Boundaries

This ecoregion includes the area of the Amazon estuary and adjacent coastal drainages, with a southeastern limit at the Rio Itapicuru basin and a northwestern limit at the Rio Caciporé drainage on the coast of Amapá, excluding the Oyapock.  The westward limit along the Amazon main stem is the confluence of the Rio Paru.  Southward the area extends into the Tocantins as far upstream as Tucuruí, and into the Xingu as far as Belo Monte.

Topography

The estuary and coastal areas lie on a flat alluvial plain, with most of the ecoregion lying below 100 m asl.

Freshwater habitats

Stretching 150 km wide and extending 300 km upstream to the mouth of the Xingu, the Amazon estuary is enormous in size and outflow, discharging an average of 180,000 m³/s. This accounts for nearly 20% of all fresh water output into oceans. Because of the magnitude of this discharge out of the main channel, there is little saltwater intrusion into the estuary. Instead, nearly all of the mixing between the river’s discharge and seawater occurs up to 160 km offshore on the continental shelf. Only during the low-water period in Marajó Bay does brackish water penetrate up to Belém.

The estuary is distinguished from the rest of the Amazon by twice daily tides that flood most of the islands in the estuary. Tidal influences extend as far upriver as Óbidos. The tide takes four to five hours to flood and eight to nine hours to ebb, discharging three million tons of sediments.  These sediments are pushed northward by the South Equatorial Current, preventing the formation of a delta at the mouth of the river. Instead, the delta merges with the northern side of Marajó Island, the largest island in the estuary. Only a small percentage of the Amazon flows south of Marajó into Marajó Bay. Another feature is a tidal bore, or pororoca, which is a large solitary wave that can travel up to 13 km upstream during spring tides.

North of the mouth is an extensive low-lying area of lakes and swamps called the Piratuba Lakes region. This area, similar to the Pantanal, is inundated by local rainfall and tides, and provides important habitats from wading birds.

Terrestrial habitats

The mouth of the Amazon is filled with numerous islands, the largest of which is Marajó Island (49,000 km²). Because there is enough freshwater to prevent significant saltwater intrusion, most islands in the estuary are covered by rainforest despite the constant tidal flooding. These tidal várzea forests are similar in stature and species to seasonally flooded forests. Marajó contains both. Whereas the western half consists of tidal várzea forests, the eastern half consists of seasonally flooded savanna and terra firme forests. The interior is inundated primarily by rain water, which remains flooded for six to eight months of the year.

Floodplain or savanna grasslands surround the periphery of the estuary, and mangroves line the coast. In addition to terra firme, igapó and várzea flooded forests line the black- or clearwater rivers (such as the Tocantins) and whitewater rivers (such as the Guamá) south of the estuary.

Palms dominate the tidal forests with such species as murumurú (Astrocaryum murumuru), jupati (Raphia taedigera), bacaba (Oenocarpus distichus), patauá (Jessenia bataua), Virola surinamensis, Symphonia globulifera, and Inajá (Maximiliana regia), as well as the economically important buriti palm (Mauritia flexuosa) and açaí palm (Euterpe oleracea).

Description of endemic fishes

The ecoregion contains 23 endemics in 13 families, including four Hyphessobrycon species and two Apistogramma species. One unique endemic is the cistern catfish (Phreatobius cisternarum), which inhabits subterranean waters north and south of the mouth of the Amazon, including on Marajó Island.

Other noteworthy fishes

Bull sharks (Carcharhinus leucas) and sawfish (Pristis pristis) are abundant in the estuary and can travel up to 5000 km and 2000-3000 km upstream, respectively. Two species of foureyes (Anableps anableps, A. microlepis) reside in tidal mud-flats and have evolved specialized divided pupils in order to see above and below water simultaneously. The rock-bacu (Lithodoras dorsalis) is a heavily armored catfish that has successfully colonized tidal forests. The ecoregion also contains 23 gymnotoid species, all of which have electrogenic or electro-sensory organs.

Ecological phenomena

The Amazon Delta provides nursery habitat for goliath catfish like the piraíba or kumakuma (Brachyplatystoma filamentosum), Laulao catfish (B. vaillantii), dourada (B. rousseauxii), and jaú or gilded catfish (Zungaro zungaro), which undergo migrations from the Orinoco and Amazon estuaries to headwaters for spawning.

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 lower Amazon and estuary is distinct from ecoregions upriver in that it is influenced by the Atlantic and semidiurnal tidal flooding. As a result it constitutes an area with a unique assemblage of species, including estuarine and freshwater fauna.

Level of taxonomic exploration

Fair