Large-scale degradation of Amazonian freshwater ecosystems

18 Pages·2015·0.79 MB·English

GlobalChangeBiology(2015),doi:10.1111/gcb.13173
RESEARCH REVIEW
Large-scale degradation of Amazonian freshwater
ecosystems
LEANDRO CASTELLO1 andMARCIA N. MACEDO2,3
1DepartmentofFishandWildlifeConservation,CollegeofNaturalResourcesandEnvironment,VirginiaPolytechnicInstitute
andStateUniversity,310WestCampusDrive,Blacksburg,VA24061,UnitedStates,2WoodsHoleResearchCenter,149Woods
HoleRd.,Falmouth,MA02540,UnitedStates,3InstitutodePesquisaAmbientaldaAmaz^onia,SHINCA5,BlocoJ2,Sala309,
Bairro-LagoNorte,Bras!ılia-DF71503-505,Brazil
Abstract
Hydrological connectivity regulates the structure and function of Amazonian freshwater ecosystems and the provi-
sioning of services that sustain local populations. This connectivity is increasingly being disrupted by the construc-
tion of dams, mining, land-cover changes, and global climate change. This review analyzes these drivers of
degradation,evaluatestheirimpactsonhydrologicalconnectivity,andidentifiespolicydeficienciesthathinderfresh-
waterecosystemprotection.Thereare154largehydroelectricdamsinoperationtoday,and21damsunderconstruc-
tion.Thecurrenttrajectoryofdamconstructionwillleaveonlythreefree-flowingtributariesinthenextfewdecades
ifall277planneddamsarecompleted.Land-coverchangesdrivenbymining,damandroadconstruction,agriculture
andcattleranchinghavealreadyaffected~20%oftheBasinandupto~50%ofriparianforestsinsomeregions.Glo-
balclimatechangewilllikelyexacerbatetheseimpactsbycreatingwarmeranddryerconditions,withlesspredictable
rainfall and more extreme events (e.g., droughts and floods). The resulting hydrological alterations are rapidly
degrading freshwater ecosystems, both independently and via complex feedbacks and synergistic interactions. The
ecosystem impacts include biodiversity loss, warmer stream temperatures, stronger and more frequent floodplain
fires,andchangestobiogeochemicalcycles,transportoforganicandinorganicmaterials,andfreshwatercommunity
structureandfunction.Theimpactsalsoincludereductionsinwaterquality,fishyields,andavailabilityofwaterfor
navigation, power generation, and human use. This degradation of Amazonian freshwater ecosystems cannot be
curbed presently because existing policies areinconsistent across the Basin, ignorecumulative effects,and overlook
the hydrological connectivity of freshwater ecosystems. Maintaining the integrity of these freshwater ecosystems
requires a basinwide research and policy framework to understand and manage hydrological connectivity across
multiplespatialscalesandjurisdictionalboundaries.
Keywords: climate change, conservation, dams, fragmentation, hydrological connectivity, land-cover change, mining, policy,
watershed
Received3July2015;revisedversionreceived29October2015andaccepted9November2015
mediated transport of matter, energy, and organisms
Introduction
withinandbetweenelementsofthehydrologicalcycle’
Freshwater ecosystems provide a range of key ecosys- (Rosenberg et al., 2000; Pringle, 2001; Freeman et al.,
tem services. They regulate climate, support nutrient 2007b). Disruptions of hydrological connectivity,
cycling, transport water and materials, and maintain referredtohereashydrologicalalterations,candegrade
water quality and natural communities (Millennium freshwaterecosystems.Damconstruction,forexample,
Ecosystem Assessment, 2005). They also provide food, disrupts river flows by changing their seasonality and
energy,fiber,andwaterforhumanconsumption,being establishing lentic (still water) conditions (e.g., Pelicice
necessary for the survival and well-being of people et al.,2014).
(Braumanet al.,2007). Hydrological alterations are escalating worldwide as
The volume,timing,quality,andvariabilityofwater human populations grow and global climate change
flows play key roles in maintaining the integrity of shifts the planetary energy and water balance
freshwater ecosystems because they control their (Vo€ro€smartyet al.,2000).Theseenvironmentalchanges
hydrological connectivity – defined as the ‘water- arewidespreadintropicalregions,particularlyinlarge
river basins such as the Congo, Mekong, and Amazon
Correspondence:LeandroCastello,tel.[phone],fax+1540 (Wohl et al., 2012). In the Amazon (the largest of these
2317580,e-mail:[email protected] basins), construction of dams, mining, land-cover
©2015JohnWiley&SonsLtd 1
2 L. CASTELLO & M. N. MACEDO
change, and global climate change are driving rapid Trade winds
Atmosphere
degradationoffreshwaterecosystemsthroughchanges
&1ltciohomantChrigkalelisminhoteg2ynldlook2fr,m0om%l2o2o0goi1osif5ctf)att.hlrgoecFlopyrAbecicslameahllaw(fsCzouaoartrnseeftsraeBtclsaeelocsaionrnesitdv,yaedsslrtar.e,avmfl2ian0onsi1wnn3cgasbos;v~iaMn6enr.t9adocomdevtdhiiesloer-- Vertical connectivity Evapotranspiration Rainfall Rainfall Evaporation Evaporation
AtlanticOcean(Coeet al.,2008). Seasonal
Maintaining the integrity of freshwater ecosystems Forests & flooding Freshwater Discharge Atlantic
requires understanding the full range of ecosystem Savannas Runoff ecosystems Ocean
effects caused by hydrological alterations occurring
Lateral connectivity Longitudinal connectivity
in adjoining freshwater, terrestrial, atmospheric, and
oceanic systems. However, predicting the impact of Fig.1 Schematicdiagramofthemainpathwaysinvolvedinthe
hydrological alterations on large tropical basins is hydrologicalconnectivityofAmazonfreshwaterecosystems.
