Boundaries

This ecoregion includes the drainage basin of the Rio Tapajós and its tributaries flowing through the northern slope of the Brazilian Shield. It is bounded to the south by the drainage divide between the Tapajós and Paraguay drainages along the Chapada dos Parecis. The northern limit is in the vicinity of Itaituba where the sedimentary basin comes in contact with the outcrop of the Brazilian Shield.

Topography

The geologic substrate ranges from soft sedimented lowlands to the hard crystalline basement of the Brazilian Shield. Elevations extend from 28 m asl along the Rio Tapajós in the north to 873 m asl in the Chapada dos Parecis. The Juruena and Teles Pires drain uplands of the the Serro do Cachimbo, Serra dos Caiabis, and Serra do Tombador, and contain a number of cataracts that cut through the granite and sandstone tablelands.

Freshwater habitats

The Rio Tapajós is the fifth largest tributary basin in the Amazon, and provides 6% of the freshwater discharged into the Amazon. Stretching 2700 km, the Tapajós is a clearwater river that rises from the Brazilian Shield. It flows north along a stable course, carries few suspended materials, has a low conductivity, and a pH from 4.5 to 7.8. The upper Tapajós and tributaries are defined by formidable rapids and cataracts whereas the lower reaches are navigable. There are also many rapids and cataracts along the middle and upper Juruena and Teles Pires rivers on the Mato Grasso. Water levels begin to rise in the upper basin in September or early October, reaching maximum levels in March or April. Near the mouth high water levels occur in May or June, and are influenced by the Amazon River. The mean annual water level fluctuation ranges between 4-5 m for the Tapajós and its tributaries. Throughout the basin low-lying areas such as the lower Juruena undergo seasonal flooding.

Terrestrial habitats

This diverse ecoregion ranges from moist forests in the north to cerrado vegetation in the south. In between lie the Mato Grasso seasonal forests, which is a transitional zone comprised of lowland forest, savanna, gallery forests, and dense thickets. Areas of white-sand igapó forest occur along the Rio Tapajós with species such as Triplaris surinamensis, Piranhea trifoliata, Copaifera martii, and Alchornea castaneaefolia.

Description of endemic fishes

Nearly a quarter of the species are endemic to the ecoregion. These are dominated by Characidae, Loricariidae, Cichlidae, and Anostomidae. Genera with a large number of endemics include Leporinus, Hyphessobrycon, Jupiaba, and Moenkhausia. Of the 12 monotypic species two are endemic to this ecoregion: Merodoras nheco and Spectracanthicus murinus.

Other noteworthy fishes

There are a number of fish that live in aquatic grasses, including Hyphessobrycon and Hemigrammus, banded knifefish (Gymnotus carapo), marbled swamp eel (Synbranchus marmoratus), and predatory characoids such as trahira (Hoplias malabaricus) and redeye piranha (Serrasalmus rhombeus).

Justification for delineation

This ecoregion falls within the Guyanan-Amazonian ichthyographic region, and more specifically within the Amazonian ichthyographic province (Gery 1969; Ringuelet 1975). The Amazon Basin was subdivided at finer scales using regionalized data on fish distributions. The Tapajós represents the fifth largest tributary basin of the Amazon, and is one of the major clearwater tributaries feeding the Amazon. It also contains a large assemblage of unique species.

Level of taxonomic exploration

Fair in large rivers, poor in headwaters.