Contents - Previous - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
Part 2 : The Brazilian Amazon
Waters and wetlands of Brazilian Amazonia: an
A fragile capitalism in a fragile environment: entrepreneurs and state bureaucracies in the free zone of manaus
Waters and wetlands of Brazilian Amazonia: an uncertain future
The Amazon river system
Human use of amazonian aquatic and wetland ecosystems
The future, a cascade of uncertainties
A broader picture: environmental perspectives in Brazil
Hilgard O'Reilly Sternberg
Sailing along the coast of South America in 1500, and reckoning their position to be forty leagues offshore, a party under Vicente Yañez Pinzón came upon a body of "fresh water of unsurpassable quality." To learn whether it extended all the way to the bottom, the seafarers adapted a barber's chafing bowl (escalfador de barbero), "so that it would open only upon reaching the seabed," which was at six fathoms. They ascertained that the fresh water at the surface was present to a depth of 2(1/2) fathoms, overlying salt water identical to that of the sea. The mariners "discarded the water they brought with them, and took on the supply needed to continue their voyage" (Ferrando, ). The "great river," source of the fresh water, named by Pinzón Santa María de la Mar-Dulce (Saint Mary of the Sweet Sea), is generally believed to have been the Amazon.¹
The existence of surface lenses of fresh water bounded by a sharp halocline, discovered five centuries ago with the aid of an improvised device, has now been substantiated by state-of-the-art techniques. These have also confirmed the fact that the Amazon River plume, overriding denser sea water, can extend more than 100 km from the mouth, across the continental shelf (Curtin, 1986a, 1986b; Curtin and Legeckis, 1986; Nittrouer and DeMaster, 1986; Gibbs and Konwar, 1986).
The Amazon discharge, estimated to average some 200,000 m³/sec (Nordin and Meade, 1986),2 affects a large region in the tropical Atlantic (Gibbs, 1970; Johns et al., 1990; Nittrouer and DeMaster, 1986; Ryther, Menzel, and Norwin, 1967). Satellite images, obtained by coastal zone colour scanners, show the distribution of near-surface pigments (e.g. chlorophyll), revealing, in an impressive way, the trajectory of the effluent. During the first part of the year, it is carried into the Caribbean Sea by the North Brazil Coastal Current and the Guiana Current. From June to January, however, an offshore retroflection occurs in the flow, and an appreciable fraction of Amazon water is transported eastward, being detectable near Africa (MullerKarger, McClain, and Richardson, 1988; Muller-Karger and Varela, 1989/1990).
The advection of Amazon sediments by the north-west-setting coastal current explains, at least in part, the sharp contrast between the coast to the north-west and that to the east of the embouchure. The former is a relatively straight, prograding littoral; the latter, a convoluted shoreline of ria-like inlets, giving the impression that submergence affected a zone previously dissected by subaerial erosion.
The Amazon river system
The source of the Amazon's huge discharge, which affects the chemistry of ocean water over thousands of square kilometres and transports sediments that build up distant shorelines and overspread remote seabeds, is a catchment area of almost six million square kilometres, two-thirds of which are in Brazil. Not all of the world's largest hydrographic basin shares the "Amazon look" (Spruce, 1908) one associates with the tropical rain forest. It includes snowcapped Andean peaks and cerrado-covered plateaus in WestCentral Brazil.³ Conversely, some adjacent areas, while Amazonian in character, do not drain into the Amazon River. Such is the case of the 800,000 Km² Tocantins basin, tributary to the Rio Pará, a cul-de-sac connected to the Amazon proper by a maze of deep narrow channels. It is also the case of about 100,000 Km² of watersheds in Amapá state that discharge directly into the Atlantic.
Whereas in the Cordilleras the pattern of the Amazon drainage is determined by the alignment of the Andean orogenic belt, the lowland westeast course of the main stem corresponds to the axis of a deep sedimentary basin. This is filled to depths of more than 5,000 metres with Palaeozoic sediments and associated magmatic intrusions. It is underlain by a very old portion of the earth's crust, a complex of Precambrian metamorphic and platonic rocks that surfaces north and south of the trough. The areas of relatively modest elevation where the ancient core is exposed are the tectonically stable, socalled "shields." There is, thus, a Guiana Shield and a Central-Brazilian (sometimes named Guaporé) Shield. The downflexed beds of the intervening structural basin are covered by extensive subhorizontal sedimentary strata, which, attributed to the Mesozoic and Cenozoic (Caputo, 1984), constitute the extensive Amazonian Plain (fig. 6.1).
