Andean Influences on the Biogeochemistry and Ecology of the Amazon River

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The Amazon River exits the Andes mountains morethan 4000 kilometers (km) from its estuary, but along itsessentially flat and serpentine path through the lowlands ofnorthern Brazil it maintains the character of an Andean river(figure 1). The indelible imprint of this distant mountain rangeon the main-stem channel of the worlds largest river has beennoted by naturalists and researchers for more than a century,but the multifaceted nature of Andean influences on the hy-drology, biogeochemistry, and ecology of theriver system haveonly come to light during the past two decades. Other fun-

damental but still obscure linkages remain to be discovered.Long before scientists took interest in the study of Ama-

zon environmentsin fact, long before Europeansdiscov-ered the rivernative peoples of the lowland Amazonrecognized the unique characteristics of Andean tributaries.Agriculture thrived on the fertile floodplains of these muddyrivers and gaverise to some of the regions first and most suc-cessful chiefdoms (Meggars 1984). Native Amazonians alsocapitalized on the rich fish stocks of Andean tributaries.Alfred Russel Wallace (1853) was perhaps the first naturalistto write about the white-water,clear-water, and black-water

river types of the Amazon basin and to relate the color oftributaries to the nature of their drainage basins (figure 2).Wallace astutely linked the sediment load of white-watertributaries to erosion in their steep Andean headwaters, andidentified clear-water rivers with the crystalline mountains

of Brazil (the Guyana and Brazilian shields). He knew thatblack-water rivers emerged from lowland sources, and hecorrectly attributed their darkcoloring to leaching of decayingleaves, roots, and other vegetable matter (Wallace 1853).Another naturalist of that time,Henry Bates (1863), marveledat the transport of volcanic pumice in the main-stem Ama-zon River and correctly assigned its origin to volcanic rangesthousands of kilometers away in the Ecuadorian Andes. Heimagined these porous stones as vehicles transporting seedsand insect eggsdownstreamandthereby dispersing organisms

far beyond their original ranges. Over the last 50 years,systematic investigations have further advanced scientistsunderstanding of the environment and distinct aquatic eco-systems of the lowland Amazon River (summarized in Sioli[1984], Junk [1997], and McClain et al. [2001]).

Steep terrain and young lithologies make the Andes animportant source of sediments and solutes to the lowerreaches of theAmazon River. The most visible characteristicsof the main-stem Amazon and its Andean tributaries arehigh discharge and heavy loads of suspended and bedload

Michael E. McClain (e-mail: [email protected]) is with the Department ofEnvironmental Studies at Florida International University in Miami, and

Robert J. Naiman (e-mail: [email protected]) is with the School of

Aquatic andFishery Sciences at theUniversity of Washington in Seattle. 2008

American Institute of Biological Sciences.

Andean Influences on the

Biogeochemistry and Ecology

of the Amazon River

MICHAEL E. MCCLAIN AND ROBERT J. NAIMAN

Although mountains often constitute only a small fraction of river basin area, they can supply the bulk of transported materials and exert strongregulatory controls on the ecological characteristics of river reaches and floodplains downstream. The Amazon River exemplifies this phenomenon.Its muddy waters and its expansive and highly productive white-water floodplains are largely the products of forces originating in distant Andeanmountain ranges. The Amazons character has been shaped by these influences for more than 10 million years, and its present form and host ofdiverse organisms are adapted to the annual and interannual cycles of Andean inputs. Although the Andes constitute only 13% of the Amazon Riverbasin, they are the predominant source of sediments and mineral nutrients to the rivers main stem, and Andean tributaries form productivecorridors extending across the vast Amazonian lowlands. Many of the Amazons most important fish species rely on the productivity of Andeantributaries and main-stem floodplains, and annual fish migrations distribute Andean-dependent energy and nutrient resources to adjacent lower-productivity aquatic systems. Mountain-lowland linkages are threatened, however, by expanding human activities in the Andean Amazon, withconsequences that are eventually felt thousands of kilometers away.

Keywords: Amazon, Andes, nutrient subsidies, land use, fisheries

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sediment. Associated with this sediment load are abundantorganic matter and nutrients. The ramifications of a high par-ticulate load are also far-reaching in their geomorphological,biogeochemical,andecological effectson thelowland river cor-ridor. Large sediment loads and flooding have created broadfloodplains, and associated nutrients support diverse andproductive floodplain forests, macrophyte beds, and lakes ofseasonal importance to thelife cycles of organisms in therivers

and adjoining uplands. In an important ecological feedback,the products of floodplain primary production eventually re-turn to the main-stem river in floodplain runoff, becomingimportant energy sources for heterotrophic communitiesliving there (Richey et al. 1990, Melack and Forsberg 2001).

Many fish also migrate annually into Andean tributariesfrom low-fertility black-water and clear-water tributaries tospawn and feed in resource-rich white-water channels andfloodplains. Upon their return, migrating fish transport or-ganic matter and nutrients that subsidize the food webs ofblack-water and clear-water rivers.

Many fundamental aspects of the geomorphology, bio-geochemistry, and ecology of the main-stem Amazon are

thereforelinked to themagnitude andvariability of water andmaterials supplied from the Andes. In fact, the dominantdownstream trend in biogeochemical and trophic charac-teristics of the main-stem Amazon and its large Andean trib-utaries is theprogressive dilution of Andean contributions by

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Figure 1. Nine major rivers flow from the Andes to form fertile corridors across the lowland Amazon (shown in bold). TheUcayali, Maraon, and Napo rivers drain southern Ecuador and northern and central Peru, converging to form the main-stem Amazonas, which becomes the Solimes River where it crosses into Brazil. The Caquet River flows from Colombia,becomes the Japur upon entering Brazil, and merges with the main stem at about 65W (west). The Putumayo Riverflows from Colombia and Ecuador to become the Ia in Brazil. The Madeira River collects the Andean tributaries flowingfrom southern Peru and Bolivia and traverses thousands of kilometers of lowland Amazon rainforest before merging withthe main-stem Amazon at about 59W.

