Boundaries

This ecoregion includes the drainage basins of the Río Paraguay (Rio Paraguai), with tributaries that flow through the southern slopes of the Brazilian Shield and tributaries that flow through the low gradient terrains of the Pantanal Matogrossense. The southernmost limit is the confluence of the Río Paraguay with the Río Paraná. The northern boundary is the drainage divide with the Amazon Basin along the Chapada dos Parecís and Chapada dos Guimarães on the Brazilian Shield. The eastern boundary is the divide with the Paraná and Araguaia basins. The western boundary is the western side of the Río Paraguay’s main channel.

Topography

The Paraguay encompasses a vast, relatively flat sedimentary plain that is bordered to the north and east by the Brazilian Shield and to the west by ancient volcanic ranges and distantly by the Andes (Heckman 1998). Within the basin lies the Pantanal, which forms a series of alluvial fans and inland deltas that have merged with the Paraguay’s floodplain (Veblen et al. 2007). Most floodplain soil is sandy with deposits of clay and limestone (Willink et al. 2000). Elevations range from 50 m to 200 m in the Pantanal, and rise to over 1200 m in the surrounding highlands (Planalto). The basin has a gentle slope, measuring 1-2 cm/km² north-south and 6-8 cm/km² east-west.

Freshwater habitats

Extending 2550 km, the Río Paraguay is the largest tributary of the Paraná, and itself the fifth largest river in South America. Its headwaters originate in the Chapada dos Parecis and flow into the Pantanal, which is the largest wetland complex in the world, stretching 140,000 km² across Brazil, Bolivia, and Paraguay (Dolabella 2000). This vast area includes permanent and semi-permanent lakes, ponds, oxbows, floodable depressions (baías), rivers, streams, swamps, and anastomosed channels (Resende 2003). The Pantanal’s gentle slope, in addition to a single outlet through the Rio Paraguay, heavy rainfall, and poorly drained soils account for massive flooding during the annual wet season when up to 78 % of the Pantanal is submerged (Hamilton et al. 2002). This amounts to 172 days of the year, with the Paraguay fluctuating by as much as 4-5 meters. During this flood season the Pantanal becomes an important nursery for fishes, macroinvertebrates, and aquatic plants (Willink et al. 2000). Flood peaks occur between May and August and low water between December and January, which is often asynchronous with the wet and dry seasons. It takes six months for the river pulse to travel from the Paraguay’s headwaters to the lower Paraguay (Dolabella 2000). However, as water levels fall, once contiguous areas separate into shallow lakes and rivers, producing a dynamic seasonal succession of biotic communities. Only the Paraguay and Cuiabá retain a steady flow despite considerable variation in their water level (Heckman 1998).

Habitat and species diversity of aquatic vegetation is high, reflecting the dynamic flood regime and nutrient cycles (Chernoff et al. 2001). Floating meadows are characteristic of the Río Paraguay. Common aquatic vegetation includes species such as pickerelweed (Pontederia lanceolata), anchored water hyacinth (Eichhornia azurea), eared watermoss (Salvinia auriculata), water lettuce (Pistia stratiotes), and floating watermoss (S. natans) (Heckman 1998).

The waters of the Paraguay are relatively shallow, meandering, and slow-moving, although around Fort Olimpo the current increases (Carron 2000). The banks are lined by gallery forests and palms, and oxbow lakes and channels are common. Water quality of the Paraguay tends to have a slightly acidic pH (6.0-6.5), low oxygen levels (<6 mg/l), low electrical conductivity (60-100 μS/cm), and water temperatures between 24-27 ºC (Chernoff 2001). 

Terrestrial habitats

This ecoregion is home to a mosaic of plant species from the Amazon and Atlantic rainforests, gallery forests, cerrado, and chaco vegetation (Resende 2003). In addition to permanently flooded wetlands are species at higher elevations that are adapted to periodic flooding. For example, seasonally flooded grasslands are dominated by herbaceous plants such as purpus’ carpetgrass (Axonopus purpusii) and acute spikerush (Eleocharis acutangula) that can tolerate the extremes of flood and fire, as well as seasonally flooded palm stands dominated by Caranday palm (Copernicia alba) (WWF 2001). There are also cerrado, chaco, semi-deciduous, deciduous, and gallery forest species that occur at higher elevations beyond the flood zone, including Psidium kennedyanum, Acacia spp., kapok (Ceiba pentandra), and pacara earpod tree (Enterolobium contortisiliquum) (Prance and Schaller 1982).

Description of endemic fishes

The ecoregion contains more than 80 endemic species in 14 families. Most of the endemics are found in the families Characidae (25 species), Loricariidae (21), and Rivulidae (12). There are also two endemic genera represented by the monotypic Mixobrycon ribeiroi and Merodoras nheco. Endemicity tends to be higher in the headwaters than in the floodplain (Willink et al. 2000).

Other noteworthy fishes

Primary production increases significantly during the flood season, resulting in the movement of fish across the floodplain to breed and feed (Lowe-McConnell 1987). Some species, like the South American lungfish (Lepidosiren paradoxa), dwarf corydoras (Corydoras hastatus), and armored catfish Corydoras hastatus, have special adaptations for aerial respiration (Lowe-McConnell 1987). Others, like Rivulus sp., lay eggs that can survive the dry season (Chernoff et al. 2001).

Ecological phenomena

A number of fish species undergo migrations in the Paraguay basin to reproduce. One of the most notable is the sábalo (Prochilodus lineatus), which typically begins its migration or “piracema” upstream around the end of the dry season. During the early wet season fishes enter headwaters and flooded areas to spawn, returning to permanent rivers as the waters recede (Resende 2003; WWF 2001). Large catfishes like the moleque (Pseudoplatystoma corruscans) also migrate to spawn in the headwaters (Resende 2003).

Justification for delineation

This ecoregion lies wholly within the Alto Paraguay and partly within the Parano-Platense ichthyographic province defined by Ringuelet (1975). It is distinguished by the Pantanal, the largest wetland complex in the world. The Pantanal was once a saltwater lake before connecting to the Paraná during the Holocene. This has resulted in a slightly different fish fauna between the upper and lower Paraguay (Resende 2003). There are even more pronounced differences in fish fauna compositions along the Paraguay, Paraná, and Río de la Plata river axis. Differences in climate patterns between the tropics and subtropics, through which these rivers flow, may account for some of the distinctiveness (Junk 2007).

Level of taxonomic exploration

Good in the large rivers and fair in the headwaters.