical pol-lutants, which can diminish over time, invasivespecies often exhibit accelerated rates of spreadfollowing initial establishment. Preventing theintroduction of new species and containing ex-isting species are central to managing their im-pacts, and some progress has been made in reg-ulating key vectors such as ballast water andrecreational boaters (60). Quantifying the eco-nomic impacts andconsequences forecosystemservices is a critical step to improve manage-ment (57).

Harvest Fish production is a valuable provisioningecosystem service generated by freshwaterecosystems. Aquatic resources, particularly shand invertebrates, but also birds, reptiles, andamphibians, are harvested for local subsistence,commercial use, recreational use, or pet trade.Recent concern about the global sheriescrisishas focusedalmost exclusively on marineecosystems, although the impacts of recre-ational shing and overshing of inland watershave garnered increased attention (61, 62).

Declining sh stocks and increasing de-mand for sh have led to sharp increases inaquaculture production in recent decades (63).Freshwater ecosystems constitute nearly half of the global aquaculture production, estimatedat 68 million metric tons (64). This inlandaquaculture boom has led to increases in theharvesting of wild sh for feed, waterpollution,altered hydrologic ows, and the accidentalrelease of nonnative species (63).

The annual commercial and subsistence harvest from inland waters, excluding aquacture and recreational shing, has increased proximately fourfold since 1950 andis currenestimated at 10 million metric tons (64), w Asia (65%) and Africa (24%) dominating (In contrast, annual harvests from marine

eries are approximately 80 million metric t(64). Fisheries that target high-value specprovide many examples of harvesting have caused sharp reductions in catch, abdance, and body size [e.g., Nile perch in L Victoria, lake trout in the Laurentian GreLakes (62)]. For multispecies sheries, mcommon in tropical waters, the limited dsuggest that intensive harvesting has often to depletion of large body-sized shes, causa shift toward the harvesting of smaller speand younger sh (62, 65).

In contrast with commercial sheriinland recreational sheries are poorly trackalthough there is growing interest in their ptential effects on aquatic resources (66). Recational shing is a multibillion-dollar indus with diverse subsectors and rapid growth drivby afuent anglers in rich countries. Post et(61) documented the widespread declinerecreational sheries adjacent to popution centers in Canada, demonstrating threcreational sheries often show depensatdynamics and that recreational shing ccollapse freshwater sheries. Fishery decliare oftenmasked and exacerbated by sh stoing (67). These recent ndings challenge tconventional belief that recreational shertend to be self-regulating and less vulnerato collapse than commercial sheries.

An assessment of inland sheries by Food and Agriculture Organization (6concluded that many inland sheries wovershed and degraded by pollution ahabitat loss. Fisheries often target large ahigh-value apex predators that have life-histtraits that make them especially vulnerableoverexploitation. Following overexploitatitarget species are often replaced by smalfaster-growing, or less desirable spec which subsequently become the new tar


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of sheries. In addition to impacting target species directly, shery overharvest can pro-duce ecosystem-level effects. Fishes can play an important role in regulating ecosystemprocesses via nutrient excretion, bioturbation,and mass migration (69). The decline of apexpredators produces trophic cascades that can

extend to the base of the food web (70). The harvest of wild freshwater shes andaquaculture make up an essential and rapidly growing source of protein for the worldspopulations (63). This situation necessitatesbalancing current demand with the protectionof freshwater ecosystems to preserve futureharvest potential and other ecosystem services.Degraded sheries areoften difcult to rebuild,and the available management tools (stockingof hatchery sh) can exacerbate the problem.Recreational and subsistence harvests are widely distributed on the landscape, not wellquantied, and difcult to regulate. Perhapsmost importantly, overharvest often actssynergistically with other human impacts.

HOW ARE FRESHWATERSCHANGING? Multiple aspects of freshwaters are affected by the drivers described above, and frequently,there are complex interactions among the re-sulting responses. We focus here on changesin physical features, biogeochemistry, ecosys-tem metabolism, biotic transformations, anddisease.

