Boundaries

This ecoregion contains the coastal drainages and a complex of lagoons south of the Río de la Plata, extending as far south as the drainage divide of the Río Colorado basin. It is limited to the west by the drainage divide of the Central Argentine endorheic basin, and to the northeast by the drainage divide between the Salado del Sur and Paraná rivers.

Topography

This ecoregion lies on the pampas (flat plains). Horizontal plains marked by slight undulations dominate the relief, although low hills also punctuate the landscape (López et al. 2002). Elevations rise from under sea level at Chasicó lagoon to over 1200 m asl at Sierra de la Ventana, which is an outcrop of Paleozoic sedimentary rocks. Another notable hilly feature is the Sierra de Tandil, which rises 524 m and is composed of metamorphic and sedimentary rocks. Across the plains soils are generally sedimentary in origin with a high organic content (WWF 2001). 

Freshwater habitats

Freshwater habitats include a few slow moving, undulating rivers as well as lagoons with fresh and salt water (WWF 2001).  Flatland streams and man-made drainage canals comprise the hydrographic network in the drier northwestern part of the ecoregion. Rivers such as Sauce Grande and Quequén Grande descend from the slopes of the Sierra de la Ventana and Sierra de Tandil. Some of the rivers flow directly or indirectly into the sea, whereas others disappear in sloughs or lagoons. The northernmost river is Río Salado del Sur, which meanders along a wide, flat valley and forms marginal lagoons before reaching the ocean at Samborombón Bay.  In this valley are 339 lagoons of varying salinity, of which 125 are temporary. The Encadenadas (chained lagoons) of Chascomús is a lacustrine system interconnected through short water courses before reaching Río Salado del Sur. To the southwest is another lacustrine system comprising 61 lagoons (37 of them temporary), of which the main ones form the Encadenadas del Oeste system. Mar Chiquita is a large saltwater lagoon around the coastal town of Mar del Plata. This coastal lagoon is influenced by the ocean and also receives waters from various streams (López et al. 2002).

Terrestrial habitats

This ecoregion lies on the humid pampas, which is composed of grasslands and xeric woodlands. The dominant grasses include Stipa, Piptochaetium, Aristida, Melica, Briza, Bromus, Eragrostis, Poa, and characteristic pampas grass (Cortaderia selloana) (WWF 2001). Xeric forests dominated by tala (Celtis tala) and Scutia buxifolia represent some of the main indigenous trees in the pampas (López et al. 2002).

Description of endemic fishes

The ecoregion contains two endemic species, the characid Hyphessobrycon togoi and rivulid Austrolebias nonoiuliensis.

Other noteworthy fishes

Species of the genera Austrolebias have remarkable reproductive features. They are also adapted to temporary water bodies that dry out during the summer months (López et al. 2002).

Occasionally species such as the streaked prochilod (Prochilodus lineatus) and Acestrorhynchus pantaneiro enter into the Río Salado del Sur due to excessive flooding of the Río Paraná, which causes freshwater to back into Samborombón Bay (López et al. 2002).

Justification for delineation

This ecoregion marks the southern boundary of Gery’s (1969) Paranean icthyogeographic region and Ringuelet’s (1975) Paranoplatensean province. It is distinguished from other ecoregions in this province by lower endemism and the absence of families and orders like Beloniformes, Lepidosireniformes, and Pleuronectiformes (López et al. 2002).

Level of taxonomic exploration

Good