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lowland tributary inputs (Devol and Hedges 2001).Even though we are beginning to understand thedynamics of Andean-derived materials in themain-stem Amazon River corridor, and the degree towhich lowland ecosystems depend on upstreaminputs, we still know little about the nature andvariability of processes that mobilize these materi-als from the Andes and modify them during down-stream transport and storage in the extensivefloodplains.

In this article, we briefly introduce the geo-morphology and ecological zones of Andean head-water regions of the Amazon, as these are poorlyknown even among scientists specializing in Ama-zon ecology. We then examine the multifacetedways in which the main-stem Amazon River isinfluenced byand depends onAndean inputs.We conclude by exploring frontiers in research link-ingAndean and lowland parts of theAmazon, con-sidering the possible impacts of increasinghuman-related development and climate change inthe Andean Amazon.

The Andean AmazonTheAndes mountains rise steeply alongthe westernmargin of theAmazon basin and stand3000 metersabove sea level (masl) in elevation over much oftheir length(figure 1).Approximately half of theAn-dean Amazon lies at elevations between 500 and

2000 masl, while most of the remainder is between2000 and 4000 masl; about 16% is above 4000 masl(table 1).The highest point in the Amazon basin isthe Nevado de Huascaran in the Cordillera Blancaof Peru, at 6768 masl,but several otherpeaks extendabove 6000 masl. Active volcanoes are prominentfeatures of theEcuadorian andBolivian Andes. Theeastern cordillera of theAltiplano,a high-elevationendorheic basin containing Lake Titicaca,forms onone of the widest sections of the Andes, spanningnearly 300 km near the lake.

Characterization of the precipitation, soils, andvegetation of theAndeanAmazon is fundamental tounderstanding Andean influences on the lowerAmazon River (figure 3).Precipitation is greatest onthe lower and mid slopes of the cordillera (500 to3000 masl) because of orographic controls on airmasses coming from the east. The wettest parts ofthe basin lie in the eastern cordillera of Colombiaand near the PeruBolivia border, where annual

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Figure 2. The main rivers of the Amazon have long been classified according to the color of their waters, which also reflects

their source. (a) The Ia (Putumayo) River is a characteristic white-water river colored by the high loads of sedimentstransported from the Andes. (b) The Negro River is the largest of the black-water rivers, tinted by high levels of dissolvedorganic matter leached from low-lying areas of sandy soils. (c) The Rio Tapajos is the most notable of the clear-water riverscarrying low levels of sediments and organic matter from the crystalline Guyana and Brazilian shields. Photographs:Margi Moss (http://brasildasaguas.com.br).

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precipitation may exceed 4 meters (figure 3a). The mostabundant soil order in the Andean Amazon is inceptisol(61%), a young, mineral-rich soil that occurs at midelevations.More developed but less fertile ultisols occupy 16% of there-gion and occur mostly at lower elevations in Peru. Mollisols,or grassland soils, are the third most abundant soil order, cov-ering 6% of the region, primarily near the PeruEcuadorborder and at higher elevations in southernPeru. Exposed rockis common at very high elevations (greater than 4000 masl)in southern Peru.

The major vegetative cover types in the Andean Ama-zonmapped using Advanced Very High Resolution Ra-diometersatellite imagery (Eva et al.1998)are submontane(700 to 2000 masl) and montane (2000 to 3700 masl) forests,which together constitute approximately 42% of the region(figure 3b, table 2). Montane herbaceous vegetation inter-spersed withshrubland andagriculture is also widespread,cov-ering nearly a quarter of the region. As of 2000, at least 40%of the region had been converted to human uses or frag-mented by these uses (JRC 2000). The most intense humanalteration has historically been at high elevations (> 3000masl), where high levels of alteration continue today; butchange is increasingly concentrated at mid and lower eleva-tions as colonization continues and roads spread across theregion (Mena et al. 2006).

The modern Amazon River is born in numerous Andeansprings, but cartographers locate the most distant source ofthe river at 5300 masl on the northern slope of NevadoMismi.From this stream, the Carhuasanta, the main stem ofthe Amazon, changes names at least nine times: from Car-huasanta to Lloqueta, Hornillos, Apurimac, Ene, Tambo,Ucayali,Amazonas, Solimes, and finally Amazon below theconfluence of the Solimes and Negro rivers. The entirenorth-south length of the Andean Amazon basin is drained

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Table 1. Elevation ranges of the Andean Amazon.

Elevation (meters Area Areaabove sea level) (square kilometers) (percentage)

5001000 111,804 18

10012000 170,514 27

20013000 117,018 19

30014000 120,671 1940015000 100,766 16

> 5000 2444 < 1

Total 623,217 100

Source: Compiled from Shuttle Radar Topography Mission90-meter data.

Figure 3. (a) Areas of higher precipitation are focused on the lower slopes of the Andes, with maximal registered precipitationin the headwaters of the Madre de Dios River in southwest Peru and the Napo River of central Ecuador. (b) Montane forestsdominate the land cover between 500 and 3000 meters above sea level and transition into natural high-elevation grasslandsabove. Compiled from Shuttle Radar Topography Mission 90-meter data and Global Land Cover 2000 data (CJRC 2000).

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by eight major riversthe Caquet, Putumayo, Napo,Maraon,Ucayali, Madre de Dios,Beni, and Mamor (figure1).

Andean influences on the loadof the Amazon main stemThe main-stem Amazon River integrates the flow of sub-basins containing distinct combinations of geology, soils,and vegetation. There are four major Andean tributaries tothe main-stem Amazon River: the Solimes, Ia,Japur,and Madeira (figure 1). (Andean tributaries

to the main stem are defined as those with head-waters above 500 masl in the Andes mountains,as-suming that the western limit of the main-stemAmazon River is setas theBrazilColombia border.)Where they intersect with the main stem, the com-bined mean annual flow of these white-water trib-utaries is approximately 90,000 cubic meters persecond: roughly half of the main-stem AmazonRivers mean annual discharge, or five times theflow of the Mississippi River (Dunne et al. 1998).