difficult due to a lack of integration of available
knowledge. Most studies of drivers on hydrological vertical (river–atmosphere or land–atmosphere) con-
alteration of freshwater ecosystems have focused on nections.
single dams in tributary watersheds, but they have Rainfall in the Amazon depends on the trade winds
largely disregarded the cumulative effects of multiple and the South American Monsoon System (Fig. 1),
dams on the hydrological connectivity of freshwater whichtransfermoisturefromtheAtlanticOceantothe
ecosystems systems. Similarly, most studies assessing Basin (Marengo et al., 2012; Jones & Carvalho, 2013).
the consequences of hydrological alteration have Average annual rainfall over the Basin is
focused on specific ecosystem components (i.e., spe- ~2200 mm yr!1 and highly seasonal (Huffman et al.,
cies composition, biogeochemical cycling), but have 2007). Between 50 and 75% of this annual rainfall
paid little attention to whole ecosystem structure and (~9600 km3 yr!1)isinterceptedbyforestsandsavannas
function. and recycled back to the atmosphere via evapotranspi-
This review provides a comprehensive framework ration (Shuttleworth, 1988; Malhi et al., 2002;
for understanding the linkages among Amazon fresh- D’Almeida et al., 2007).Theremainder falls over fresh-
water ecosystems, drivers of hydrological alteration, waterecosystems,ordrainsthroughforestsandsavan-
ecosystem responses and feedbacks to these changes, nas (i.e., surface runoff) and enters a vast network of
and the role of management policies. The framework streams, lakes, and rivers, transporting terrestrial
hinges on four research questions: (i) What is the role organicandinorganicmaterialsintofreshwaterecosys-
of hydrological connectivity in maintaining the struc- tems. Downstream flows transport these materials and
ture and function of freshwater ecosystems? (ii) How discharge an estimated ~6700 km3 yr!1 of freshwater
and to what extent are dams, land-cover change, min- intotheAtlanticOcean(Coeet al.,2008).
ing,andglobalclimatechangealteringthehydrological
connectivity of Amazon freshwater ecosystems? (iii)
Influenceonfreshwaterecosystems
What are the consequences of these hydrological alter-
ations for freshwater ecosystems at the Amazon Basin Rainfall and geomorphology control the physical and
scale? and (iv) What deficiencies in existing policies chemical properties of rivers (Sioli, 1984; Junk et al.,
may hinder protection of freshwater ecosystems from 2011; Hess et al., 2015). Whitewater rivers originate in
hydrologicalalteration? the Andes Mountains andcarry heavy sediment loads.
Clearwater rivers originate in the southeastern region
ofthebasinanddraintheweatheredsoilsoftheBrazil-
Hydrologicalconnectivity
ianandGuiananShields,carryingsomedissolvedmin-
erals but few suspended sediments. Blackwater rivers
Macroscalepatterns
(e.g., the Negro River) drain the sandy, nutrient-poor
The hydrological connectivity of Amazon freshwater soils of the central Amazon, carrying few suspended
ecosystems operates in four dimensions, one temporal sedimentsbuthighlevelsofacidityandtannins.These
and three spatial (Fig. 1; adapted from Ward, 1989). In rivers form a diverse array of freshwater ecosystems
the temporal domain, connectivity refers to seasonal throughouttheBasin(Fig. 2).
and interannual changes in water flows (e.g., rainfall). As intermittent rainfall flows from land into stream
Inthespatialdomain,itconsistsoflongitudinal(head- channels, it creates aquatic–terrestrial interfaces (re-
water–estuary),lateral(river–landorstream–land),and ferredtoasriparianzonesofsmallstreams)thatarethe
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
DEGRADATION OF AMAZON FRESHWATER ECOSYSTEMS 3
Fig.2 Drivers of hydrological alteration in Amazon freshwater ecosystems. Figure by Paul Lefebvre (adapted from Castello etal.
2013b).
principal zone of exchange of water, nutrients, sedi- 1989). These floodplains, which can span tens of kilo-
ments, and organic matter between terrestrial and meters (Hess et al., 2003), are fertile and productive in
freshwater ecosystems (Junk, 1993; Godoy et al., 1999; whitewater rivers due to their heavy sediment loads.
McClain & Elsenbeer, 2001; Naiman et al., 2005). The annual rise and fall of river waters induce lateral
Despite their size, small streams and their riparian exchanges of organic and inorganic materials between
zones are numerous and likely the most extensive river channels and floodplain habitats that influence
freshwaterecosystemoftheBasin(Junk,1993;Beighley most biogeochemical processes in these ecosystems
& Gummadi,2011). Althoughtheir extent is unknown, (Junket al.,1989;Melacket al.,2009).
headwater streams are thought to represent two-thirds Local precipitation andinputs fromstreams andriv-
of total stream length in typical watersheds and thus ersformseveralextensivewetlandsindepressedorflat
underpin basinwide freshwater connectivity (Freeman areas of the basin. As river networks traverse large
et al.,2007a). inland depressions in the Basin, they form extensive
Inthelowerreachesoflargerivers,seasonalinunda- wetlandsintheMaran~on-Ucayaliregion(Peru),Llanos
tion cycles (i.e., flood pulses) with mean amplitude of deMoxos(Bolivia),andBananalIsland(Brazil),among
10 m (to as high as 15 m in the Purus River) control others (Kalliola et al., 1991; Hamilton et al., 2002). Sea-
floodplain ecosystems supporting diverse forest stands sonal rainfall and high water tables form swamps and
and aquatic macrophyte communities (Junk et al., flooded savannas in interfluvial regions (e.g., in the
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
4 L. CASTELLO & M. N. MACEDO
Negro Basin; Junk, 1993). Precipitation and seasonal along river floodplains in a 1.77 million km2 region of
inundation driven by flood pulses and tides create a the Central Amazon has been estimated at
diversewetlandmosaiconMarajo"Islandintheestuary ~298 Tg C yr!1, of which ~210 Tg C yr!1 are out-
(Smith,2002). gassedascarbondioxide(CO )andsubsequentlyrecy-
2
cled as net primary production (Melack et al., 2009).