Physical TransformationsDam construction is perhaps the most conspic-uous anthropogenic modier of riverine sys-tems andtheir hydrology. Natural owregimesand hydrologic connectivity of approximately 60% of the worlds large river basins are now affected by dams (20). Dams often stabilize dis-charge patterns by reducing the magnitude andduration of high ows and by supplementinglow ows (71). The net global effect of im-poundments has been to reduce the average an-nualdischarge to the ocean asa resultofstorage,

evaporation, and withdrawal, leading in somecases to complete desiccation of the aquaticecosystem(72). Other water management prac-tices such as levee building and ditching gener-ally increase baseow and ood magnitudes by rapidly routing water to channels andconningow to these conduits (73, 74), whereas inter-

basin transfers increase ows in one region at the expense of discharge in another. The hydrology of many freshwater sys-

tems is profoundly affected by agriculture.Over the past three centuries, the expansionof pasture and nonirrigated croplands has in-creased streamow approximately tenfold by reducing ET. Conversely, the expansion of irrigation-based farming is associated with de-clining streamow (75). At the global scale,thecombined effects of agricultural water with-drawals and reservoirs have been estimated at amodest 3.5% reduction in the long-term aver-age riverine discharge to the ocean. However,among regions, this decline varies substantially,from no measurable change (e.g., in parts of Southeast Asia) to an average annual dischargedecrease of 35% or more (representing 16% of the global land area excluding Greenland and Antarctica) (76).

Finally, thehydrologic consequencesof landuse and reservoir construction manifest withinthe context of changing climate conditions. Analyses of long-term discharge records havelinked regional climate dynamics to a variety of river-ow trends, including both increasedand decreased annual discharge and shifts inseasonal ow magnitudes (12). Climate-drivenhydrologic changes have also been accompa-nied by temperature warming and decreasedice duration (77). Warming is most rapid inthe mid- and high latitudes of the NorthernHemisphere, and surface temperatures in sev-eral lakes are increasing faster than regional airtemperatures (78). Surprisingly, some of thebest examples of lake warming are providedby the worlds largest lakesSuperior (79), Tanganyika (80), and Baikal (81)a puzzlingtrend because these very large masses of wa-ter would be expected to warm slowly. Warm-ing trends and previous ice-out dates (and in


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some cases, ice-free winters) increase the sta-bility of thermal stratication, thus limiting theupwelling of nutrient-rich waters and primary productivity (80) and modifying planktonicpopulation and community dynamics (81, 82).

FRESHWATER CONSERVATION PRIORITIES

Certain species groups and ecosystem types are particularly sen-sitive or vulnerable to anthropogenic impacts and, thus, are par-ticularly threatened. Below we highlight several examples:

Diadromous shes: Diadromous shes migrate betweenfreshwater and marine systems to complete their life cycle. A recent assessment described dramatic and widespread declines inthe diadromous sh stocks of the North Atlantic (159). Thirty-two of the 35 stocks showed overall declines, and 68% (24/35)of the stocks showed abundance declines of 90% or more. Allspecies underwent population extirpations. A suite of threatsdams, alteration of owregimes, overshing, invasives,pollution,and climate changeare responsible for the declines (159).

Megashes: A large number of the worlds largest freshwater shes, the megashes, are threatened with extinction(http://megashes.org/ ). Thesespecies often inhabit large riverecosystems, are highly migratory, and inhabit large geographicrangescharacteristics that make them particularly vulnerableto overharvest, habitat destruction, pollution, dams, and alteredow regimes. These impacts all threaten the extinction of thesespecies. Accordingly, the plight of the megashes also serves asan indicator of the threats faced by manyof the worlds large riverecosystems.

Floodplains: Floodplains are among the most dynamic, pro-ductive, and diverse ecosystems on Earth. In developed regions,most rivers have been channeled and leveed, oodplains havebeen converted for agricultural and urban use, and natural ow regimes have been altered through river regulation. This combi-nation has led to widespread elimination of oodplain functionand the associated oodplain aquatic ecosystems, both of whichare dened by lateral connectivity with the river. Though ood-plains cover 1.5% of land surface, they provide> 25% ofall ter-restrial ecosystem services including nutrient removal, ood reg-ulation, water supply, and shery production. The tremendous value of these ecosystem services highlights the need to protect and restore oodplain ecosystems. Yet, oodplains ecosystemsare being lost at a rapid rate, particularly in Asia and parts of thedeveloping world (86).

Changes to the physical structure and gemorphic processes of freshwater environmeare closely linked to hydrologic modicatioand they similarly vary within and amregions. For example, riverine transport sediment has both increased (owing to humland use, particularly agriculture) and

creased (owing to retention by reservoirs) (8Changes in ow and sediment regimes caa variety of channel form adjustments (eincision, bed armoring, siltation, bank failure which, in turn, often trigger managemeresponses intended to stabilize channels (ehardening of stream banks by addition of laboulders). Anthropogenic modications coccur in lakes as well: Littoral habitats modied by shoreline structures; armorof banks; and removal of riparian vegetaticoarse woody habitat, and aquatic plants. Salterations generally decrease sh abundanbiomass, and diversity (84).