TheAndes cover only about 13% of theAmazon

basin upstream of bidos, and Andean tributariesmay flow through hundreds to thousands of kilo-meters of lowlands (below 500 masl) before con-necting withthemain stem.Yet most measurementsof Andean contributions to the main-stemAma-zon have been made at the main-stem confluencesof the four Andean tributaries. Clearly these rivershave accumulated water, particulates, and solutesfrom the lowlands before reaching the main stem,and therefore one must be careful to consider whatpart of these loads actually derived from the Andes

rather than from the lowlands. In the case of water,wenoted thatthe combined flow of the Andeantrib-utaries amounts to approximately half of main-stem flow, but the volume of water actuallyoriginating in the Andes is probably roughly pro-

portional to the areal coverage of the Andes. Although annualprecipitation on the lower slopes of the Andes exceeds theAmazon average, higher valleys of the Andes are more arid,and thus the average precipitation for the entire range is notlikely to be greatly different from precipitation for the basinas a whole. But while Andean contributions of water to themain-stem Amazon may be proportional to area, contri-butions of sediments and solutes are disproportionatelygreater. Moreover, energy and nutrients carried from theAndes by the river appear to largely drive main-stem pro-ductivity, both directly and indirectly through biophysicalfeedbacks with the massive lowland floodplain.

Inorganic sediments and solutesFour decades ago, Ronald J. Gibbs wrote that the Andeanmountainous environment controls the geochemistry of theAmazon River (Gibbs 1967). He had sampled the Amazonmain stem and 16 of its major tributaries and had comparedtotal particulate and solute concentration data for the wet anddry seasons against nine environmental parameters. On thebasis of strong correlations with the environmental param-eter mean relief, Gibbs concluded that the Andes were thesource of 82% of the total suspended solids exported by theAmazon River. The importance of Andean sources of sus-pended sediment to the main-stem Amazon River was re-affirmed bythe subsequent work of Robert Meade and others,who concluded that between 90% and 95% of thesuspended

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Table 2. Land cover of the Andean Amazon basin.

Area AreaLand-cover class (square kilometers) (percentage)

Forest (includes areas 329,574 53of fragmented forest)

Grassland and shrubland 215,755 34(includes pasture)

Wetland 231 < 1

Cropland 71,216 11

Dryland 6375 1

Water 1832 < 1

Ice 1031 < 1

Urban 394 < 1

Totals 626,408 100

Note: The difference in the total area reported in tables 1 and 2 isdue to grid size differences of the initial raster data sets.

Source: Compiled from Global Land Cover 2000 data (JRC 2000).

Figure 4. The disproportionate loads of sediments carried by the mainAndean tributaries are evident when comparing the inflows of (a) waterand (b) sediments to the main-stem Amazon river from its major tribu-taries. Inputs at the top of each diagram represent the contributions ofthe Amazonas/Solimes River flowing from Peru. Data were compiled byR. H. Meade from water-discharge data listed by Carvalho and da Cunha(1998) and from the sediment-discharge data of Dunne and colleagues(1998).

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sediment load of the main stem derived from the Andean trib-utaries (figure 4; Meade 1984, Meade et al. 1985).

Returning to the question of how much of the water andsuspended particles carried by the Amazon River originatefrom the Andes mountains, we speculated that less than aquarter of the water originates in the Andes but that mostsuspended sediments could originate in mountain areas.Loads of suspended and bed sediments measured along theentire lengthof the Madeira River, from itsAndean headwatersto its confluence with the main stem, show a sharp decreasein sediment load (as much as 60%) at the base of the Andes,a decrease in the mean diameter of suspended particles in thepiedmont region, and a progressive decrease in the meandiameter of bed sediments (Guyot et al.1999)all indicatorsof a declining energetic capacity to transport materials.Thesecharacteristics indicate that Andean rivers supply more thanenough sediment to account for the total load of sedimentsin the lowland sections of theAndean tributaries.Conclusiveevidence of an Andean source is found in the mineralogicaland isotopic composition of the suspended sediments. Themineral composition of sediments in the main-stemAmazoncorrelates well with that of the Ucayali and Maraon riversin the Andes (Gibbs 1967). Measurements of neodymium,strontium, and lead isotopic ratios reaffirm that Andeansources account for an overwhelming proportion of themain-stem sediment load (Allegre et al. 1996).

Andean-derived suspended sediments bring a large flux ofminerals into the main-stemAmazon River, butthey also bringother elements and materials. Andean tributaries deliver an

order of magnitude more particulate nitrogen (1170 mega-grams [Mg] per year) and phosphorus (806 Mg per year) tothe main stem than their lowland counterparts (119 and 43Mg peryear, respectively;Richeyand Victoria 1993).Most par-ticulate nitrogen is likely to be organic, whereas phosphorusis mainly phosphate strongly adsorbed to iron and aluminumoxide surfaces (Berner and Rao 1994). The availability ofthis phosphorus to main-stem organisms is not known, butsignificant amounts of phosphorus are released from Ama-zon sediments upon entering the estuary and may be avail-able to organisms on the floodplains (Melack and Forsberg

2001). The question of whether particulate nitrogen andphosphorus actually derive from the Andes or from someintermediate river section is tied to theorigin of thefractionswith which they are associated. The tendency of phosphatetoadsorb to mineral surfaces links this nutrient to the Andeansources of the mineral sediment, but the organic associationof nitrogen is tied to that of the particulate organic fraction,which is less well understood.

