Boundaries

This ecoregion in southern Brazil and eastern Uruguay drains the southern slopes of the Serra Geral. It also includes most of the Serra do Sudeste formation in Rio Grande do Sul that drains into the Lagoa dos Patos system. Along the coast it extends roughly from Tramandaí, Brazil in the north to Piriápolis, Uruguay in the south. The northern and western boundaries are defined by the divide with the Uruguay River basin.

Topography

The coastal plain is a flat to gently undulating Holocene and Pleistocene barrier system. The barriers are not islands, but in fact consist of two peninsulas composed of dune fields (Dillenburg & Hesp 2009). Alluvial fan sediments derived from igneous and metamorphic rocks of the Precambrian shield and sedimentary and volcanic rocks of the Paraná basin rest further inland (Tomazelli et al. 2000). Elevations range from sea level to over 1200 m asl in the Serra Geral (Hijmans et al. 2004), where deep and narrow valleys dominate the landscape.

Freshwater habitats

This ecoregion includes a variety of aquatic habitats, including brackish-salt coastal lagoons, freshwater coastal lagoons, swamps, marshes, seasonal and permanent freshwater ponds, rivers, and streams. Lagoa dos Patos is the largest coastal barrier-lagoon system in South America (Dillenburg & Hesp 2009). With an area of 10,000 km², Lagoa dos Patos is a relatively shallow estuarine lagoon with one permanent outlet called the Rio Grande, located at Cassino. Waters are brackish a short distance above this outlet. Salt marshes (marismas) border the estuaries and provide nutrients and productivity to the system (Marques et al. 2004). Lagoa Mirim (3,770 km²) is smaller and empties into Lagoa dos Patos through the Canal de São Gonçalo. Both of these lagoons are remnants of an ancient depression enclosed by dune field barriers built by high energy winds and currents. Their waters are affected by semidiurnal tides with a mean range of 0.5 m (Tomazelli et al. 2000).  The rivers draining into the laguna complex are meandric with well-developed fringing floodplains in their lower reaches and steep and fast in their mid and upper reaches, except for the Rio Gravataí that runs mostly through a system of low-lying wetlands along the coastal plain.

The ecoregion supports two Ramsar sites: Lagoa do Peixe and Bañados del Este. Lagoa do Peixe, located on the peninsula near Mostardas, is a large brackish to saline lagoon that is an important wintering and staging area for migrating birds, and is home to large concentrations of waterbirds. Bañados del Este is a large wetland complex in Uruguay comprised of freshwater (e.g. Lagoa Mirim) and brackish lagoons, as well as parts of several rivers. It is an important area for local and migratory shorebirds (Charadriidae and Scolopacidae).

Terrestrial habitats

This ecoregion is home to four terrestrial ecoregions: Uruguayan savanna, Alto Paraná Atlantic forests, Araucaria moist forests, and Atlantic coast restingas. Although the coastal habitats are most notable, a majority of the southern and western portion of the ecoregion is Uruguayan savanna (Pampas) composed of grasslands, some gallery forests in the south, palm savannas, and outcroppings of submontane forests. Alto Paraná Atlantic forests are Brazilian Atlantic semi-deciduous forests that occur along the foothills of the Serra Geral on the northern side. Above 400 m Araucaria moist forests are dominant. These forests are a mix of Atlantic moist forest types with an emergent layer of Brazilian Araucaria (Araucaria angustifolia).  Along the coast are pockets of Atlantic coast restingas, composed of sandy dunes, shrubs, and low forests (WWF 2001).  The upper reaches of the Serra Geral and Serra do Mar are dominated by high elevation grasslands.

Description of endemic fishes

One-third of the fish in this ecoregion are endemic. Of these, Cyprinodontiformes, particularly rivulids, represent almost half. The one endemic genus is represented by the sea catfish, bagre (Genidens planifrons), which occurs in Lagoa dos Patos drainage and the Atlantic Ocean. Fourteen of the sixteen Austrolebias species are endemic, as are all six Rineloricaria species, and four out of five Cynopoecilus species. The two remaining Austrolebias species (A. nigrofasciatus, A, juanlangi) are near-endemic.

Other noteworthy fishes

Four species found in fresh, brackish, and marine waters include pejerry (Odontesthes argentinensis), Argentinian silverside (Odontesthes bonariensis), Atlantic sabretooth anchovy (Lycengraulis grossidens), and bagre (Genidens planifrons).

Justification for delineation

This ecoregion lies primarily in Gery’s (1969) East Brazilian faunal region and southeastern province and Ringuelet’s (1975) ríos costeros S.E. Brasil ichthyographic province, with the southern tip crossing into the Paranean province. It contains a unique assemblage of species of the southern Atlantic coastal drainages.

Level of taxonomic exploration

Good