Boundaries

This ecoregion includes the Amazon floodplain and tributaries that flow through the low elevation terrains of the Amazon sedimentary basin between the Brazilian Shield to the south, the Guiana Shield to the north, and the Andean piedmont to the west.  It extends from the confluence of the Rio Paru on the east to 250 m elevation in the west along the Napo, Pastaza, Marañon, and Ucayali drainages. It also includes the mid-lower portions of the Madeira basin downstream from Porto Velho. The northern and southern boundaries are defined by the contact zone between the Precambrian shields and the sedimentary deposits of the lowlands, which is characterized by the presence of rocky outcrops and the occurrences of pronounced rapids.

Topography

The present form of the Amazon floodplain is a relatively recent alluvial plain with Tertiary and Pleistocene sediments of riverine and lacustrine origin that were deposited from the Andes as well as the Guiana and Brazilian shields. Many large rivers dissect this relatively flat landscape, which ranges in elevation from sea level to roughly 250 m. Those draining the Andes carry the vast majority of sediments into the Amazon; the Madeira, for example, contributes nearly 50% of all sediments that flow through the Amazon.  The dominant landscape feature is the extensive and diverse fringing floodplain that extends along the river channels.

Freshwater habitats

Carrying one-sixth of all freshwater transported by rivers, the Amazon River and its tributaries represent the largest river system on earth. From the confluence of the Ucayali and Marañon rivers in Peru to the Atlantic Ocean, the Amazon is approximately 3750 km long. The river network is characterized by meanders, oxbows, and thousands of streams, many of which flood annually. The main channel is between 2-5 km wide through much of its course, and the width of the floodplain is around 5-20 km, but extends 30 km in some places; this floodplain stretches more than 100,000 km². Most of the lakes in the region are broad shallow floodplain lakes that form during the flood period and may persist for several years. The rainy season affects the timing of peak flood such that southern tributaries flood before northern tributaries. Around Manaus, the peak flood occurs between May and June, just after the rainy season. The water level varies in the region with amplitudes ranging from a few meters up to 15 m, with the greatest variation occurring in the central Amazon around Manaus. The mean annual discharge at Óbidos is 162,000 m³/s.

Many whitewater rivers (Madeira, Purus, Juruá, Marañon, Ucayali) that drain the lowland basin, including the Amazonas main stem, carry suspended solids (both organic and inorganic) and rich nutrients from the Andes, and have a high pH and conductivity. In contrast, clearwater (Tapajós, Xingu, Tocantins) and humic-rich blackwater (Negro) rivers originate from the crystalline shields of Guiana and central Brazil and have a lower pH and conductivity, as well as negligible sediment loads. The various types of river discharge as well as the gradients and geology determine the various fish habitats in the region.

Basins of two of the largest tributaries, the Purus and Juruá, occur within this ecoregion. These are the second and third largest tributaries after the Madeira, and also contain the largest tributary floodplains. These rivers contain thousands of meanders and oxbow lakes, as well as extensive areas of flooded forest and floating meadows.

Terrestrial habitats

The main types of vegetation in the ecoregion include seasonally inundated or flooded forests (várzea and igapó), swamp forests (which are semi-permanently or permanently flooded), terra firme forests, and floating meadows. The areas surrounding the Amazonian rivers flood annually, lasting six months on average. Várzeas (flooded by whitewater rivers such as the Solimões, Amazon, Madeira, Purus, and Juruá) are generally more fertile than surrounding areas due to the deposition of fertile alluvial soils from the Andes. Igapó forests (flooded by black or clearwater rivers such as the Tapajós) grow on nutrient-poor white sandy soils. Common tree species in flooded forests include tachi (Sclerolobium aureum), sorva (Couma utilis), Exellodendron coriaceum, and Piassava palm Leopoldinia piassaba. Terra firme forests grow at higher elevations and include trees such as the Brazil nut (Bertholletia excelsa). Many typical várzea species are also found in terra firme forests.

Floating meadows, composed of aquatic grasses like Paspalum and Chinochloa, are widespread in the middle Amazon, and support a large number of species like marbled swamp eel (Synbranchus marmoratus), banded cichlid (Heros severus), and Hyphessobrycon and Hemigrammus characoids.

Description of endemic fishes

Currently there are 206 endemic species and nine endemic genera (Hypodoras, Klausewitzia, Micromyzon, Miuroglanis, Parapteronotus, Pareiodon, Pariosternarchus, Pseudorinelepis, and Stichonodon).  There are more than 43 endemic characids, followed by 39 loricariids, 23 callichthyids, and 20 cichlids.

Other noteworthy fishes

There are 84 described Gymnotiformes throughout the ecoregion, of which 27 are endemic. This unique order has evolved specialized electro-sensory and electrogenic organs for communication and navigation. The largest species is the electric eel (Electrophorus electricus), which is the only species of the genus Electrophorus. It is also the only species that is capable of generating an electrical discharge for hunting and self-defense.

Ecological phenomena

Characins and catfishes account for nearly all of the fruit and seed eating fishes in the Amazon Basin, and are the most important seed dispersal agents within the flooded forests. Characins, the most morphologically diverse group of vertebrates, have adapted dentition to crush seeds. Notable characins include pirhanha (Serrasalmus) and tambaqui (Colossoma macropomum), which use the floodplain for feeding and nursery habitats. Catfishes (siluroids) are also important seed dispersers, and include species such as pirarara (Phractocephalus hemeliopterus).

Changing water levels have caused many species to undergo feeding and spawning migrations. For example, the large dourada (B. rousseauxii) and Laulao catfish (Brachyplatystoma vaillantii) migrate the length of this ecoregion from the Amazon estuary to headwaters as far as Columbia, Bolivia, and Peru. Other migrating species include paraíba (B. filamentosum) and jaú or gilded catfish (Zungaro zungaro). A number of characins also undergo spawning migrations, including the genera Brycon, Semaprochilodus, Leporinus, and Triportheus.

Justification for delineation

The Amazonas Lowlands ecoregion falls within the Guyanan-Amazonian ichthyographic region, and more specifically within the Amazonian ichthyographic province (Gery 1969; Ringuelet 1975). It contains a unique composition of rich and diverse assemblages of large river lowland fauna affiliated with the extensive seasonally flooded floodplain of the Amazon main stem.

Level of taxonomic exploration

Good in large rivers, fair in mid-size and lowland streams, and poor in headwaters.