Two features of the Andes enhance their importance tothe solute geochemistry of the Amazon River and to its eco-logical characteristics. First, the Andes contain the only sig-

nificant deposits of evaporites and carbonates in theAmazonbasin (Stallard and Edmond 1983).High fluxes of Ca2+ (cal-cium), Mg2+ (magnesium),HCO3

(bicarbonate), and SO42

(sulfate) ions occur in rivers draining carbonate deposits,and high fluxes of Na+ (sodium) and Cl (chloride) ions

occur in rivers draining evaporite deposits. Rivers drainingbasins containing carbonates generally have total cationcharges of 450 to 3000 microequivalents (eq) per liter (L),and rivers draining basins containing evaporites may havetotal cation charges of greater than 70,000 eq per L near thesalt sources (Stallard and Edmond 1983). The rich mineralcontent of Andean tributaries underpins the ecologicalproductivity of downstream reaches.Black-water and clear-water tributaries draining lowland portions of the basin, bycontrast, have total cation charges below 300 eq per L andare characteristically considered to have low ecosystem-scaleproductivity. The second distinguishing feature of theAndesis the intensity of its weathering regime, which increases theconcentration of ions in solution. Among the Amazon trib-utaries that drain basins dominated by less-weatherablesilicate rocks, Andean rivers have consistently higher totalcation concentrations (Stallard and Edmond 1983).

Few data exist that would allow us to estimate the pro-portional contribution of major ion fluxes to the main stemfrom the Andes. Robert Stallards work demonstrates thatsolute concentrations are elevatedin Andean rivers,butwith-out measurements of discharge it is not possible to calculatefluxes. Furthermore, one-time flux measurements are notrepresentative of annual or interannual contributions to themain stem. Unfortunately, no suitable data exist for Colom-bian, Ecuadorian, or Peruvian Andean tributaries, and thusno estimation can be made regarding the Andean contribu-tion of major ions to flow in the Solimes River from thesecountries.We may speculate,however, on thebasis of the high

ion concentrations in Andean rivers, that the Andean con-tribution to the main-stemsolute loadis dominant,especiallyfor certain elements found preferentially in Andean litholo-gies. For the headwaters of the Madeira River in Bolivia,An-dean fluxes can be estimated with some confidence, thanksto a 10-year data set (Guyot et al. 1992). Over the period ofthese data, the specific flux of total dissolved solids from An-deanbasinswas 80 Mgper km2 per year, while thespecific fluxfrom lowland Bolivian basins was 7 Mg per km2 per year. Theheadwaters of the Madeira River contain few carbonate andevaporite deposits in comparison with the headwaters of the

Solimes River in Peru.Thus it is likely that the Peruvian An-descontribute an even larger percentage of the major ions de-livered to the main stem.

Organic matterAndean-derived suspended sediments carry a significantamount of organic matter, 90% of which is made up of par-ticles less than 63 micrometers (m) in diameter (Richey etal. 1990). Variations in the fluxes of fine particulate organiccarbon (FPOC; particles < 63 m) along the main stem cor-relate closely with variations in suspended sediment fluxes,

suggesting a close physical association. In fact, the vast ma- jority of FPOC (> 90%) cannot be physically separated frommineral material and is therefore probably physically boundto it (Keil et al. 1997). This physical association has beenshown to reduce therate of organic matter decomposition and

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enhance its preservation. Total organic carbon is approximately1%, by mass, of suspended sediment in the main stem, con-stituting a flux of 5 to 14 teragrams (Tg) of carbon per yearto the Atlantic Ocean (Richey et al. 1990).

Measurements show that more than 90% of particulateor-ganic carbon (POC; > 0.5 m) in the main-stem AmazonRiver comes fromAndean tributaries, buthow much actuallyoriginates in the Andes Mountains? POC behaves moreor lessconservatively in the main stem, suggesting that it resistsdecay and is derived from distant sources (Richey et al. 1990).Just how refractory and how distant the sources are can be es-timated from a suite of molecular, elemental,and isotopictech-niques used to characterize the organic matter and to trace itback to its sources (Hedges et al. 1986, 2000, Aufdenkampeet al. 2007). Concentrations of total lignin-derived phenols,carbon-to-nitrogen ratios, and stable carbon isotope ratiospoint to terrestrial plants, and more specifically the leaves ofterrestrial plants, as the main source of main-stem organicmatter.Algae and aquatic plants, so abundant on the exten-sive Amazonian floodplain, are important sources of labileorganic matter, fueling microbial metabolism in the mainstem, but do not persist in the system (Richey et al. 1990).Thedepletion of carbohydrates and the increasing abundancesof nonprotein amino acids and diagnostic lignin-derivedphenols confirm that the organic matter is highly degraded,especially the FPOC fraction. Moreover, these characteristicsignatures extendup theMadeira andSolimes riversand intothe Andean foothills (Hedges et al. 2000, Aufdenkampe etal. 2007). Richey and colleagues (2002) estimated that the

main-stem Amazon River transports only 7% of the organicmatter supplied to the river basinwide, supporting the find-ing that it also transports the most degraded and recalcitrantmaterials.

The isotopic data, however, provide the most definitiveinformation on theageandgeneral source area of particulateorganic matter in the main stem and its Andean tributaries.For main-stem FPOC to have a true Andean source, much ofit would have to be hundreds to thousands of years old. Thisis because little main-stem FPOC (and little of the fine sed-iment with which it is associated) is transported directly

from the Andes; most is stored for varying periods of time inpoint-bar and floodplain sediments (Dunne et al. 1998).FPOC does, in fact, have the lowest levels of bomb carbon-14 (14C) of any organic matter fraction in the main-stemAmazon (+19 14C per thousand []), suggesting an aver-age turnover time of hundreds of years (Hedges et al. 1986).Allowing for the dilution of the bomb 14C signal by youngerorganic matter, this implies that a significant portion ofmain-stem FPOM may be Andean.

The actual proportion of FPOC of Andean origin has beenapproximated using delta carbon-13 (13C) stable isotopic

ratios as a fingerprintof its origin. The13

C of plant leavesis positively correlated with elevation, and ratios in thePeruvian Andes have been found to range from about 30at 1000 to 2000 masl to 26 at 4000 masl (Townsend-Small et al.2005, 2007).Thevalues of leaves from prominent

floodplain and upland forest trees along the main-stemriveralso average 30, indicating that there is no clear isotopicseparation of leaf13C between lowland forests and Andeanforests below 2000 masl of elevation (approximately 50% ofthe Andean Amazon area; table 1). Unlike plant leaves,how-ever, there is a clear separation of FPOC13C between Andeanand lowland rivers, and this separation can be used toestimatethe relative proportion of each in the main stem. FPOC inpurely lowlandrivers has13C values consistently near 28.5(Quay et al. 1992). The 13C of FPOC discharged in themain-stem Amazon River at bidos is 27.4 and thusindicates a mixture of theAndean and lowland sources.If thePeruvian value for13C of FPOC exiting the Andes (approx-imately 26.5) is taken as the Andean end member and28.5 is taken as the lowland end member, FPOC at bidosis a mixture of 50%Andean FPOC and 50% lowland FPOC.Alternatively, if the Bolivian end member of 25.5 is used,FPOC at bidos is a mixture of 33% Andean and 67%lowland FPOC (Quay et al. 1992, Hedges et al. 2000).