As the seas retreated during the last glacial stage, coming to a stand more than 100 m lower than that of today, the Amazon adjusted to the change by deepening its bed. With this, erosion advanced up tributaries, frequently exploiting a lattice of rectilinear and right-angled fractures (Bemerguy and Costa, 1991; Bremer, 1971, 1973; Projeto RADAMBRASIL, 1978; Sternberg, 1950, 1955; Sternberg and Russell, 1952). This incision left standing above the valley bottoms all but a fraction of the central sedimentary plain, a vast area of relatively flat or gently undulating landscapes known as the terras firmes.
When sea level rose again, it caused the flow of the Amazon to back up. Thus, was created a freshwater gulf thousands of kilometres long, with ramifications into the lower reaches of tributary valleys. This drowned river system is the theatre of the ongoing alluviation that has shaped the presentday riverine wetlands, or várzeas (fig. 6.2).
Volcanic renewal of nutrients, through convection in the upper mantle and the creation of new rock, occurs near active plate margins. Brazilian Amazonia, however, lies in an essentially stable province that experiences only modest epeirogenic activity (Sternberg, 1950, 1975; Sternberg and Russell, 1952). With the exception of those developed on rare, and generally small, basaltic outcrops, terra firme soils are derived from nutrient-deficient parent rocks, further impoverished by intense biochemical weathering. Overall stability and low energy relief have favoured the accumulation in situ, over a time span estimated to reach some ten million years, of the depleted regolith (Fyfe et al., 1983), reported to extend down to a depth of 10 to 60 metres (Costa, 1993). Locally, it has been identified at more than 300 m below the surface, as in the Serra dos Carajás (Coelho, 1986), which may have been uplifted after the weathered mantle had been formed. Nutrients released at deep-seated weathering fronts are, of course, unavailable to plant roots. If Amazonian soils developed on rocks of the basement, or oldlands, are poor, those generated on the clastics of the central plain are even more so. Yet, in spite of inherent geological and climatic constraints to the formation and maintenance of fertile soils, lush rain forests clothe not only shield areas but also much of the sedimentary axial plain.
Figure 6.1 The Amazon (features mentioned in text).
The youngest strata of the Amazonian Plain, including
non-Hooded terras firmes and (not differentiated here) recent
alluvial wetlands, overlie a deep, sediment-filled trough.
Dippiug toward the axis of the structural basin, older beds are
largely buried under the superficial accumulations. The whole is
underlain by a complex of very ancient metamorphic and igneous
rocks. Exposed north and south of the axial plain, this worndown
platform constitutes the Guiana and Brazilian
"shields." The Cis-Andean Plains, essentially a mantle
of debris from the cordilleras and front ranges, decline
eastward, merging with the Amazon Plain or lapping over the
Guiana and Brazilian oldlands.
Inset: "Amazônia Legal," an operational region, originally defined in 1953 for the purpose of implementing the development activities provided for by the 1946 Constitution.
Abbreviations: Ita, Itacoatiara; MtA, Monte Alegre; Óbi, Óbidos; P.Velho, Porto Velho; Stm, Santarém; Tap, Taperinha; VGr, Vargem Grande. PM, Pongo de Manseriche; PTL, Pantanal. Bol, Bolivia; Col, Colombia; Ec, Ecuador; FG, French Guiana; G. Guyana; P. Peru; Sur, Surinam; yen, Venezuela. Abbreviations in inset AC, Acre; AM, Amazonas; MA, Maranhão; MT, Mato Grosso; PA, Parã; RO, Rondônia; RR, Roraima; TO, Tocantins.
Figure 6.2 Sealevel, rising after the last glacial, backed up the Amazon River, drowning the main stem and the terminal sections of its lowland tributaries.
Sediment-poor effluents have been unable to alluviate the
resulting lake-mouths; such is the case of the Tapajos River,
partially closed off by an alluvial deposit of the trunk stream.
Despite its huge sediment load, even the Amazon has left
extensive areas of its giant freshwater embayment to be filled.