Other manipulations of river channels clude dredging and installation of hydraulic d vices to maintain navigation channels and baccess, channel redesigns (e.g., ditching, chnel relocation), and wood removal associa with deforestation and/or human developme(85). The most extreme form of modicatinvolves elimination of specic habitats (edisconnectionof oodplains via levee constrtion) or of the entire ecosystem, and such terations are common. Floodplains, which heavily used by humans because of abundecosystem services (see the Freshwater Cons vation Priorities sidebar), are frequently mied. Floodplain loss can be extensive; forample, as much as 90% of oodplain habihave been eliminated from Europeanrivers (86).

Expansion of urban areas is associated wengineered adjustments in channel form, flowed by conversion to canals or ditches (and ultimately burial (88). Headwater streain the United States have been buried undoverburden materials from mountaintop miing(89). Such physical changes have substanconsequences for structure and, more broadbiodiversity of aquatic communities.


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Biogeochemistry and Nutrients The capacity of freshwater ecosystems to trans-form or retain added solutes is a function of in-puts, hydrology, and biogeochemical process-ing. As noted above, inputs and hydrology arechanging. Biogeochemical processing rates arealso subject to change as a result of altered tem-

perature regimes, chemical composition, andbiota. A shift in any one driver is unlikely toaffect just one chemical constituent alone, andsimilarly, a change in one individual chemicaltypically reects multiple and often interactingcauses that vary geographically. Modicationof any one chemical can trigger a cascade of responses in other chemicals. Here we provideselect examples of ongoing, large-scale changesin the biogeochemistry of freshwater systems.

Freshwaters exhibit geographically ex-

tensive changes in concentrations of key nutrients (N and P), inorganic and organicC, sulfate (SO4), total dissolved salts, andmicropollutants. Nutrient-enriched runoff from agriculture and urban land is a majorcause of eutrophication, i.e., the overenrich-ment of aquatic ecosystems with nutrients ororganic matter, leading to harmful bloomsof algae, deoxygenation, loss of economically valuable species, and human health problems(32, 90). Seitzinger et al. (91) estimated that

global riverine loads of N and P increasedby approximately 30% between 1970 and2000. Although riverine loads increased on allcontinents, the greatest increases and largest total loads occured in southern Asia. Whereasriverine uxes of inorganic P declined in North America and Europe over this 30-year period,they increased in Africa, South America,Oceania, and South Asia (91). Organic C poolsand uxes are also changing, as soil disruption,altered plant cover, use of organic fertilizers,

and efuent discharges are inuencing organicC quantity and quality in aquatic systems (92).Chemicals transferred to aquatic environ-

ments via the atmosphere affect large regionsand include SO4, nitrate (NO 3), and contam-inants such as mercury. SO4 emissions fromcoal burning and industrial activities date from

Large lakes: The 189 largest lakes (> 500 km2 each) cont 68% of the planets liquid surface freshwater (158). Llakes are of tremendous importance to humans. They are mnets for human settlements and provide a wide range of vices: transportation, shing, power generation, waste dispirrigation, and domestic and industrial water supplies. Co

quently, the worlds large lakes have been disproportionatelteredbyhumanactivity.Many areancient andsupport largenubers of endemic species. The Great African Rift lakes (Vic Tanganyika, Malawi) are heavily impacted by rapid human ulationgrowth, deforestation, soil erosion, andovershing. Insion of water hyacinth and Nile perch in Lake Victoria havemajor effects including the widespread decline of native In contrast, the worlds oldest lake (25 million years old), Baikal is remote, and although it has been affected by induand domestic pollution and overshing, its water quality remhigh, andnone of the1,5002,000 endemic specieshave been

(158).

the start of the Industrial Revolution, andaquatic consequences became apparent duringthe 1960s and 1970s (93). Policies that cur-tailed emissions in Europe and North Americaprompted a decline in SO4 concentrations andthe recovery of pHin many lakes and rivers (94,95). However, both biotic and chemical recov-ery has been delayed or incomplete in some af-fected regions (96, 97). Three additional chap-ters to the acid rain story are now emerging.First, long-term inputs of acid and SO4-richprecipitation has triggered a cascade of otherchemical trends in many acid-sensitive freshwaters that are now becoming apparent. Theseinclude declines in base cations (98) and in-creasesindissolvedorganic carbon(DOC) (99).Second, rather than diminishing as a globalproblem,acid rain andacidicationhave movedfrom North America and Europe to other con-tinents. Acid rain is now a notable environmen-tal issue inChina (100) and will likely emerge inother developing industrial areas. Third, eventhough SO4 emissions have decreased in someparts of the world, emissions of oxidized nitro-gen (NO x), mercury, and other pollutants fromfossil fuel combustion have not. Inputs of theseconstituents are increasing globally, especially


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in southeast Asia, and are producing a range of ecological and, in the case of mercury, humanhealth effects (101).

