Boundaries

This ecoregion includes the Rio Negro drainage basin from Manaus in the east to the slopes of the eastern cordillera of the Andes in Colombia. It includes the lower Rio Branco downstream from Caracaraí and the lower Rio Jauaperi basin.  It is bounded in the north by the mountainous divide between the Negro and the Orinoco and upper Rio Branco drainage. The southern limit is formed by the low-lying divide between the Rio Negro tributaries and the Rio Solimões drainage.

Topography

The Rio Negro drains three main areas: northern tributaries originating in the southern Guiana Shield, western headwaters originating in the Colombian Andes, and right-bank tributaries originating in the lowlands. The majority of this ecoregion lies on the lowland plateau, which is between 100-250 m in the west, sloping down to less than 50 m elevation in the east. The upper portion of the basin is dominated by granitic outcrops of the Guiana Shield. The Tapirapecó and Imeri mountain ranges run along the Brazil-Venezuela border, with the highest elevation at Pica da Neblina, which rises over 3000 m asl.

Freshwater habitats

Low gradient rivers and streams run through sedimentary soils formed mainly by podsols subjected to seasonal flooding. The Rio Negro’s water is extremely poor in mineral content, with conductivity as low as 8 µS, and is extremely acidic, with pH’s ranging from 2.9 to 4.2. The largest blackwater river in the world, the Rio Negro provides roughly 14% of the average annual discharge into the Amazon, second only to the Madeira. Its main tributary, the Rio Branco, is by contrast a whitewater river. Although not as turbid and the Amazon or Madeira, it is muddy during the flood season. The sediments are visible 200 km downstream of the confluence with the Negro.

The headwaters of the Negro originate in the foothills of the Andes. In the upper reaches the flood season occurs between May and September, peaking in July. Water level fluctuations in the lower reaches are dictated more by the Amazon, and occur earlier. Here, the flood season lasts from February through July, with peak water levels in June. The average annual river fluctuations range between 4-5 m in the upper reaches and 10 m in the lower reaches. It is estimated that an area of 30,000 km² within the Negro basin is seasonally flooded between 4-8 months of the year. The largest floodplains occur along right-bank tributaries, as well as among the network of islands along the middle and lower Negro.

There are many insular environments along the main stem of the Rio Negro, including more than 600 islands in the lower and mid-section of the river. Along the main stem and tributaries are vast fringing floodplains covered by flooded forests and inundated campinas and caatingas (campinarana), as well as many floodplain lakes and oxbow lakes along the channels.  During the dry season vast sand beaches are found along the entire extent of the rivers. River bottoms are rocky with gravel and large boulders. Rocky outcrops and cataracts reveal evidence of the Guiana Shield at points along the Negro’s middle reaches.

Terrestrial habitats

The ecoregion is home to large areas of igapó forest due to the high number of blackwater streams and rivers in the ecoregion. This forest type occurs on land that is seasonally flooded each year and has sandy, oligotrophic, or nutrient-poor, soils. Abundant tree species include Virola elongata, Eschweilera longipes, E. pachysepala, and Pithecellobium amplissimum. There are also areas of várzea forests, which occur on the floodplains of whitewater rivers, and tend to contain suspended material and high nutrients. Terra firme, a third type, never floods and has a similar floristic composition to várzea. Scattered across these lowland forests are large areas of campinarana (campina) that form a patchwork of vegetation types from herbaceous savannas to closed-canopy forests. These occur around circular swampy depressions of sandy, nutrient-poor podzols. Some of the species limited to campinarana include Virola parvifolia, Compsoneura debilis, and Pithecelobium leucophyllum, among others.

Description of endemic fishes

More than 90 species are considered endemic to the Rio Negro basin, including the distinct Denticetopsis sauli from the lower Pamoni River in the Cassiquiare Canal area. There are also six monotypic genera – Tucanoichthys, Ptychocharax, Atopomesus, Leptobrycon, Niobichthys, and Stauroglanis) – currently found only in this basin.

Other noteworthy fishes

The Rio Negro is home to more than 100 species that are sought after for the aquarium trade. The iridescence of some species like the endemic cardinal tetra (Paracheirodon axelrodi) may be an adaptive trait to the Negro’s black water. 

Ecological phenomena

Ecological phenomena include large migrations of doradid catfishes, characids such as Brycon, and prochilodontids of the genus Semaprochilodus. The jaraqui (Semaprochilodus insignis), for example, migrates from blackwater to whitewater rivers to spawn. There are also unique assemblages of species on leave-litter deposits, and many miniaturized forms.

Justification for delineation

This ecoregion falls within the Guyanan-Amazonian ichthyographic region, and more specifically within the Amazonian ichthyographic province (Gery 1969; Ringuelet 1975). It is differentiated from other Amazonian ecoregions by a very diverse and differentiated assemblage of species with pronounced endemism and unique adaptations to low pH waters.

Level of taxonomic exploration

Good in large river channels, fair in midsize streams, and poor in the headwaters draining the Guiana Shield.