This is evidenced by the intricate pattern of channels and
natural levees that enclose thousands of shallow bottomiand
lakes. Some lacustrine basins are partly bounded by upland
bluffs, as is the case of Lake Maicuru, also known as Lago Grande
de Monte Alegre or, simply, Lago Grande (upper right). (Projeto
RADAMBRASIL, 1972, Semi-controlled mosaic of radar images
executed in 1971-1972, Sheet SA-21-Z-B. Original scale 1:250,000.
Departamento Nacional da Produçãe Mineral, Rio de Janeiro.)
Inset: An attempt, made "in the face of every kind of protest," to create new land by silting up Lago Grande and the surrounding marshes, began in 1950, when the Piapó, a 4-km-long natural channel through the levee of the Amazon (Sioli, 1951), was cleared and straightened with the use of manual labour. Following up with mechanical excavators, several artificial channels were cut, and were named after public figures. Novais Filho, the first to be excavated, was completed in 1953, reaching a high-water inflow into Lake Maicuru of 275 m³/sec in 1954, when the aggregate discharge of all six cuts came to 670 m³/sec (Camargo, 1958). According to a recent denunciation, the so-called "siltation canals," instead of accomplishing their purpose, are allegedly bringing about the expansion of Lago Grande, which, it is claimed, is swallowing up neighbouring lakes and threatening the levee that separates it from the Amazon (O Liberal, 1989). (Excerpted and re-drawn from Camargo, approximate scale added, width of canals exaggerated. Differences in the size and shape of the lake as represented on a sequence of maps, air photos, and satellite images cannot be used to follow its evolution. This is because of the diffuse boundary between lakes and marshes, as well as the fact that surveys made at different, and mostly unspecified, river stages reflect seasonal contraction and expansion.)
This fact indicates an efficient adaptation to oligotrophic conditions, as in the case of "direct nutrient cycling" through the agency of mycorrhizal fungi, described a quarter-century ago by Went and Stark (1968). Recent research suggests, further, that exogenous nutrients are being imported in the form of aerosols. Chemical "fingerprinting" of the allochthonous particulates and satellite observation of dust plumes over the Atlantic point to material originating mainly on the African continent, nearly 5,000 km away (Swap, Garstang, and Greco, 1992; Talbot et al., 1990). A nutrient cycling pathway, recently reported from the Rio Negro headwaters in Venezuela, would appear particularly well adapted to utilize such windborne particulates: plants with upward-growing roots intercept stem flow and absorb the precipitated nutrients before they enter the soil solution, where they might be lost to competitors or become generally unavailable (Sanford, 1987).
Provenance and characteristics of Amazonian waters
The source for most of Amazonia's waters is evaporation from the Atlantic Ocean. The moisture precipitated over the basin is wafted upvalley by an easterly flow of air, driven by the convergence of trade winds from the Azores and South Atlantic highs (Salati and Vase, 1984). Average precipitation over Brazilian Amazonia amounts to 2,200 mm per annum; accentuated relief introduces marked spatial variations in the Andean portion of the drainage. Thus, the upper Madeira basin exhibits extremes of less than 500 mm and more than 7,000 mm (Roche et al., 1990). Westward advection of oceanic moisture does not occur in a continuous flux. Preliminary observations suggest that a considerable proportion of the precipitation that falls on the basin is "recycled" water, returned to the atmosphere by evapotranspiration from the rain forest (Lettau, Lettau, and Molion, 1979; Salati and Vase, 1984).
Reflecting the paucity of nutrients in the sedimentary areas whence they flow, the relatively short streams that drain the central plain are among "the most electrolyte-poor natural waters on earth" (Fittkau et al., 1975), and carry almost no solid load. Those that rise in the flanking, lithologically complex, shield areas are somewhat more favoured geochemically. But it is the youthful Andean belt, estimated to represent a mere 12 per cent of the total area of the basin (Gibbs, 1967), that is believed to supply more than 90 per cent of the sediments and nutrients carried by the Amazon River (Meade et al., 1985). This load is diluted downstream by the influx of large volumes of sediment-poor waters from the deeply weathered oldlands or from the central plain. The Rio Negro, for instance, has been calculated to contribute 20 per cent of the Amazon's outflow, but less than one per cent of its sedimentary discharge (Meade, Nordin and Curtis, 1979).