Mercury is just one example of a growingnumber of micropollutants that are becom-ing more concentrated and more widely dis-tributed in freshwater environments. Products

from industrial, farming, and mining activitiesentering surface waters range from heavy met-als to organic compounds such as polychlori-nated biphenylsandpolycyclic aromatic hydro-carbons, and they can contribute to biologicalconsequences such as direct toxicity, mutage-nesis, developmental disruption, and reducedprimary production in aquatic environments(102, 103). A few toxins, such as hydrocar-bons andmethylmercury,are magnied incon-centration as they move through food chainsand therefore are most harmful to top preda-tors includinghumans. Pesticides from agricul-ture, including genetically engineered insecti-cides (104), can change freshwater invertebratecommunities. Pharmaceuticals and personal-care products arefoundinsurface watersat con-centrations that may affect freshwater organ-isms (105, 106), whereas health risks to humansare plausible but poorly understood (107).

Many freshwaters are becoming increas-ingly saline, particularly in urban and dry-land settings. Multiple processes are involvedin salinization of freshwaters, including point-source discharge of treatedefuents, irrigation,marine saltwater intrusion due to overpumpingof coastal aquifers, sea-level rise, and, in somecold climate regions, road salting (108), but ir-rigation andwater abstraction in water-stressedenvironmentsisthemostwidespreadcause(72).Chloride and sodium can interact with otherchemical constituents, andsalinizationhas beenassociated with increased base cation concen-trations in lakes (109), altered DOC ushingfrom soils, and interactions with other metalcontaminants (110). Chloride concentrationsoften surpass chronic and acute toxicity levels(108); thus, they represent a major stressor of resident biota.

Rates of biogeochemical processes are oftenmodeled as a function of temperature and

concentrations of substrates. The total amouof chemical processing is, in turn, dictaby the time available for these processeoccur. As discussed in sections above,these factors are undergoing anthropogenmodication. Residence times have been altnatively expedited or delayed, temperatures

often increasing, and concentrations of soreactants are declining while those of mmore are increasing.

Examples of shifting biogeochemical rain freshwaters are accruing in the literatuBecause they increase residence time, res voirs substantially inuence sediment as weC, N, P, and silicon (Si) uxes from contineto oceans (83, 91). Increased nutrient inppromote primary production and ecosysterespiration, and they reduce the efciencynutrient uptake because biological demandsaturated (111). Shifting ratios of C, N, anfavor different biogeochemical transformatioas well as the species composition of aqucommunities (112).

Ecosystem Metabolism and Carbon Balance Although freshwaters cover only a smpercentage of Earths land surface, they pa disproportionately large role in the C cybecause net C uxes per unit area are greafor freshwater systems than for the surrouning land (113). Freshwater systems tendhave high rates of carbon dioxide (CO2) amethane (CH 4) emissions to the atmosphe Whereas freshwater CH4 emissions offapproximately 25% of the continental sink organic C (114), lakes and reservoirs contributo the C sink through C storage in stablong-lasting sediments.

Current land storage of organic C is esmated to be approximately 1,500 Pg in sand 560 Pg in biomass (115, 116). Over Holocene, lake sediments accumulated approimately 420820 Pg C (113). The current stage rate in freshwater ecosystems is at l0.23 Pg C year 1 (113). By comparison, durthe 1980s1990s, land-use change is thought


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have released 0.33 Pg C year 1 to the atmo-sphere, whereas terrestrial ecosystems storedbetween 0 and 5 Pg C year 1 (16). Organic Cin lake sediments is preserved for a consider-ably longerduration (tens of thousandsor more years) than organic C in terrestrial ecosystems(decades to centuries).