These natural carbon fluxes are comparable in scale to
Ecosystemservices
netcarbonemissionsattributedtoland-coverchangein
The hydrological connectivity of Amazon freshwater the Brazilian Amazon during the 1990s (Houghton
ecosystems enables the provision of several services et al.,2000),makingthemanimportantpartoftheglo-
that are vital for local, regional, and global communi- balcarboncycle.
ties.Keyecosystemservicesincludebiodiversitymain- Seasonal inundation promotes secondary productiv-
tenance; water quality, climate and flow regulation; ity by allowing fish populations to exploit plant-based
nutrientandcarbon(C)cycling;andfoodandfiberpro- resources in the floodplains (Lagler et al., 1971;
duction. The diversity of freshwater ecosystems found Goulding, 1980; Castello, 2008a). Fish migrate laterally
in the Basin sustains a wealth of life forms. According onto the floodplains during rising river waters to
to available estimates,the Basincontains between 6000 avoid predators and feed on nutritious plant materi-
and 8000 fish species (Schaefer, 1998; Reis et al., 2003), als (Welcomme, 1985; Gomes & Agostinho, 1997; Cas-
of which only about 2320 have been described to date tello, 2008a,b). Conversely, declining waters force fish
(Abell et al., 2008). About half of those fish species are to migrate back to river channels and lakes, where
thought to inhabit river floodplains, while the rest water quality is generally poor and fish are more vul-
occupy headwater streams, where geographicisolation nerable to predation (Welcomme, 1985; De M"erona &
promotesendemism(Junk&Piedade,2004).Thediver- Gascuel, 1993; Arantes et al., 2013). These lateral
sity of bird and tree species is similarly high, with an migrations are performed by resident floodplain spe-
estimated1000flood-toleranttreespeciesandover1000 cies (e.g., Cichla spp.), as well as migratory species
bird species inhabiting the lowland forests of the Cen- that travel longitudinally along river channels (e.g.,
tralAmazon(Junk,1989;Stotzet al.,1996).Muchofthis Prochilodus nigricans; Ribeiro et al., 1995; Barthem &
diversityoccursalongrivernetworks,asecologicalcor- Goulding, 2007). Some large-bodied catfish species
ridors with specific environmental conditions deter- migrate exclusively along river channels from the
mine species occurrence and mediate movement estuary to the headwaters (e.g., Brachyplatystoma rous-
through the landscape (e.g., Van Der Windt & Swart, seauxii), but they prey on floodplain-dependent spe-
2008). cies (Barthem & Goulding, 1997). The productivity of
As rainwaters drain through terrestrial ecosystems, Amazon river floodplain fish populations sustains
riparian zones regulate water quality by filtering the high mean per capita fish consumption rates of 40–
organic and inorganic materials they carry (Alexander 94 kg yr!1, well above the global average of
et al., 2000). Terrestrial inputs are transported down- 16 kg yr!1 (Isaac & Almeida, 2011).
stream,deposited,andremobilizedinriverfloodplains Spatialandseasonalpatternsofwaterflowsinfluence
until they are discharged into the ocean (Wipfli et al., many other animals at different points in their life his-
2007; McClain & Naiman, 2008). During this transport, tories. Turtles (Podocnemis spp.), caimans (e.g., Melano-
freshwater ecosystems regulate water flows, buffering suchusniger),otters(Pteronurabrasiliensis),anddolphins
flows during high discharge periods and maintaining (Inia geoffrensis, I. boliviensis, and Sotalia fluviatilis) have
them during low discharge periods. This regulation of life cycles dependent on seasonal colonization of the
flowspromotessoilinfiltration,rechargesgroundwater floodplains during high waters (Martin & Da Silva,
stores, and facilitates regular river navigation and 2004; Martin et al., 2004; Fach"ın-Ter"an et al., 2006; Da
hydropowergeneration. Silveira et al., 2010, 2011). Many terrestrial animals
Seasonal inundation induces the constant recycling inhabit riparian zones year-round or during the dry
of nutrients in river floodplains, leading to primary season to access water and feed on fruits, leaves, and
production rates (~17 Mg C ha!1 yr!1) that are five otheranimals(Naiman&Decamps,1997;Bodmeret al.,
times higher than those of upland forests (Melack & 1999).Riparianzonesalsoserveasimportantmigratory
Forsberg, 2001; McClain & Naiman, 2008). About 93% corridors for wide-ranging terrestrial species such as
ofthisproductionoccursinleveeforestsandC4macro- jaguars (Panthera onca), tapirs (e.g., Tapirus terrestris),
phyte communities (i.e., Echinochloa polystachya; Pie- and peccaries (e.g., Tayassu pecari; Lees & Peres, 2008;
dadeet al.,1991),whichinwhitewaterriversreachone Keuroghlian & Eaton, 2008). Some terrestrial and
of the highest primary productivity rates on the planet migratory bird species use wetlands as seasonal feed-
(Melack & Forsberg, 2001). Net primary production inggrounds(Petermann,1997).
