Boundaries

This ecoregion includes the lower portion of the Paraná River drainage downstream from the Guaíra Falls (Salto de Sete Quedas) and Itaipú Dam, a small portion of the Paraguay River downstream from the confluence of the Bermejo River, the lower portion of the Iguaçu River below Iguaçu Falls, as well as diverse water courses that flow directly into the Río de la Plata. It is limited to the west by the mountain ranges of the Sierras de la Candelaria (the drainage divide between the Pasaje-Juramento-Salado system and the central endorheic basin). Its eastern boundary is the drainage divide between the Paraná and Uruguay drainages.

Topography

The lower Parana forms part of a sedimentary basin bordered in the west by the Andes and in the east by the Brazilian Shield. Elevations range from sea level to over 6000 m asl in the Andes, although most of the basin lies below 200 m asl among flat plains and low undulations.

Freshwater habitats

Occupying 3.1 million km², the Plata basin is the second largest basin in South America. It comprises seven ecoregions [332, 333, 342, 343, 344, 345, 346], including the Paraguay, Uruguay, and Paraná, which is the largest, and itself sub-divided into three ecoregions [344, 345, 346]. This ecoregion, which includes the high, middle, and lower Paraná, follows the river from its confluence with the Iguaçu River downstream to the Atlantic Ocean. Above the confluence with the Paraguay, the river is rocky with patches of sedimentary clay and sandy substrate.  Around Posadas the river widens onto an alluvial fan with anastomosing creeks and oxbows, meanders, floodplains, sandbanks, and islands, as well as narrow passages. Below the Paraguay, the river widens into a plains river with a wide floodplain composed of backwaters, oxbows, and many islands. It becomes more turbid and divides into branches below the Salado River, and then forms anastomosing channels south of Santa Fé (Resende 2004). The river ends in a delta that is more than 300 km long and 60 km wide (Bonetto 1986a). Its outflow combines with the Uruguay to form the Río de la Plata, which is a saline estuary that receives roughly 57,000,000 m³ of silt each year. The water volume discharged from the Río de la Plata is 22,000 m³ per second, and 65,000 m³ per second at peak flow (Resende 2004). The total annual water volume discharge is 500 million m³ (Bonetto 1986a).

The flood season varies between the Paraguay and upper Paraná, thus moderating the flow and extending the flood season by the time flood waters reach the middle and lower Paraná. High water occurs along the middle and lower Paraná between March and April, followed by low water season between August and October, with water levels varying from 2-6 m. During the flood season the Paraná overflows its banks, reaching ponds and lakes on islands in the floodplain, thus providing important habitats for young and juvenile fish (Lowe-McConnell 1987). During the dry season many of the lakes become isolated or dry up completely. The floodplain of the middle and lower sections of the Paraná are especially productive due to the variety of habitats providing food and refuge. The seasonal regularity in the flood pulse is important to the ecology of the river system. In contrast, variations in river water levels do not significantly affect the Río de la Plata due to its length and breadth, although tidal influences have a significant influence (Resende 2004).

Terrestrial habitats

This large ecoregion crosses eleven terrestrial ecoregions that include Atlantic forests, chaco, espinal, and pampas, as well as small patches of yungas, puna, and steppe in the Andes. Flooded savanna occurs along seasonally flooded islands of the middle and lower Paraná, including the Paraná Delta and Río de La Plata. Characteristic trees along the floodplain include Humbolt’s willow (Salíx humboldtiana), river alder (Tessaria intergrifolia), and ceibo (Erythrina crista-galli) (WWF 2001).

Description of endemic fishes

Fifty-three species (15%) in 16 families are endemic to the ecoregion. The Characidae family (33%) contains the most endemics, followed by Loricariidae (15%) and Pimelodidae (11%). Only one genus, Ramnogaster (R. melanostoma), is restricted to this ecoregion. The number of endemics is markedly lower than the upper Paraná [345], which presently has 131 recorded endemics. The distinction between the two ecoregions is due to the barrier of the former Guaíra Falls (Bonetto 1986b).

Other noteworthy fishes

Noteworthy fishes include the aimara (Hoplerythrinus unitaeniatus) and trahira (Hoplias malabaricus) of the Erythiniae family, which are predatory and can tolerate low oxygen levels. As a result, they assume dominance during periods of low water on the floodplain.

One of the most noteworthy fish in the Paraná basin is the sábalo (Prochilodus lineatus). It is abundant throughout the Paraná basin and may comprise more than 50% of the fish biomass in the lower Paraná (Bonetto 1986b). It is able to migrate distances up to 1000 km and can overcome rapids and falls (Sivasundar et al. 2001).

Ecological phenomena

Migratory species in the Paraná are potamodromic, undergoing long migrations to spawn. Some examples include the sábalo or streaked prochilod (Prochilodus lineatus), dorado (Salminus brasiliensis), and boga (Leporinus obtusidens). Most migrations are reproductive, migrating upstream to spawn during high water. The eggs and larvae float downstream with the current to colonize and grow in lakes in the alluvial plain. Adults undergo downstream mirgrations to feed in the alluvial plain (Lowe-McConnell 1987; Iriondo et al. 2007).  Some migrations are also temperature induced. For example, the large spotted sorubim or surubí (Pseudoplatystoma corruscans) undertakes spawning migrations in spring, and another migration in winter in search of favorable temperatures (Bonetto 1986b).

Justification for delineation

This ecoregion falls within Gery’s (1969) Paranean region, and was further refined by Ringulet (1975) as the Paraná-Platense icthyofuanistic province. The fish fauna of this ecoregion has a low affinity with the fishes of the upper Paraná due to the historical barrier presented by the Guaíra Falls (Bonetto 1986b).

Level of taxonomic exploration

Good