Related to the particulate and dissolved load of Amazonian waters are their optical properties,(5) which Native Brazilians incorporated in their fluvial nomenclature. Those appearing black are especially striking, and several Amazonian streams bear the Tupi designation Ipixuna ("black water") or include the diagnostic suffix -una. Early Europeans were equally impressed by the seeming inkiness of the water, "negra como tinta" (Carvajal 1942) of one major affluent, since known as the Rio Negro (fig. 6.3).
Alfred Russel Wallace (1853) perceived a simple threefold division of Amazonian streams into "white-water rivers, blue-water rivers, and blackwater rivers," and recognized the association between optical properties and source areas. Issuing most characteristically from the Andean Cordillera and its foreland, so-called "white"-water rivers, such as the Madeira or the Amazon itself, are actually muddyyellow to light reddish-brown because of the inorganic particulates they transport in suspension. Their pH values tend to lie close to neutral. Streams carrying minimal amounts of suspended sediments may be either clear-water or, if tinged by organic matter, "black"water rivers, such as the Negro. Clear-water streams, of which the Tapajós is an example, may appear blue or green. Typically, they rise in the worndown Guiano-Brazilian oldlands, and carry little dissolved salts and suspended solids. Their pH values vary widely. The earliest reference to the provenance of tropical black-water streams from areas of bleached sands or podzols may have been that of Lochead (1798). Imbued with fulvic and humic acids derived from the breakdown of plant tissue, the tea-colored water, which appears jet black in a stream or lake, may have pH values below four (Sioli, 1975).
Variation in water level and its ecological significance
The fact that summer rainfall maxima in the southern and northern hemispheres are out of phase offsets, to some extent, the peak discharges of major right- and left-bank tributaries of the lowland Amazon.6 Storage provided by a vast floodplain also contributes to damp the seasonal variation of main stem discharge. The estimated highflow/low-flow ratio of, at most, 3:1, is significantly less than that of other large rivers of the world (Nordin and Meade, 1986). Even so, the average yearly amplitude reaches 10 m at which, situated 20 km up the Rio Negro, provides a reasonable approximation of the stage level in the trunk stream at the confluence.
The rise and fall of the waters, the great "drama of nature," in the words of Martius (Spix and Martius, 1831), is of supreme ecological consequence. The invasion of the várzea by floodwaters provides an opportunity for seed or fruit to bob away, leaving behind pests concentrated near the parent tree. Many floodplain plants are success fully adapted to dissemination by water. Ducke(1949) compared strategies for dispersal in species of the same genera belonging, respectively, to upland and bottomland habitats. In addition to mechanisms that involve the aquatic fauna (referred to below), he noted, among features that are adaptive in areas subject to inundation, indehiscence of fruit, favouring flotation, as contrasted with dehiscence in corresponding upland species.
Variation in river level is no less important for hydrophytes. In the case of white waters - or even black or clear waters, when seasonally invaded by silt-bearing floods - penetration of light is blocked by the very turbidity that denotes the presence of mineral nutrients. Here, buoyant plants are optimally adapted: regardless of water-level oscillations, they can use the radiant energy available at the surface. Vast areas are covered by "floating meadows" (Gessner, 1959; Junk, 1970, 1973) made up mainly of grasses, but sometimes bearing full-sized trees. The matupás, preferred habitat of the manatee (Trichecus inunguis) and home to a complex aquatic fauna, may also shelter amphibious and terrestrial animals, e.g. paces (Agouti paca), capybaras (Hydrochoerus hydrochaeris), peccaries (Tayassu pecari), and tapirs (Tapirus terrestris).
Both free-floating plants and those that, rooted in the floodplain, are capable of pacing the rising waters play a key role in the nutrient and energy budget of the várzea, by capturing throughflowing nutrients. In one study that exemplifies the capacity of such macrophytes to concentrate critical elements, levels of potassium and phosphorus in the plants were, respectively, eight and four times higher than in the supporting soils and waters (Howard-Williams and Junk, 1977). With falling stage, the plant tissue deposited on the emerging land substantially enriches the soil.
The periodic appropriation of the várzea by floodwaters also has profound implications for Amazonian aquatic wildlife. During high water, many kinds of fish and other components of the riverine fauna, such as turtles, pass freely into flooded forests, which serve as breeding grounds and provide an appreciable part of the annual food intake. This is the time when a number of species develop the fat stores that carry them through the "physiological winter" of low water (Lowe-McConnell, 1967; Marlier, 1967), acquiring the energy necessary for gonad development and spawning migrations (Junk, 1985a).