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
DEGRADATION OF AMAZON FRESHWATER ECOSYSTEMS 5
Productive whitewater river floodplains allow Ama- tible to international forces. For example, multilateral
zonianstogenerateimportanteconomicactivitieswhile development initiatives, including the Initiative for the
diversifyingtheirdiets.Huntingalongriparianzonesis Integration of the Regional Infrastructure of South
widespread(Bodmeret al.,1999;Parryet al.,2014),asis America (IIRSA) and the South American Council on
explorationofpalmfruits(e.g.,Euterpeoleracea),timber Infrastructure and Planning (COSIPLAN), have
(e.g.,Calycophyllumspruceanum),andfish(e.g.,Arapaima invested heavily in the construction of waterways,
spp.;Pinedo-Vasquez et al., 2001; Brond"ızio, 2008; Cas- hydroelectric dams, and other infrastructure in the
tello & Stewart, 2010). Together, these activities often Amazon.Theresulthasbeenlarge-scaledisruptionsto
contribute as much as two-thirds of rural household the hydrological connectivity of Amazon freshwater
income(McGrathet al.,2008,2015;Ewel,2009). ecosystems via a variety of mechanisms, including: (i)
storageofwaterinhydroelectricreservoirs;(ii)changes
inseasonalflooddynamics;(iii)reducedrainfall,water
Driversofhydrologicalalterations
quality, and evapotranspiration at regional scales; and
Amazon freshwater ecosystems are becoming increas- (iv)increasesinthefrequencyandintensityofextreme
ingly degraded due to human development activities, weatherevents(i.e.,droughts,floods;Fig. 3).
includingtheconstructionofdams,mining,land-cover
change, and global climate change (Fig. 2). Many of
Dams
these activities were historically driven by domestic
markets and national development interests, which Some of the most direct impacts on streams and rivers
prompted construction of roads and conversion of stemfromdams(Fig. 4a).Storageofwaterinreservoirs
native forests and savannas to croplands and range- regulates water flows, blocks animal movements, and
lands (Laurance et al., 2001; Nepstad et al., 2014). disrupts downstream transport of materials. Water
Although these domestic forces remain strong, the storageinreservoirscandrasticallyalterstreamorriver
growing engagement of Amazonian countries in thermal regimes, depending on the depth from which
export-oriented markets for agricultural and mineral waterisreleasedandthereservoir’sphysicalcharacter-
commodities has made the region increasingly suscep- istics (e.g., depth, surface area; Olden& Naiman,2010;
Food availability
Economic activity Improved
human
wellbeing
Global climate Electricity
Land-cover change Mining Dams
change
Reduced evapotranspiration Decreased Water storage
water quality in reservoirs
Reduced rainfall;
increased droughts Attenuated
seasonal floods
Increased Biodiversity
local runoff loss
Decreased
regional runoff Decreased
Increased Increased floodplain Disrupted
flooding floodplain fires productivity migrations
Reduced water
for human use,
navigation, and Decreased Decreased fish yields
power generation crop yields and carbon cycling
Decreased human wellbeing
Fig.3 Interactions among freshwater ecosystems, drivers (Red rectangles), hydrological alterations (Blue rectangles), and ecosystem
impacts(Yellowrectangles).Dashedlinesdenoteeffectsnotaddressedbythisreview.
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
6 L. CASTELLO & M. N. MACEDO
(a) Natural conditions Postdam conditions
Increased
Evaporation
evaporation
Discharge Reservoir Lowered
water discharge
storage
Seasonal flow Decreased flow variability,
variability and timing altered timing, reduced floods
(b) Natural conditions Local scale disruptions Regional scale disruptions
Decreased Same Decreased Decreased
ET Rainfall
ET rainfall ET rainfall
Runoff Increased runoff Decreased runoff
Discharge Increased discharge Decreased discharge
Fig.4 Schematicdiagramdepictingthemainhydrologicalalterationscausedbydamsandland-coverchangeonAmazonfreshwater
ecosystems. (a) Relative to undisturbed conditions (Left), dams store water in reservoirs, lower discharge and flow variability, alter
floodseasonality,anddecreasehigh-floodmaxima(Right).(b)Relativetoundisturbedconditions(Left),localland-coverchange(Mid-
dle)generallydecreasesevapotranspiration(ET),increasingrunoffanddischargebutnotrainfall.Land-coverchangeatregionalscales
(Right)maydecreaseETsufficientlytoalsodecreaserainfall.Runoffanddischargemayexperienceanetincreaseordecrease(+/ ),
!
dependingonthebalancebetweenrainfallandET(rainfall–ET=runoff).