Intensive metabolism or storage of organicC in freshwaters may also be signicant forregional C budgets. In the Northern High-land Lake District of Wisconsin, a region of 7,000 km2, freshwater wetlands and lakescover only 20% and 13% of the land area, re-spectively, but account for> 90% of the storedorganic C (117). In Amazonia, upland forestsappear to be accreting organic C, but for theregion as a whole, the C budget is roughly bal-anced owing to large CO2 emissions from wet-

lands and rivers (118).Changing climate and land use are likely toaffect processing and storage of organic C infreshwaters. Building of reservoirs for irriga-tion, ood control, or hydropower increasessedimentation and C storage while produc-ing energy, thereby offseting greenhouse gasemissions (119). However, reservoirs also emit greenhousegasestotheatmosphereatratesthat can offset the benets of reservoirs (120). Sedi-ment organic C burial rates by reservoirs rangefrom 150 to 17,000 g C m 2 year 1, far greaterthan sedimentation rates in natural lakes andoceans(121). Globally, impoundments buryap-proximately four times as much organic C asthe worlds oceans (121). Warming temper-atures increase aquatic ecosystem respiration(122, 123) and are likely to decrease rates of or-ganic C storage while increasing rates of CO2and CH 4 emission to the atmosphere.

Biotic Transformations Although freshwater composes less than 1%of the planets surface, the 126,000 knownfreshwater animal species make up approxi-mately 10% of described animal species andone-third of global vertebrate diversity (124).Freshwaters support 15,000sh species (45%

of the global total)a disproportionate con-centration of global shdiversity (http://www.iucn.org/what/tpas/biodiversity/ ). A numberof other measures indicate that freshwaterecosystems support a disproportionately highlevel of species diversity relative to their area.Rates of endemism can be high for fresh-

waters, especially for ancient and physically isolated ecosystems (125). High levels of en-demismincrease species vulnerability to extinc-tion. Aquatic biodiversity remains poorly de-scribed, even for better-known groups such as vertebrates. For example, each year, approxi-mately 200 new sh speciesare described (126).

Aquatic biota are subject to a wide range of interacting threats. Direct alteration of aquaticecosystems as well as their low-lying and in-tegrative position within catchments makesaquatic biota sensitive to altered land-use prac-tices. Sediment and nutrient loading fromnonpoint-source pollution, invasive species,and altered ow regimes due to impoundment operations were the leading threatsto imperiledfreshwater taxa in North America (127).

Some of the most detailed data regardingspecies imperilment is for North America andtheUnitedStates.Freshwatertaxahaveahigherrate of imperilment than do terrestrial or ma-rine taxa (128), and the taxonomic groups withthe greatest proportion of at-risk species areall freshwater. The list is topped by freshwater mussels (69% at risk or extinct), cray-shes (51%),stoneies (43%), freshwatershes(37%), and amphibians (36%). High-proleterrestrial taxonomicgroupshave lower rates of imperilment (birds14%, mammals 16%) (129).High rates of imperilment for North Ameri-can freshwater shes have been recently cor-roborated by an American Fisheries Society as-sessment (39% imperiled: 230 vulnerable, 190threatened, 280 endangered, and 61 extinct orextirpated) (130).

The Red Lists for freshwater species by the Internation Union for Conservation of Nature (IUCN) parallel imperilment trendsfor the United States, although the IUCNassesses fewer taxa. The 2009 IUCN Red List notes 1,147 freshwater shes (37% of species


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assessed) and 1,369 species of freshwaterinvertebrates (131). Listed invertebrates aredominated by the handful of assessed and rela-tively well-studied invertebrate taxa: mollusks,crayshes, and odonates. For these groups,approximately 8% of species are classiedas imperiled, and 73% of listed invertebrate

species are from North America, Europe, Australia, and New Zealand, reecting thegreater amount of recorded biological data inthese countries. The majority of groups havenot been assessed, but if overall rates are com-parable with those for assessed groups, then12,000 freshwater invertebrate species may beextinct or imperiled (131). High rates of im-perilment are documented for other freshwatertaxa such as amphibians, freshwater-reliant reptiles, andwaterbirds (16). The LivingPlanet Index developed by the World Wildlife Fund(http://www.wwf.panda.org ) provides a com-posite measure of trends in biota. The Fresh- water Living Planet Index, based on trends for1,463 freshwater vertebrate populations (458species), declined by 35% since 1970morerapidly than either terrestrialor marine indices.

Human activities have caused speciesextinction rates that far exceed backgroundrates. Current and projected extinction ratesfor freshwater faunal groups in North Americaare ve times those of terrestrial taxa andare on par with species extinction rates fortropical rainforests. Freshwater extinctionrates were estimated to be 1,000 times greaterthan background rates (128). Researchers havedocumented 123 North American freshwatertaxa as extinct (128), including approximately 60 mussel species (131). Global extinctions aredifcult to tally, but range from 95 to 290 shspecies (132).