Interestingly, the 13C of FPOC in each of the majorAndean tributaries (the Solimes and Madeira rivers) wherethey meet the main stem is 26.8. This suggests that theserivers carry FPOC that is largely of Andean origin and accountfor 82% of the FPOC input to the main stem. If only 30% to50% of FPOC entering the Atlantic Ocean is of Andean ori-gin, then there is a 50% to 70% reduction in Andean-derivedFPOC in the main-stem section of the river. This reductionprobably occurs through sediment exchange with the flood-plain and gradual decomposition of Andean organic matter

while in storage. Recentresearch using a dual-isotope approach(14C and 13C) estimated the degree of mineralization ofAndean-derived FPOC with transport downstream andconcluded that nearly all Andean FPOC was mineralized inthe river and floodplain system (Mayorga et al. 2005). Takentogether, the Andes largely regulate the particulate load tothe main-stem Amazon River, not simply with respect to itsparticulate mineral load but also with respect to associatednutrients and organic matter.

The four major Andean tributaries contribute approxi-mately 50% of the dissolved organic matter (DOM) input to

the main stem (Richey et al. 1990), but unlike particulateorganic matter, this DOM appears to derive largely fromlowland sources. Neither mass-balance nor chemical-tracerapproaches support important Andean contributions ofDOM to the lowland or main-stem Amazon. DOM accu-mulates in swampy environments that arecommon through-out the lowland Amazon, and in rivers and streams that drainareas of spodosol soils (McClain and Richey 1996). In thecentral Brazilian Amazon, fluxes of DOM to groundwater inthe spodosols characteristic of the Rio Negro subbasin areapproximately 20 times greater than those in the oxisols

characteristic of much of the rest of the lowland Amazon(McClain et al. 1997). In the Rio Negro basin, high ground-waterDOM concentrations (approximately 3000 micromolesof carbon) also appear in surface water draining spodosols,whereas in oxisol terrains, fringing wetlands appear to be

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important sources of DOM. DOM concentrations are uni-formly low in the few studies on Andean rivers (Guyot andWasson 1994, Hedges et al. 2000, Saunders et al. 2006). In theMadeira subbasin, there is a distinct increase in DOC con-centrations in rivers below 500 masl, and this additionalDOC appears to derive from floodplains and wetlands suchas those of the Bolivian Llanos de Mojos (Guyot and Wasson1994).

Andean influences on the productivityof the main-stem AmazonThe productivity of the main-stem Amazon is tied to theproductivity of its floodplain, a system built of Andean-derived materials and fueled by mineral nutrients from theAndes (Melack andForsberg 2001).Over a 2010-km reach ofthe Amazon main stem, the mean lateral flux of sediments(1570 to 2070 Tg per year) between the channel and adjoin-ing floodplain exceeds the downstream flux (1200 Tg per

year), andapproximately 500 Tg per year of upstream-derivedsediment and associated nutrients accumulate on the flood-plain and in channel bars (Dunne et al. 1998). This processbuilds thefertile floodplain soils alongAndeantributaries andthe main stem. By contrast, floodplains along non-Andean,lowland tributaries are farmore depleted in mineral nutrients.The Amazon River maintains year-round lateral exchangeswith its floodplain, and especially with its abundant lakes.The floodplain is a highly productive system, with an estimatedregional net production of 113 Tg of carbon per year occur-ringoveranareaof 67,900km2, from theBrazilianColombian

border to near the rivers mouth (figure 5; Melack and Fors-berg 2001). This translates to 17 Mg carbon per hectare per

year, which exceeds the productivity of upland Amazonforests by a factor of five; in fact, the Amazonian floodplainis among the most productive ecosystems on Earth. The

majority of primary productivity is attributed to macrophyte(65%) and floodplain forest (28%) communities. Subtract-ing estimates of carbon loss to respiration and burial, about90 Tg carbon per year are available for export to the main-stem river, where the additional carbon fuels respiration(Melack and Forsberg 2001, Mayorga et al. 2005).

A portion of the supply of Andean nutrients to the flood-plain can eventually be traced back into the main stem not onlyas labile organic matter but as part of myriad organisms thatmove between thefloodplain and channel.Large numbers offish move onto the floodplain annually to exploit its pro-ductivity and utilize its habitats (Goulding 1993). In fact,annual movements onto the floodplains of Andean-influencedwhite-water rivers are the most common form of migrationamong Amazon fishes and are critical to maintaining theregions fisheries (Goulding et al. 1997). Of the 24 species intheBrazilian Amazon that are most important to humans (innutritional and economicterms), most migrate as part of theirlife cycle, and most rely to some extent on the resourcesdelivered from the Andes (Araujo-Lima and Ruffino 2004).One of the most sought-after fish is the tambaqui(Colossomamacropomum). This omnivorous/frugivorous fish occursover the length of white-water rivers but only in the lowerreaches of black-water rivers. It feeds in flooded forestsduring high water and migrates back into the channelduring low water. Tambaqui, like many other species, spawnsalong the margin of white-water rivers, and the larvae arewashed onto floodplains by the rising waters. There theyfeed and seek shelter beneath the ubiquitous macrophyte

beds (Araujo-Lima and Goulding 1997).A number of othercharacids important to Amazon fisheries (Brycon spp.,Mylossoma spp., Myleus spp.) also follow this migrationpattern (Araujo-Lima andRuffino 2004),using thefloodplainfor feeding and nursery habitats and for transporting

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Figure 5. Nutrients and mineral substrates carried by Andean tributaries and deposited on floodplains fuel the highest

primary productivity rates per hectare in the Amazon basin. This schematic illustrates the balance of organic carbon onthe main-stem Amazon floodplain between 70.5W (west) and 52.5W (refer to figure 1 for extent). This balance indicatesthat large quantities (approximately 90 teragrams) of organic matter are returned to the river channel annually to fuelin-channel respiration. All quantities are for total organic carbon unless otherwise noted. Source: Melack and Forsberg(2001) and Richey and colleagues (1990). Abbreviations: DOC, dissolved organic carbon; POC, particulate organic carbon.