Macedo et al., 2013). Reservoirs can also reduce river facilitate road construction in flat areas, or enable
discharge as stored water evaporates or is diverted for small-scale energy or irrigation. In 2007, an estimated
other uses (e.g., irrigation). Flow regulation by dams 10 000 such impoundments existed in the headwaters
also disrupts lateral connectivity by decreasing sea- of the Xingu Basin alone (Macedo et al., 2013), averag-
sonal flow variability, most notably by attenuating ing one impoundment per seven kilometers of stream
floodmaxima(Poffet al.,1997;Poff&Hart,2002). length. The cumulative impacts of many small dams
Dam-induced hydrological alterations are increas- maybesignificant,particularlyinthesensitiveheadwa-
ingly common in the region as dams of all types and tersregionswheretheyaremostcommon.
sizesproliferate.Althoughthereisuncertaintyinavail-
able data, the total installed power generation capacity
Land-coverchange
in the basin is expected to double from ~18 000 mega-
watts (MW),provided by154 largehydroelectric dams Land-cover change, particularly the conversion of
currently in operation, to ~37 000 MW with the com- native forests and savannas to other land uses (e.g.,
pletionof~21additionaldamsnowunderconstruction agriculture, pastures), alters the surface water balance
(Fig. 2; Table 1; ANEEL, 2012; Proteger, 2012; Castello andpartitioningofrainfallintoevapotranspiration,dis-
et al.,2013b).Constructionofanestimated277planned charge, and soil moisture (Abell et al., 2007; Brauman
hydroelectric dams (now in the initial planning stages) et al., 2007; Sterling et al., 2012; Wohl et al., 2012). In
couldaddasmuchas~58 000 MWofinstalledcapacity general, crops and pasture grasses use less water than
in the region, although many dams operate below native vegetation due to their lower height, less com-
capacity. Most hydroelectric dams are relatively small plex canopy, shallower rooting depth, and lower leaf
(<100 MW) and are or will be located in the Araguaia- area index (Calder, 1998; Giambelluca, 2002). As a
Tocantins, Tapajo"s, and Madeira tributary basins result, at local scales, deforestation tends to decrease
(Table 1).Althoughglobalattentionhasfocusedonthe evapotranspirationandincreaserunoffandstreamdis-
construction of large hydroelectric dams, the most charge (Sahin & Hall, 1996; Andreassian, 2004; Coe
abundantdamsin the Amazonareactually small farm et al., 2011; Hayhoe et al., 2011). Over large spatial
impoundments, which are constructed in headwater scales,deforestationislikelytoreduceregionalrainfall
stream reaches to provide drinking water for cattle, andalterrainfallseasonality(Buttet al.,2011;Spracklen
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
DEGRADATION OF AMAZON FRESHWATER ECOSYSTEMS 7
Table1 Amazon hydroelectric dams by installed potential, gold mining activities have existed in the Amazon for
country,andsubwatershed decades, they have recently surged following a 360%
increase in gold prices after 2000 (Fig. 2; Nevado et al.,
Operational Construction Planned
2010;Swensonet al.,2011;Asneret al.,2013;DeMiguel
Damcapacity et al., 2014; Marinho et al., 2014). Artisanal miners
<100MW 135 14 206 extract gold by dredging sediments from the river bot-
100–1000MW 14 4 56 tom and using mercury (Hg) to amalgamate fine gold
>1000MW 5 3 15
particles, thereby altering stream and river morphol-
Country
ogy, increasing suspended sediment loads, and pollut-
Brazil 138 16 221
ing waters via the release of Hg. Mercury can be
Peru 7 2 30
transformed by microorganisms into Methylmercury
Ecuador 5 2 17
(MeHg), which is a powerful endocrine disruptor that
Bolivia 4 1 8
Colombia 0 0 1 can damage the nervous system, be assimilated into
Subwatershed living tissue, and become magnified in food webs via
Araguaia-Tocantins 56 2 101 bioaccumulation(Zhang&Wong,2007).
Madeira 43 8 44 Large-scale mining for iron ore, bauxite, oil, and
Tapajo"s 33 6 73 gas impacts freshwater ecosystems (both directly and
Ucayali 6 1 15 indirectly) by promoting deforestation, dam construc-
Xingu 6 1 2
tion, and roads in remote regions. Because smelting
Maran~on 5 3 22
of iron ore and bauxite is energy intensive, the steel
Amazondrainage 4 0 8
and aluminum industries have motivated the con-
Negro 1 0 1
struction of many dams in the Amazon (Switkes,
Purus 0 0 6
2005; Fearnside, 2006), including the Tucuru"ı Dam,
Napo 0 0 4
Caqueta-Japur"a 0 0 1 which flooded an area spanning 2860 km2 and dis-
placed more than 24 000 people (WCD, 2000). Where
hydroelectric power is insufficient to meet mining
et al., 2012; Yin et al., 2014), which in turn would demands, smelters consume charcoal that is produced
decrease stream and river discharge (Fig. 4b; Brui- by burning native vegetation (Fearnside, 1989; Sonter
jnzeel,2004;Stickleret al.,2013). et al., 2014a,b). The Caraj"as Mining Complex (Par"a,
Land-cover change has affected about 1.4 mil- Brazil), for example, is the world’s largest iron ore
lion km2 (~20%) of the Amazon Basin (Hansen et al., mine with large stores of bauxite, copper, manganese,
2013; Fig. 2), primarily driven by expansion of cattle and gold. Since construction in the 1970s, the Greater
ranching and crop production into native forest and Caraj"as Project has led to the construction of a rail-
savanna regions (Nepstad et al., 2014). Most land- road, many roads, and a large hydroelectric dam, all
cover change to date has occurred in the headwaters of which have led to significant land-cover changes.
of the Araguaia-Tocantins, Xingu, and Tapajo"s Rivers Leases for oil and gas extraction drive similar land-
(i.e., the ‘arc of deforestation’); more recently, it has cover changes and infrastructure development. Today,
extended to the south and southwestern regions of more than two-thirds of the Peruvian and Ecuadorian
the basin. High rates of deforestation observed in the Amazon are covered by oil and gas leases (Fig. 2),
early 2000s decreased significantly after 2005, particu- many of which overlap protected areas and indige-
larly in Brazil (Macedo et al., 2012; Nepstad et al., nous reserves in remote regions (Finer et al., 2008). As
2014). They remain low relative to historic rates, but energy demand grows, controversial projects like the
have shown an increasing trend since 2012 (INPE Camisea gas pipeline in Peru are likely to become
2014). Growing demands for agricultural products more common, particularly in the Andean Amazon
and weakening of environmental legislation in some (Finer et al., 2008).