A species-by-species view of the statusof freshwater biological resources may not be optimal for considering freshwater biota,particularly for invertebrates, which are highly diverse and not well-known (131). There hasbeen growing interest in classifying and assess-ing freshwater communities and ecoregions at broad geographic scales. Efforts to assess thestatus of wetland plant communities or associa-

tions (classication based on the U.S. Natio Vegetation Classication System) in No America indicated high rates of imperilmeOf the 1,560 wetland plant communiti60% are considered at risk, 12% criticimperiled, 24% imperiled, and 25% vulnable (http://www.natureserve.org/servlet/

NatureServe?init =

Ecol ). Freshwater ecogions provide a starting point for assessand prioritizing conservation efforts (132Rather than focus on the status of individspecies, conservation efforts may be meffectively focused on unique community ecosystem types as well as on hot spotspecies diversity and endemism such as ancrivers, oodplains, groundwater ecosysteand large or ancient lakes (see FreshwConservation Priorities, sidebar above).

Species extinctions are generally duethe extirpation of many individual populati(133). Rates of population extirpation of fre water taxa are not well-known. Using histical data for rivers in Mexico, Mercado-Set al. (134) reported native-species loss rate10%30% perdecade anda concomitant 10%20% increase per decade in nonnative specNumerous other studies report similar trenof extirpation of native species and increing incidence of nonnative species (135). Tis a classic example of biotic homogenizat whereby many species are declining as a reof human activity and are being replaced bsmallnumberof tolerantandexpanding speci Theconsequence is that biotic communities bcome increasingly similar over time. Three processeshabitat alternation, invasions, aextinctionsinteract in ways that tend to fther biotic homogenization (56).

Species replacement can alter ecosystprocesses (136). A variety of mechanismsinvolved, including alterations in habitat strture and in species abundances resulting frchanges in predation and competition. Theare numerous examples of how loss or gaispecies impacts ecosystems. For example,clines of a migratory tropical sh decreadownstream transport of organic C and icreased primary production and respirati


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(137). McIntyre et al. (138) found that nutri-ent cycling rates and N:P ratios were highly sensitive to changes in the sh community.Consumer-drivenchanges in nutrient availabil-ity and N:P can have strong impacts on au-totrophic productivity and species composition(69). Changes to the top predators in lakes have

altered primary production, ecosystem respira-tion, the direction and magnitude of CO2 ex-changewiththeatmosphere,andsedimentationrates (139).

Disease Emergence and Transmission Water availability is a major determinant of human health, particularly in regions of lim-ited economic development (9). Risk of water-borne diseases increases when encounters be-tween aquatic vectors and humans are high, asoccurwhenwaterisscarceandpeopleaggregatearound limited water points or when water isabundant and aquatic vector populations (suchas mosquitoes) are widespread (140). Water-bornediseases areclosely associatedwith absent or compromised waste-management infras-tructure and water pollution (141). Contami-nation of domestic water supplies with humanor animal feces increases the risk of cholera,other diarrheal diseases, and a range of para-sitic infections. Ironically, construction of im-poundments also increases the risk of diseasessuch as schistosomiasis (bilharzia) and malariaby creating or expanding the habitat for aquaticdisease vectors such as snails and mosquitoes(142). Deforestation; land-use change; and thedevelopment ofdams, canals, andirrigationsys-tems are associated with increases in the habitat available for parasites and vectors and with hu-man morbidity and mortality related to emer-gent parasitic diseases (143). Biodiversity lossfrequently increases the incidence of humanand wild-organism diseases (144).

Whereas relationships between water andhuman diseases are well studied, the dynamicsandecological signicance ofdiseases ofaquaticorganisms have only recently begun to garnersignicant attention. Diseases play an impor-tant role in aquaculture andshproductionand

are a factor in the global decline in amphibians. Vertebrate models dominate the literature, al-though examples are also known for disease im-pacts on aquatic invertebrates (145).

As with species invasions, disease spreadinvolves transport, establishment, and expan-sion of pathogens and vectors in new habi-

tats. In the case of sh, introduction of nonna-tive species has been a major pathway for dis-ease spread. For example, in European freshwaters, aquaculture has been associated withthe introduction and translocation of 94 knownpathogens from 13 different taxonomic groups(146).Reports of antibiotic-resistant bacteria inand around aquaculture facilities are increasing(147). Pathogenicity, host susceptibility, anddisease spread can be affected by the trophicstatus of an infected water body; although spe-cic mechanisms vary, nutrient enrichment isa common attribute of disease outbreaks acrossa range of freshwater systems and taxonomicgroups (145). Eutrophication often increasesinfection risk by acting in concert with otherenvironmental stressors such as agrochemicals(148) that weaken host resistance.

