Boundaries

This ecoregion includes the entire Orinoco Delta complex in Venezuela extending inland to Barrancas along the main stem of the Orinoco, all of the small rivers draining directly into the delta, and adjacent Atlantic coastal drainages in northwestern Guyana.  The latter categories include east flowing rivers entering the Gulf of Paria (e.g., San Juan and Guanipa) and northern Orinoco Delta (e.g., Uracoa and Tigre); small north flowing tributaries entering the delta east of Barrancas (e.g., Manaco, Aguirre, Barima); and coastal rivers entering the Atlantic between the eastern mouth of the Orinoco Delta and the Essequibo drainage divide (e.g., Barama, Pomeroo and Waini).

Topography

Lowlands of the Orinoco plains gently slope from the delta toward the highlands, where peaks reach 2400 m. The floodplains are composed of sediments carried by the Orinoco and tributaries draining the Andes Cordillera, llanos, and Guiana Shield. Other sediments are carried by coastal currents from the Amazon River and deposited in the delta.

Freshwater habitats

The ecoregion comprises seasonally flooded interconnected swamps and lakes, as well as small streams to large rivers united in a braided complex. These are influenced by semi-diurnal tides, with the tidal influence extending up to 200 km inland from the coast. The estuarine and marine influence is greatest in the north, although marine waters also penetrate along channels 60-80 km inland. Water types (white, black, clear) vary depending on the time of year. These inputs, in addition to the combination of fresh, brackish, and marine waters, contribute to a heterogeneous environment that supports a large number of species. The Orinoco deltaic plains form one of the largest wetland complexes in South America, and together are a highly productive nursery, feeding, and breeding area for a number of freshwater and marine organisms. Types of wetlands include marine, estuarine, marsh, and riparian wetlands.

Terrestrial habitats

This ecoregion encompasses a mosaic of seasonally flooded grasslands, seasonally flooded forests, palm communities, mangroves, swamp forests, marsh forests, llanos, and deciduous and evergreen forests.

Description of endemic fishes

The endemic fish fauna comprises four species from four families: Manamo anchovy (Anchoviella manamensis, Engraulidae), Roeboides araguaito (Characidae), Micropoecilia bifurca (Poeciliidae), and Henonemus triacanthopomus (Trichomycteridae).

Other noteworthy fishes

The Orinoco Delta harbors a diverse assemblage of estuarine fishes, including ariid catfishes, sciaenids, and engraulids.

Ecological phenomena

The Orinoco Delta provides nursery habitat for migrating goliath catfish like the piraíba or kumakuma (Brachyplatystoma filamentosum), Laulao catfish (B. vaillantii), and dourada (B. rousseauxii), as well as the jaú or gilded catfish (Zungaro zungaro).

Justification for delineation

This ecoregion lies within the Orinoco ichthyographic province outlined in Ringuelet (1975) and Gery (1969). It constitutes an area with a unique assemblage of species, including estuarine and freshwater fauna.

Level of taxonomic exploration

Good