countries have increased pressures on the region’s
native vegetation, especially in the Andes (Guti"errez-
Climatechange
V"elez et al., 2011) and Cerrado savannas (Soares-Filho
et al., 2014). Increases in atmospheric greenhouse gas (GHG) con-
centrationsaredrivingglobalclimatechangesthatwill
likely exacerbate the impacts of ongoing hydrological
Mining
alterations on freshwater ecosystems (Fig. 3; Melack &
Mining involves rapidly expanding operations to Coe, 2013). Climate predictions for the future of the
extract gold, oil, gas, bauxite, and iron ore. Although Amazon generally indicate that temperatures will
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
8 L. CASTELLO & M. N. MACEDO
increase, promoting melting of snow and ice in the ganic materials, freshwater community composition,
AndesMountains(IPCC,2014)andreducingthewater and productivity (Fig. 3). These changes may hinder
storage and discharge buffer capacity of freshwater theprovisionofkeyecosystemservicesbycausingbio-
ecosystems (Junk, 2013). Projections also indicate that diversity loss and increasing disturbances such as
total rainfall will likely decrease, while seasonal vari- floodplain fires and extreme droughts and floods.
ability will increase and extreme weather events (i.e., Decreases in hydrological connectivity can also drive
droughts, floods) will become more frequent and sev- changes in water quality, carbon cycling, and fish
ere (Mahli et al., 2007; Malhi et al., 2009; IPCC, 2014). yields, and may limit the availability of water for
Such dry–warm conditions would likely dampen humanuse,navigation,andpowergeneration(Fig. 3).
annual flood pulses and increase the frequency and
severityoflow-watereventsinlargerivers(Costaet al.,
Disruptedbiogeochemicalcycles
2003). Low-order rivers could experience dramatic
changes in discharge and flood pulses, while many The biogeochemistry of freshwater ecosystems is gov-
perennial headwater streams may become intermittent ernedprimarilybyhydrology,soiltype,nutrientavail-
(Junk,2013). ability, and terrestrial inputs of organic and inorganic
Regional land-cover changes have been linked to matter. Biogeochemical cycling, in turn, is largely con-
decreasesinwaterrecyclingandincreasedlandsurface trolled by biota, temperature, light availability, and
temperatures, driving local climate changes beyond water chemistry. All of these factors vary geographi-
those attributed to atmospheric GHG concentrations cally throughout the Amazon, and changes to any of
(Silv"erioet al.,2015).Bycausingnear-termshiftsinthe them can indirectly affect others. In temperate water-
energyandwaterbalance,land-coverchangesmaypro- sheds,conversionofforeststocroplandshasbeenasso-
voke shifts in regional rainfall regimes, increased land ciatedwithincreasedstreamflowandnutrientloading,
surface temperatures, and changes in river flows (Pan- causing large-scale eutrophication (Carpenter et al.,
day et al., 2015). Land-cover change in the western 1998; Schindler, 2006). However, little is known about
Amazon, for example, has been linked to decreased howsimilarchangesaffecttropicalsystems,wheresoils
precipitation,longerdryseasons,andhigheramplitude requiredifferentfertilizationregimesanddifferintheir
of seasonal water flow (Lima et al., 2013). Cumulative capacity toretain and cyclenutrients. Inthe southeast-
land-cover changes in the western Brazilian Amazon ern Amazon (Xingu Basin) fertilizer use in soy crop-
(Rondo^niastate)havebeenlinkedtodelaysintheonset lands has not affected stream nutrient concentrations
ofthewetseason,decreasingitslengthbyanestimated duetothehighbindingcapacityofregionalsoils(Neill
sixdaysperdecade(Buttet al.,2011;Yinet al.,2014). et al., 2013). On the other hand, land-use practices in
Climateandland-usechangeoftenactsynergistically thesameregionhavedegradedriparianvegetationand
(Fig. 3). Regional deforestation appears to amplify the led to the establishment of thousands of small farm
magnitude of droughts, making them dryer and more impoundments,whichtogetherhavewarmedheadwa-
severethantheywouldbewithfullforestcover(Bagley terstreamsby2–3 °Candincreaseddischargefourfold
et al., 2014). Severe droughts, in turn, can fuel further relative to streams in forested watersheds (Hayhoe
land-cover changes by killing trees directly (Lewis et al.,2011;Macedoet al.,2013).
et al., 2011) or triggering more widespread and intense Inadditiontothesephysicalchangestostreamwater,
wildfires (Brando et al., 2014), both of which release mining exploration, agricultural development, and
carbon stored in vegetation back to the atmosphere. dam construction can introduce new pollutants into
These changes are expected to disproportionately freshwaterecosystems.Mercury,forexample,isoneof
impact dryer transitional forests (~40% of the Amazon severalpollutantsthatareproducedoraccumulatedin
basin) and their associated freshwater ecosystems reservoirs, dispersed downstream, and magnified in
(Brandoet al.,2014). food webs (Schwarzenbach et al., 2006; Ashe, 2012;
Marinho et al., 2014). The anoxic conditions commonly
found in dam reservoirs increase natural levels of
Ecosystemimpacts
MeHg. For example, MeHg levels in water, plankton,
Hydrological alterations trigger a wide range of and fish downstream of the Balbina Dam on the
impacts on Amazon freshwater ecosystems, many of Uatum~a River have been shown to be higher when
whichhavecomplexfeedbacksandsynergisticinterac- reservoir water is stratified, because stratification fos-
tions. The available information indicates that the ters the anoxic conditions required for methylation
cumulative impacts of dams, land-cover changes, min- (Kasperet al.,2014).