FRESHWATER CHANGE AND HUMAN WELL-BEINGEcosystem services are the benets that humans derive from nature (16). Ecosystemservices from freshwater include ows that are withdrawn (e.g., for irrigation or municipaluse) and in situ ows (e.g., for hydropoweror transportation) (149). In addition to waterows, freshwaters also provide sh, waterfowl,edible plants such as wild rice, and other nat-ural products for human use. Freshwaters areassociated with several processes that regulatethe condition of ecosystems or the capacity of ecosystems to provide services (16). Floods canbe moderated or intensied, depending on thephysical conguration of landscapes and thecapacity of the land to hold water. Nutrient ows can be regulated by the capacity of riparian ecosystems to retain sediment and nu-trients. Disease can be regulated by ecosystemprocesses, such as the association of cholera


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Natural capitalSoilVegetationFreshwater HabitatBiotic interactions

Human use and managementInstitutions, incentives, regulationsMarketsOperatorsEquipment

Human-driven changes:

land cover; ecosystem heterogeneity;water infiltration, runoff, and quality;

carbon storage; nutrient flows; soil fertility; biota

Ecosystem services:food and fiber production, freshwater, flood regulation,

nutrient regulation, carbon sequestration, recreation, aesthetics

Figure 3Freshwater ecosystem services are derived from natural capital as well as the human-use and managemepractices of a region.

outbreaks with phytoplankton blooms (150).Finally, freshwater ecosystems provide culturalbenets in terms of education, enjoyment of nature, recreation, or spiritual values.

Freshwater ecosystem services interact with other ecosystem services through thedynamics of natural capital (Figure 3 ). Naturalcapital is the capacity of a particular place orregion to provide ecosystem services in boththe present and the future. Natural capitalincludes, e.g., the regions soils, vegetation,freshwater, habitat, as well as biota and theirinteractions. The dynamics of natural capitaldepend on external factors such as climate, in-ternalphysical andbiotic processes, andhumanactions. Human use and management affect not only the current status and future trends of natural capital, but also the ecosystem servicesderived from it. Depending on human use andmanagement, a regions natural capital can becongured quite differently and yield distinct sets of ecosystem services. Efforts to maximize

food and ber production often degra water quality, groundwater recharge, and tcapacity of landscapes to moderate oods (

Up to one-third of Earths human poulation is affected by water scarcity and attendant problems associated with foproduction, human health, and diversion labor simply to meet water needs (16). Natiaffected by water shortage may choosepurchase water-intensive commodities on tglobal market. The resulting trade in virtu water may lead to signicant efciencie water use (Figure 4 ). However, dependenon the virtual water trade may decrease gloexibility to respond to severe famines cauby drought or crop failures (151).

ARE THERE LIMITS? Access to adequate water is recognized byUnited Nations as a basic human right (15South Africa has legalized water priorities


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Maize Wheat Rice Barley Beef Pork Chicken

Global trade in virtual water in 2000

Commodity

A n n u a

l w a

t e r

t r a d e ,

k m

3

0

1 0 0

2 0 0

3 0 0

4 0 0

Virtual waterReal water

Figure 4Global trade in virtual water in 2000 (Reference 16, chapter 7, table 7.8). Virtual water is the water that would have been usedimporting country to produce the commodity. Real water is the water that was in fact used by the exporting country to producecommodity.

human and ecosystem needs by establishing areserve that provides an allocation of 25 litersper person per day to meet basic human needs(5). The ecological reserve is dened as the

water necessary to maintain ecosystems neededto ensure sustainable development. Conceptsof peak renewable water (ow constraintslimit water availability), peak nonrenewable water (extraction rates exceed recharge ratesof groundwater systems), and peak ecological water (aggregate loss of natural capital from water extraction exceeds the total value fromhuman use of water) clarify trade-offs in water-allocation decisions (153).