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resources back to the river as they migrate. Isotopic tracershave shown that C3 macrophytes, floodplain trees, andphytoplankton account for 82% to 97% of the carbon in 35species of adult fishes examined (Forsberg et al. 1993).Phytoplankton, while accounting for a small proportion ofthe total primary productivity on floodplains, represents theprimary source of carbon to characiform fishes (Araujo-Lima et al. 1986).

Migrations are also important in distributing theenhancedproductivityof Andean-influenced white-water riversandtheirfloodplains to less productive black-water and clear-waterenvironments. ManyAmazon fish migrate from black-waterand clear-water rivers to the main stem and other white-water rivers to spawn. In fact, all commercially importantspecies appear to spawn only in white waters (Goulding et al.1997). During times of the year other than the spawningseason, some move back into black-water and clear-waterenvironments,andin the event of predation or death,theor-ganic matter and nutrients of their bodies serve as subsidiesto these less productive ecosystems. Jaraqui (Semaprochilodusspp.) is an example of a fish that migrates from black-waterrivers into white-water rivers to spawn (figure 6a). Thesepredictable migration routes are stalked by larger predatorsthat congregate at the confluences of black-water and white-water rivers, such as the Amazon River dolphin,or boto (Iniageoffrensis).

Many other fish use the main stem and its Andean tribu-tariesas migrationcorridors,most notably largepredatory cat-fish (Pimelodidae) moving upriver to Andean spawning

areas. Catfish making long-distance migrations are quanti-tatively the most important predators in the river system,andthey are also the most important species to fisheries alongtherivers length (Barthem and Goulding 1997). The most re-markable of thesemigrations is that of the dorado,or dourada,catfish (Brachyplatystomaspp.; figure 6b), which travels as faras 5000 km in one direction (Goulding et al. 2003). Statisti-cal data on size classes along the entire length of theAmazonRiver reveal that dorado spawn in headwater regions (in-cluding Andean foothills) and that the young are washeddownstreamto nurseryareasin theAmazon estuary(Barthem

and Goulding 1997). Preadult dorado move upriver again,completing the approximately 8000-km migration over sev-eral years. Dorado and a number of other migrating catfishare heavily fished along the river, so their numbers are sig-nificantly reduced by the time they reach the rivers of the pied-mont and Andean foothills.

In Andean piedmont regions, characins emerge as themost important fishery species in biomass; the most im-portant among these is Prochilodus nigricans, known asboquichico in Peru.Boquichico is a fine-particle feeder that in-gests detritus and algae,and has a maximum length of lessthan

40 centimeters.Duringthe low-water season, it lives in flood-plain lakes and channels of the Amazon piedmont, but at theinitiation of rising water it leaves thefloodplain and migratesen masse upAndean tributaries to spawn (Diaz-Sarmiento andAlvarez-Len 2004). Collectively, thefishmigrations illustrate

the critical connections between theAndes and downstreambiotic communities andecologicalprocesses,as well as theim-portance of maintaining both lateral and longitudinal con-nectivity throughout the Amazon.

Enormous sediment loads, fluxes of nutrients and refrac-tory organic matter, and ultimately the fertility of the ex-pansive floodplains reflect the many influences of distantAndean mountain ranges on the main-stem Amazon andother white-water tributaries (figure 7).The rivers characterhas been shaped by these materials for more than 10 million

years, and its present form and host of diverse organismsare adapted to the annual and interannual cycles of Andeaninputs. It is safe to say that the ecology of the modern Ama-zon main stem has been built on substrates and nutrients de-

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Figure 6. Migrations of many Amazon fish are strongly in-fluenced by the pursuit of resources and habitats tied toAndean tributaries. (a) The jaraqui (Semaprochilodusinsignis) is an example of species that, as adults, livemostly in black-water rivers or lakes, but migrate towhite-water rivers to spawn. Juvenile jaraqui also usewhite-water floodplains as their nurseries. (b) Thedourada (Portuguese) or dorado (Spanish) catfish(Brachyplatystoma spp.;B.rousseauxii in photo) are thefarthest-migrating species known in the Amazon. Theyhatch in the Andean foothills, use the Amazon estuary astheir nursery, and then migrate thousands of kilometersup Andean tributaries to spawn. Photographs: MichaelGoulding.

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rived from the Andes, and that the decoupling of the main-stem Amazon from its mountain headwaters would lead todramatic changes in therivera pattern reflected in many ofthe worlds other great rivers.

Andean processes regulating fluxesto lowlands: A research frontierTheAndes exert strong influences on themain-stemAmazon,and these influences strengthen as onetravels upstream alongthe major Andean tributaries. But what processes regulate thefluxes of Andean derived materials, and how do theseprocessesvary spatially and temporally in the Andean Amazon? Un-fortunately, little research to date addresses these questions,and obtaining regional numbers is exceedingly difficult.Nevertheless, current rates of land-use change in the An-dean Amazon are among the highest in the Amazon basin;40% or more of the region already has been significantlyfragmented and otherwise affected by human alterations(Eva et al. 1998).How will land-use change and possible flowregulation alter fluxes of particulates and solutes to the low-land Amazon,andwhat other forms of contamination mightbe emitted by growing mountain populations? Research

addressing these human-related questions is still relatively re-stricted spatially in theAndeanAmazon, but such research isessential forthecoming decadeif effective regionalagreementsare to be forged about the future of the Amazon basin.