Huber (1909) recognized the role of phytophagous fish in the dissemination of várzea plants. Ducke (1949) took the matter one step further, suggesting that the frequent tartness of floodplain fruit, in contrast with the insipidness or sweetness of upland counterpart species, may have been selected for by the aquatic fauna, which, by passing viable seed, contributes to plant dispersal. The correlation acquired an additional mutualistic dimension in the light of evidence that, in the course of evolution, at least some fishes have lost the capacity to synthesise ascorbic acid (Chatterjee, 1973). It has been suggested (e.g. by Gottsberger, 1978) that the relative homogeneity of the wetland vegetation may be due to the alimentary regime of Amazonian fishes.
Riverine, estuarine, and coastal wetlands
The chemistry and fluctuations in stage of Amazon waters exert a decisive influence on the riparian and aquatic vegetation, as well as on the faunistic components of the ecosystems involved. Their sediment burden governs the evolution of the várzeas. Thus, the small load of rivers like the Tapajós and Xingu accords with the fact that they have yet to silt up their drowned lower courses. Other tributaries, rich in sediments, like the Juruá and Purus, describe meandering courses on the alluvium they themselves deposited.
This is not the pattern of the main stem. The incomplete filling of its drowned valley has produced an intricate mosaic of open water and wetlands. Lenticular alluvial islands split the stream-bed into a master channel and one or more laterals, or paranás (fig. 6.4). As the Amazon and major paranás shift back and forth, they leave behind gently curving strands of higher ground, restingas. Owing, among other reasons, to compaction of sediments, areas from which active channels have pulled away are lower then the youthful riverside levees. Major inundations submerge the entire alluvial plain; even during lesser floods, water flows into the bottoms through low sections in the natural levees. With their capricious windings and ramifications, the depositional strips laid down by distributary channels partition the floodplain into inumerable sub-basins, occupied by seasonal or permanent lakes (figs. 6.2 and 6.5) (Sternberg, 1956).
A widely-used estimate, giving the Amazonian várzeas a total of 64,400 Km² (Camargo, 1958), has recently been revised upward, on the basis of airborne radar images. According to the new computation (Sipper, Hamilton, and Melack, 1992), wetlands along the main stem add up to 92,400 Km² (11 per cent of which is covered by lakes), and, along major tributaries, to another 62,000 Km², making a total of 154,400 Km² in Brazilian Amazônia.7 The várzeas, covered by grasslands (campos de várzea) and forests, some of which are more or less permanently inundated (igapós), represent a small fraction of the Amazon Plain. But they contain some of the region's most productive ecosystems, and stand in sharp contrast to the generally poor, non-flooded, terra firme.
Figure 6.4 Stretch of the Amazon River, showing lenticular islands dividing the stream-bed into a master channel and one or more side channels, or parabolas Selected soundings (in metres) from the 1967 hydrographic survey by the Brazilian Navy; datum is mean lower lowwater stage. (Taken, together with most wafer surface lines, from Brasil, Rio Amazonas, Cartas de Praticagem da Flotilha do Amazonas, Marinha do Brasil, Sheets P 4106 A and B. 2nd ed., 1970. Original scale 1:100,000. Approximate outline of terra firme lakes, and boundary between uplands and floodplains, from acrid photographs.)
In terms of net primary production per unit area (e.g. g/m²/yr), Amazonia's estuarine and coastal wetlands are probably comparable to the várzeas. Mangrove forests, for example, provide privileged nurseries and feeding grounds for aquatic fauna.
The continuum of riverine, estuarine, and coastal wetlands reflects the remarkably subdued gradient of the Amazon River after it escapes from the Cordilleras through the Pongo de Manseriche, at 200 metres a.s.l., some 4,000 km from its mouth. It is thought to descend about 2-3 cm/km near Iquitos, and less than 1 cmlkm below Obidos (Nordin and Meade, 1986), towns situated more than 3,000 km and 1,000 km from the sea, respectively. As a consequence, a far-reaching tidal influence has been observed, at least as early as 1743, by La Condamine (1745). This scientist noted at Fort Pauxis, site of today's
Óbidos, where the river is constricted to a channel about 2,400 m wide, that "the flux & reflux of the Sea reaches as far as these narrows ... From Pauxis to the sea,... over two hundred and some leagues .... the River must not descend more than 10(1/2) feet."
Contents - Previous - Next