ing, and global climate changes can substantially alter Reservoirs often flood large forested areas, killing
biogeochemical cycling, transport of organic and inor- treesthatproducelargequantitiesofmethane(CH )as
4
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
DEGRADATION OF AMAZON FRESHWATER ECOSYSTEMS 9
they decay. As a result, tropical reservoirs can have river floodplain ecosystems. Over 50% of floodplain
high concentrations of CH and CO in their deeper forestsintheLowerAmazonregionweredeforestedby
4 2
anoxic layers (Kemeneset al., 2007), although there are 2008 (Reno" et al., 2011), compared to ~20% of upland
few reliable estimates of the rate at which these GHGs forestsin2012(Hansenet al.,2013).Inadditiontoredu-
are emitted to the atmosphere. Estimates from the cing biodiversity, deforestation of riparian areas
Balbina hydroelectric dam (Amazonas state, Brazil) reduces filtration of terrestrial inputs flowing into
suggest that emissions from within and downstream streams and rivers, causing erosion, lowering water
of the reservoir totaled 3 Tg C yr!1 (Kemenes et al., quality, and altering aquatic primary production (Wil-
2007, 2011). Other studies of tropical dams likely liams et al., 1997; Neill et al., 2001). In whitewater riv-
underestimate GHG emissions because they exclude ers, floodplain deforestation reduces the abundance of
downstream fluxes (St Louis et al., 2000; Demarty & C3 plant communities that sustain herbivorous and
Bastien, 2011). Sediment deposition in reservoirs (par- detritivorousanimalpopulations, as wellasC4 macro-
ticularly in whitewater rivers) has the potential to trap phyte communities that are key biological producers
C, lowering CO and methane (CH ) emissions that (Araujo-Limaet al.,1986;Forsberget al.,1993).Riparian
2 4
would normally occur from biological processing deforestation also removes structures that provide
downstream(Smithet al.,2001).ItisunclearwhetherC habitatforaquaticbiota(e.g.,macrophytes,woodydeb-
storage in sediments could be sufficient to compensate ris) and reduces shading of small streams, often
emissions, but Amazon reservoirs are likely net pro- increasing incident sunlight and water temperature,
ducers of GHGs (St Louis et al., 2000; Fearnside, 2004; which may directly affect species composition and
Kemeneset al.,2011). metabolism (Bojsen & Barriga, 2002; Sweeney et al.,
2004;Macedoet al.,2013).
Alteredsedimentdynamics
Changinginundationregimes
Damsandland-coverchangesaffectriverdischargeand
sediment transport and mobilization, which are key Disruptionofseasonalinundationregimesimpactsspe-
determinants of river geomorphology, but their net cies composition and biogeochemical cycling in river
effects are scale dependent and context specific. In the floodplains. Floodplain forest trees have a number of
Upper Xingu Basin, for example, a fourfold increase in adaptations to cope with the physiological stress
stream flow in agricultural watersheds had little effect caused by regular seasonal flooding (Haugaasen &
onsedimentloadsorthemorphologyofsmall headwa- Peres, 2005). Global climate change (coupled with
ter streams (Hayhoe et al., 2011). On the other hand, in large-scaleland-coverchange)isexpectedtoshiftthese
the Araguaia River Basin a 25% increase in annual dis- hydrological regimes by decreasing mean annual rain-
charge (due to large-scale land-cover change) increased fall and increasing the frequency of extreme weather
bed load transport by 31% and completely restructured events (e.g., droughts and floods). Such changes in the
theriver’smorphology(Latrubesseet al.,2009;Coeet al., inundationregime,particularlyreducedfloodmaxima,
2011). In contrast, hydroelectric projects on whitewater could reduce selection for flood-tolerant species and
rivers such as the Madeira are expected to trap large alterthespeciescompositionoffloodplainforests(Nils-
amountsofsediments,reducingsedimenttransportand son & Berggren, 2000). Reduced flood maxima could
potentiallyalteringriverfloodplainmorphology(Fearn- reduce lateral exchanges between river channels and
side, 2013). Such changes in sediment dynamics and floodplains, decreasing nutrient recycling and associ-
water temperature may affect incubation and develop- atedbiologicalproductivity(Nilsson&Berggren,2000)
menttime,sexdetermination,growthrates,andmetabo- and hence altering regional Cbudgets, including GHG
lism of some species, particularly ectotherms (e.g., fish, emissions.Reducedfloodmaximacanalsoincreasethe
reptiles). Thenestingoutcomes of turtle speciessuch as frequency, severity, and ecological impact of fires,
the giant Amazon river turtle (Podocnemis expansa) and given that floodplain trees lack many traits associated
yellow-spotted sideneck turtle (Podocnemis unifilis) have with fire and drought resistance (Brando et al., 2012;
been linked to river dynamics, temperature, and the Flores et al., 2012). In the Middle Rio Negro, for exam-
grain size of sediments in the nesting area (Lubiana & ple, severe droughts have caused fires that killed over
FerreiraJu"nior,2009;FerreiraJu"nior&Castro,2010). 90%offloodplainforesttrees,whichshowedlittlesign
ofregenerationevenadecadelater(Floreset al.,2012).