Concerning sediment fluxes, it is important to note that in-stantaneous loads in lowland rivers are largely decoupledfrom those in mountain rivers. Where lowland Andean trib-utaries remain white with high sediment loads year-round,mountain rivers are generally clear during the dry seasonand white only during storm-runoff events (Townsend-Smallet al. 2008). Their sediment fluxes may fluctuate greatly ondaily or weekly timescales in response to individual storm andlandslide events (Guyot et al. 1999), whereas lowland riverfluxes, like their hydrographs, fluctuate according to damp-enedseasonal cycles.Meandering lowland riversmaintaintheirsediment loads by continually resuspending and depositingmaterials within their channels (Meade et al. 1985,Dunne etal. 1998), effectively mining sediments accumulated in thepiedmont over long timescales through discretedepositionalevents (Aalto et al.2003). To understand mountain-lowlandlinkages, one therefore needs to consider erosional processesover a broad range of timescales.

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Figure 7. Andean influences on the ecology and biogeochemistry of the Amazon may be grouped into three interacting sets ofprocesses. Andean exports of water, sediment, nutrients, and organic and biological material exert fundamental control and

produce the white-water characteristics of Andean tributaries and the mainsteam Amazon itself. Floodplain building bythese Andean-derived materials provides the substrate and nutrition fueling productive flooplain forests, macrophyte beds,and lakes. Fish migrate throughout these systems and along tributaries, capitalizing on the productivity of white-waterriver systems and transferring a small quantity of Andean-derived energy and nutrients to nutrient-poor black-waterand clear-water systems.

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At timescales stretching into millions of years, and at thespatial scale of the entire mountain range, climate seems toexert a fundamental control on erosion processes in theAndeanAmazon. Montgomery andcolleagues (2001) analyzedthe topographic, climatic, and tectonic variability of the en-tire Andes cordillera and concluded that morphologyis more

closely related to climate than to tectonic processes.Erosion from the mountain range over the past 25 millionyears has come predominantly from the northern AmazonAndes (north of 15 south), where historical rates of erosionare up totwice as high as in the drier southern portion of theAmazon Andes (southern Peru and Bolivia). Linked to thislong-term erosional history, a striking and relevant geomor-phological characteristic of the high Andes is a shift fromsteep-sided, V-shaped valleys to gently sloped, U-shapedvalleys between 3000 and 3500 masl. Although much re-duced in size today, glaciers have been important in shaping

highAndean valleys. Moreover, the gentle valley slopes exposedby glacial retreat result in reduced physical erosion in thehighest portions of the Andes.

At subregional spatial scales and shorter timescales, vege-tation may assume a first-order control of erosion rates.Erosion rates in the Beni and Mamor river basins of Boliviarange from521 to6000metric tons per km2 per year and from310 to 2600 metric tons per km2 per year, respectively (Guyotet al.1988). Topography, lithology, rainfall, and vegetation allplay roles in explaining differences in erosion between basins,but vegetation plays the dominant role. Rates of erosion are

greatest in the southernmost basins, where vegetation issparse. In the north, where rainfall is greater but subbasinsareheavily forested, erosion rates are considerably lower.

The controlling influence of vegetation on erosion at bothsubregional andhillslope scales is significantbecause land-usechangeis themost prolific form of anthropogenic disturbancein the Amazon (figure 8). Erosion is less intense in densely veg-etated parts of the Andes, despite high rainfall on erosion-prone slopes.The stabilizing effects of natural vegetation arelost,however, followingdeforestation,and land managementpractices become important variables in explaining fluxes of

sediments,organic matter, and nutrients from newly createdagricultural fields and pastures. Studies conducted in mid-elevation (2000 to 2500 masl) valleys of the Peruvian Ama-zon find increased fluxes of sediments, organic matter, andnutrients in rivers draining valleys with greater proportionsof agriculture and pastures (Waggoner 2006). Similar trendshave been observed in the Napo River basin of Ecuador,where clear correlations were found between overall riverhealth andthe level of anthropogenic alterations (Celi 2005).Continued investigations of land-use impacts on stream andriver sediment loads are one of the most pressing research

needs in the Andean Amazon today. Studies of land-useimpacts on rivers and streams should emphasize riparianzones,both because they are control points for land-to-rivermaterial transfers (Naiman and Dcamps 1997,Naiman et al.2005) and because they are favored for agriculture in the

Andean Amazon as a result of the relative fertility of their soils(McClain and Cossio 2003).

It was recognized earlyon that concentrations of major ionsand trace elements in Andean Amazon rivers were linked tothe lithologies of the major subbasins, and subsequent workhas supported this link (Sobieraj et al. 2002).Themost focusedimpacts that humans have on major ions and trace-elementfluxes from the Andes is through mining, which is wide-spread at higher elevations. Contamination of soils and veg-etation by heavy metals has been documented near mines anddownstream of mining operations (Hudson-Edwards et al.2001).Accumulationsof metals in river invertebrateshave evenbeen measured downstream of the point at which contami-nation of bottom sedimentsis no longer detectable (Bervoetset al.1998).Mercury contamination from placer gold-miningoperations is a significant concern in manyAmazonian areas,and mercury accumulations in fish and in the hair of river-ine people have been linked to gold-mining operations as faras 150 km upstream in the upper Beni subbasin of Bolivia(Maurice-Bourgain et al. 1999). Although of considerablelocal concern, the current impacts from mining appear to belimited to river reaches immediately downstream of miningsites. Expansion of mining activities, however, may eventuallylead to significant changes in the fluxes of heavy and tracemetals to adjoining Amazon lowlands. Quantifying thecomposition, magnitude, and ecological consequences ofincreased heavy metal fluxes is an important need in the An-dean Amazon.

