Boundaries

This ecoregion includes the entire Essequibo River basin in Guyana and part of southeastern Venezuela where it drains the eastern margin of the Guiana Shield (source at 600 m) and low country south of the Orinoco Delta.  The Essequibo is separated from the Caroní (Orinoco Basin) in the west and southwest and Branco (Amazon Basin) to the south by moderate divides of little more than 500 m elevation. It is also separated from adjacent small rivers of coastal Guyana by low divides of only a few meters. It is limited to the east by the drainage divide between the Essequibo and Demerara rivers.

Topography

The ecoregion encompasses a low coastal plain, forested hills, and highlands. Some of the ranges include the Pakaraima, Kanuku, and Acarai mountains, from which the Essequibo River rises. Elevations range from sea level to nearly 2400 m asl.

Freshwater habitats

The uplands include moderate gradient small streams and rivers with rapids and cataracts. Kaieteur Falls (251 m) on the Potaro River is impressive with a flow rate of 663 m³/s. Along the coastal plain are low gradient rivers, lakes, and swamps with seasonal flooding.

Terrestrial habitats

Moist forests cover a majority of the ecoregion, with marsh forests along rivers.

Description of endemic fishes

The Essequibo contains 58 endemic species including a handful of Rivulus species (Rivulidae), Bryconamericus hyphesson (Characidae), Lithogenes villosus, and Parotocinclus collinsae (Loricariidae).  Bryconops colaroja (Characidae) is endemic to the Cuyuní Basin (Chernoff & Machado-Allison 1999) and the monotypic Mazarunia mazarunii (Cichlidae) is known only from the Upper Mazaruni (Kullander 1990). Farlowella rugosa is near-endemic to this ecoregion and the Guianas [311]. Endemic genera are: Acanthocharax, Mazarunia, and Skiotocharax.

Other noteworthy fishes

The only goliath catfish that occurs in this ecoregion is the Laolau catfish (Brachyplatystoma vaillantii).

Ecological phenomena

The ecoregion contains the potamadromous barred sorubim (Pseudoplatystoma fasciatum).

Justification for delineation

This ecoregion lies in the Guyanan ichthyographic province outlined in Ringuelet (1975) and more broadly within the Guyanan-Amazonian ichthyographic  region (Gery 1969; Ringuelet 1975). It has a unique composition of endemic Guiana Shield taxa that are considered the most primitive sister groups to many fish taxa in South America.

Level of taxonomic exploration

Fair in larger rivers, still poor in headwaters and small streams.