Disruption of seasonal inundation regimes by dams
Deforestationofriparianareas
also disrupts the migrations of fish and other fauna
Human settlements and development activities have (Jackson&Marmulla,2001).MostdamsintheAmazon
disproportionatelyimpactedstreamriparianzonesand areconstructedinthemiddleandupperreachesofriv-
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173
10 L. CASTELLO & M. N. MACEDO
ers (Fig. 2), affecting resident and long-distance ecosystems, and combined they fail to address the full
migrants whose home ranges encompass the area. range of drivers of hydrological alteration (Fig. 5). The
Dependingonriverandreservoircharacteristics,atten- main limitation of these policies is their disregard for
uation of seasonal inundation regimes and reductions the role of hydrological connectivity in freshwater
in high-flood maxima can restrict access to floodplain ecosystemstructureandfunction.
food and habitat resources for fish populations far
downstream. Many other animal groups (e.g., turtles,
Protectedareas
caimans, otters, dolphins) are similarly affected by
attenuatedinundation.Lateralandlongitudinalrestric- The Brazilian Amazon enjoys a relatively high level of
tions of fish migrations by dams have led to dramatic protection, with a network of nature reserves, indige-
fishery impacts in the Araguaia-Tocantins Basin and nouslands,andsustainableuseareascovering~54%of
elsewhereintheworld(Ribeiroet al.,1995;Limburg& its area (Soares-Filho et al., 2010; Castello et al., 2013b).
Waldman, 2009; Fei et al., 2015). Continued hydroelec- Although it is touted as the model of Amazonian con-
tric development in the Amazon is thus likely to servation, this protected area network has limited
disrupt the ecosystem roles of many animals and capacity to protect freshwater ecosystems because its
reduce fisheries yields, with the potential to threaten designwaslargelybasedonthebiogeographyofterres-
regionalincomeandfoodsecurity(Castelloet al.,2015). trial taxa, with little regard for hydrological connectiv-
Such hydrological alterations also limit animal disper- ity (Peres & Terborgh, 1995; Abell et al., 2007). A large
sal and recolonization after extreme events, increasing proportionofheadwaterstreams,rivers,andotherwet-
the likelihood of biological extinctions over the long land types are unprotected, and many freshwater
run(Hess,1996;Fagan,2002),particularlyinheadwater ecosystems within protected areas are vulnerable to
streamswithhighspeciesdiversity. upstream threats (e.g., dams) outside their boundaries
(Pringle, 2001; Hansen & Defries, 2007). Furthermore,
manyprotectedareasoverlapcompetinglanddesigna-
Establishmentofreservoirconditions
tionsoraregovernedbylawsthatallowmining,forest
By replacing lotic habitats with lentic ones, the storage exploration, or hydroelectric development within their
ofwater inreservoirsthreatensspecialist endemicspe- boundaries (Ver"ıssimo et al., 2011; Ferreira et al., 2014).
cies and favors generalist species, leading to biotic Forexample,theoriginaldesignofBrazil’sBeloMonte
homogenization and reduced biodiversity (Poff et al., Hydroelectric Complex contemplated five separate
1997; Liermann et al., 2012). As a result, Amazonian reservoirswithinfederalindigenousreservesupstream
reservoirs are often heavily vegetated with macro- of the Belo Monte Dam in the Xingu River. Although
phytesanddominatedbyspeciesadaptedtolenticcon- the five reservoirs are not yet being constructed, they
ditions (Junk & Mello, 1990; Gunkelet al., 2003). Inthe mayeventuallybebuilttoallowBeloMontetofunction
Araguaia-TocantinsRiverBasin,forexample,construc- at capacity (Stickler et al., 2013). Brazil’s Congress is
tionoftheTucuru"ıDamledtoadominanceofpredator alsodebatingseveralnewlaws(i.e.,the‘MiningCode’,
species, and increased the abundance and biomass of PEC215,PL 3.682) thatcould openprotected areas and
detritivorous Prochilodontids and planktivorous indigenousreservestominingexploration.Inaddition,
Hypophthalmus spp. (Ribeiro et al., 1995). It is often Amazonian protected areas have been increasingly
argued that reservoirs create additional habitat, but downgraded, downsized, degazetted, and reclassified
habitatqualitymaybepoorerthanthenaturalhabitats since 2008, mainly to enable the generation and trans-
they replace. For example, today the 4500 km2 Balbina mission of hydroelectric power (Bernard et al., 2014;
Reservoir supports giant otter (Pteronura brasiliensis) Ferreiraet al.,2014).
populationstwiceaslargeasthosebeforeconstruction,
butfourtimessmallerthanthosepredictedbyavailable
Climateandland-usepolicies
habitat(Palmeirimet al.,2014).
BrazilandPeruregulateforestcoveronprivateproper-
ties. The Brazilian Forest Code requires landowners in
Policylimitations
the Amazon biome to conserve native vegetation on
Somepoliciespertinenttofreshwaterecosystemconser- 80%oftheirpropertyinforestedregionsand20–35%in
vation exist, including laws governing protected areas, Cerradoregions.Italsodesignatesriparianforestbuffer
conservation of forests on private properties, water zonesasAreasofPermanentPreservation(Soares-Filho
resource management, and environmental licensing of et al., 2014). Peru’s Forest and Fauna Law also man-
hydroelectric dams (Fig. 5). However, each of these dates the conservation of a 50 m riparian buffer zone
policies has limited capacity to protect freshwater along rivers and lakes. By requiring conservation of
©2015JohnWiley&SonsLtd,GlobalChangeBiology,doi:10.1111/gcb.13173