The dependence of lowland river corridors on sediments

and nutrients derived from theAndes requires unobstructedconnectivity between the two regions. No major Andeantributary to the Amazon is currently dammed, althoughBrazil is pursuing plans to build two major dams on theMadeira River. Hydroelectric installations arecommon,how-ever, on streams and small rivers close to major mining op-erations, to urban areas, or to other significant humansettlements. Peru has five significant hydroelectric projectsunder way in its Amazon region, and the Peruvian Ministryof Energyand Mineshas identified dozens more potentialdamsites, some on prominent rivers such as the Maraon, Hual-

laga, Tambo,andUrubamba. Dams trap large volumes of sed-iment, and could cause major readjustments over the longterm in the geomorphology of downstream river sectionsand the eventual sediment starvation of some downstreamreaches.Theill effects of dams on river organisms and riparianenvironments are well known (e.g., Dudgeon et al. 2006)and could be especially destructive in the Andean Amazon,where biodiversity is high and many fish species migrate an-nually between mountains and the lowland rivers and flood-plains. Far too little is known at this point about the extentto which riverine organisms and riparian environments rely

on open linkages between mountains and adjacent lowlandsin the western Amazon. It is therefore impossible to predictwhat the short- and long-term consequences of widespreaddam building would be.We suspect,on the basis of evidencepresented here and evidence from other regions with

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numerous dams, that eventually the consequences would besevere, as they have been for other rivers (e.g., the ColumbiaRiver in the United States).

A wild card in all discussions of future scenarios in theAndean Amazon is theeffect of climate change,includingthefeedbacks between land use and climate. There is alreadystrong spatial variability in todays Andean climate, dueto the

areas topographic complexity. Even though the response ofAndean environments to El Nio/La Nia events is compli-cated, thetrend is toward heavier than normal rainfall (Kane2000), resulting in increased landslide intensity. This maynot be the case,however, in the future.Rainfall in theAndeanAmazon is sensitive to the water balance of the lowlandAma-zon, and this balance is expected to change in predictableways.Because rain in the Andean Amazon is ultimately derived fromthe Atlantic Ocean, it must be transported across the lowlandAmazon basin in westward-moving air masses. During thiswestward movement, moisture cycles between the atmos-

phere and land surface, and estimations are that roughly55% of the rain falling in the Amazon basin is derived fromevapotranspiration within the basin (Marengo and Nobre2001). For the eastern slopes of the Andes, the percentage ofrainfall derived from evapotranspiration is probably higher.

Consequently, continued deforestation should lead to re-duced levelsof precipitationin theAndean Amazon (Chagnonand Bras 2005).

Both elevated carbon dioxide (CO2) and the conversion offorest to managed uses are predicted to reduce evapo-transpiration andthus theamount of water moving westwardtoward the Andes. Elevated CO2 alone is predicted to reduce

evapotranspiration in the Amazon by about 4% through re-ductions in stomatal conductance,andthis should also reducerainfall. Conversion of forest to pasture across the entireAmazon basin is predicted to reduce evapotranspiration byas much as 20% (Lean et al. 1996). These changes in theregional water balance will certainly affect terrestrial andaquatic ecosystems of theAndean Amazon and thereby fun-damentally alter the mountain-to-lowland fluxes discussedhere. As investigations of these questions proceed at a basinscale, and as confidence in predicted changes increases, An-dean policymakers should carefully examine local impacts.

The Amazon River system is unique in many waysbecause of its size and orientation along the equator, but thecontrols by its Andean headwaters are not unique. In fact,many of the mountain-lowland linkages we have discussedshould be relevant to other major river systems. Similar

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Figure 8. The Oxapampa Valley in central Peru illustrates a number of the forces threatening the ecological healthof Andean and downstream river reaches, including the deforestation and cultivation of steep slopes and the urbandevelopment of narrow valley bottoms. Future damming of valleys such as this could significantly affect downstreamfluxes of sediments and nutrients. Photograph courtesy of Thomas Saunders.

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controls are certainly observed in the adjoining OrinocoRiver system (Edmond et al. 1996, Jepson and Winemiller2007) and are likely to be important in the major riversdraining the Himalayas, namely the Indus, Ganges, Brahma-putra, and Mekong.The fundamental ecological importanceof these linkages stresses the need to manage even theworlds

largest rivers in a basin context.Although our knowledge of the nature and magnitude ofmountain-lowland linkages in the Amazon basin can serve toinform research and management in the Amazon and inbasins around the world, much remains to be learned.Research in recent decades has illuminated the nature andmagnitude of mountain-lowland linkages along the main-stem Amazon river, but investigations in the Andes lag farbehind. Researchers still know little about the fluxes of sed-iments and associated nutrients from the Andes on a re-gional scale, and even less about the spatial and temporalvariability in those fluxes. We know equally little about the de-gree to which river organisms depend on habitat and otherresources of Andean rivers during annual and multiyear mi-grations. In themidst of our incomplete ecological knowledge,the Andes are being rapidly transformed into a managedlandscape where rivers are modified and where montaneforests and high-altitude grasslands are converted to pas-tures and agricultural fields. Filling these knowledge gaps isan immediate scientific challenge with important ramifica-tions for the sustainability of the Amazon River basin as awhole. Brazil, the downstream beneficiary of Andean inputsfrom its upstream neighbors, should take special interest inthese issues. Over the long term, the most productive com-ponents of the Brazilian Amazon River system are also themost vulnerable to poor management decisions in the Andes.Brazils own plans for large-scale hydroelectric development,new road building,andagricultural intensification should paysimilar consideration to theimportant hydrological and eco-logical linkages uniting the larger basin.

AcknowledgmentsWe wish to acknowledge our colleagues and collaborators intheAndeanAmazon who have informed andinfluenced ourunderstanding of Andean-Amazon linkages, especially JayBrandes, Remigio Galarraga, Michael Goulding, Jean LoupGuyot, Carlos Llerena, Jos Efrain Ruiz,Richard Chase Smith,and Amy Townsend-Small. We thank the Inter-AmericanInstitutefor Global Change Research,theUS National ScienceFoundation,and theAndrew W. Mellon Foundation for sup-porting our research in the Amazon basin. Daniel Gann andAnna Boyette providedcritical support with graphics.MichaelGoulding, Margi Moss, and Thomas Saunders contributedphotos. This manuscript was improved by the comments ofJohn Melack and three anonymous reviewers.

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