Pollution and water ows are closely cou-

pled given that concentrations of a pollutant equal its mass divided by the water volume. Ineconomic terms, pollution decreases the supply of clean water and adds to the costs of wateruse. Thus many countries have established up-perlimits for pollutionof freshwaterby N,P,ortoxic materials. These upper limitsare basedon

expectations of water ows sufcient to dilutepollutants to acceptable concentrations. Policy instruments for the management of water sup-ply must also take account of changing climate,

land use, and chemical inputs to freshwaters. Are there natural upper limits for human useof freshwater resources? Between 5% and 25%of global freshwater extraction exceeds the re-newable supply, althoughthere is greatvariabil-ity among regions (16). Sustainable water with-drawals computed for the driest 10% of yearsare substantially lower than those computed forthe average year (25). Thus planning for aver-age conditions is of little use in drought-proneregions.

Because freshwater is heterogeneously dis-tributed andexpensive to transport, it is difcult to establish an upper limit for human con-sumptive use of freshwater at the global scale.If overconsumption of water is widespread,however, it will be difcult to meet needs by moving water from place to place. Thus a


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global pattern of overconsumption would raiseconcern about exceeding global limits. Any upper limit for water consumption, regional orglobal, must allow for environmental ows tomaintain natural capital that supports essentialecosystem services. A safe upper limit for hu-man consumptive use of freshwater worldwide

was suggested to be 4,0006,000 km3 year(154). Current consumptive water use approximately 2,600 km3 year 1 is within thboundary. However, global requirements f water to meet food and biofuel demand mbring consumptive use close to the propoupper limits by mid-century (155, 156).

SUMMARY POINTS

1. Freshwaters, alongwith land transformed foragricultureuse,are themost extensively andrapidly altered ecosystems on the planet. Responses by freshwater systems are also broain scope, encompassing changes in physical structure, chemistry, ecosystem processebiotic characteristics, and diseases.

2. Major drivers of change include climate, hydrologic modication, land use, chemicainputs, invasive species, and harvest. All drivers play a role, but some have already hsubstantial effects on freshwaters (e.g., changing hydrology, channel form, land usechemical inputs, species introductions, harvest), whereas the impacts of others have no yet reached their peak impact.

3. The projections of long-termdirectional climate changesuggest massiveeffects on freshwaters within a few decades. Because temperature and precipitation patterns affect fresh water ows, rates of nutrient cycling and toxin breakdown, as well as thermal niches organisms, the effects of future climate change could impact all aspects of freshwaters

4. The drivers of change and the responses of freshwaters interact. For example, variationin storm intensity affects streamchannel form, inputsof nonpoint pollutants, habitat, andtransport of invasive species anddiseases. Deforestation, urbanization, and infrastructuredevelopment often lead to increases in the host vectors of human parasites, resulting iincreased disease rates. Reservoirs, in addition to their hydrologic and biogeochemica

effects, facilitate the spreadof nonnative species andareassociatedwith spreadof diseasesuch as schistosomiasis (bilharzia).

5. Human impacts on freshwater ecosystems sometimes follow a predictable sequenceOverharvest often precedes other anthropogenic effects on aquatic ecosystems, produc-ing declines in populations of the most valuable shes. These changes have often beefollowed by increases in pollution, invasive species, and habitat destruction. This suite stressors inhibits the future recovery of many degraded sheries, such that in the absenceof harvest, many major sheries may never return to their previous abundance.

FUTURE ISSUES1. Substantial uncertainties remain about the effects changing climate and land use have o

surface and groundwater ows, water quality, and aquatic biota.

2. Agricultural policies and practices will have powerful effects on surface and groundwter ows, ood frequency and spatial pattern, water quality, human and wild-organismdiseases, and living aquatic resources.


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3. Basic research is needed to determine the factors that control transport, the establish-ment and expansion of invasive species, and aquatic diseases related to humans and wildorganisms.

4. Effective policies, practices, and technologies must be invented and implemented tocontain the spread of, or extirpate, harmful freshwater diseases and invasive species.

5. There is a critical need for research to developa basic understanding, technologies, policy instruments, and institutions that can maintain the ow of freshwater ecosystem services without degrading the natural capital that creates these services. This research will gobeyond the natural science disciplines we draw from here and embrace engineering andpolicy sciences as well.

6. Appropriate policy instruments, economic incentives, and technologies are needed toconstrain pollution by N, P, and other chemicals within boundaries for acceptable waterquality.

DISCLOSURE STATEMENT The authors are not aware of any biases or conicts of interest that might be perceived as affectingthe objectivity of this review.

ACKNOWLEDGMENTS We thank Mimi Chapin for her work on Figure 2 and Helen Sarakinos for helpful commentson drafts. Financial support was provided by the National Science Foundation through North Temperate Lakes Long-Term Ecological Research program and other research grants to theauthors.

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