The Lower Grande - Bravo freshwater ecoregion, FEOW ecoregion 135, is a temperate coastal-river ecoregion centered on the lower Río Bravo / Rio Grande of northeastern Mexico and southern Texas. It includes the lower river corridor, tributaries, springs, aquifers, resacas, oxbows, reservoirs, irrigation canals, riparian wetlands, and the coastal transition toward the Gulf of Mexico and Laguna Madre. In Mexico, it is associated mainly with the lower Río Bravo region of Coahuila, Nuevo León, and Tamaulipas.

The ecoregion is warmer, lower, and more subtropical than the upper Río Bravo and Río Conchos regions. Its habitats include mainstem river channels, pools, runs, spring-fed streams, thornscrub tributaries, resacas, floodplain wetlands, reservoirs, irrigation drains, estuarine lower reaches, and brackish coastal margins. FEOW emphasizes that the basin is being dewatered and salinized, with brackish-water influence expanding upstream and contributing to the loss of freshwater species.

The fish fauna combines Nearctic, Gulf Coastal Plain, Chihuahuan Desert, northeastern Mexican, spring-associated, and estuarine-tolerant elements. Important groups include cyprinids, catostomids, ictalurids, centrarchids, cyprinodontids, poeciliids, atherinopsids, gars, silversides, killifishes, estuarine-associated fishes, and cichlids. The ecoregion is not a tropical fish-diversity center like the Grijalva-Usumacinta or Papaloapan, but it is highly distinctive because it lies at the meeting point of desert rivers, subtropical lowlands, springs, salinity gradients, and the Gulf coast.

Cyprinids and catostomids are important in flowing-water and larger river habitats, while catfishes, sunfishes, gars, killifishes, and livebearers occur in river channels, reservoirs, resacas, floodplain habitats, springs, and coastal transition zones. Poeciliids and cyprinodontids are especially relevant in springs, lowland margins, vegetated habitats, warm, shallow waters, saline habitats, and coastally influenced settings. As flows have declined and salinity has increased, the fish community has shifted in some areas from freshwater species toward more brackish-tolerant or lowland forms.

The Zona Carbonífera is one of the distinctive features of the ecoregion. FEOW describes it as a subterranean aquifer with springs and caves that support a distinctive cave fauna. These groundwater and spring habitats are highly vulnerable to aquifer overuse, pollution, and drying. For FFM, the Lower Grande - Bravo should therefore be treated not only as a river ecoregion, but also as a spring, cave, aquifer, and resaca system.

Cichlids are native to the Lower Grande-Bravo ecoregion, but the native cichlid fauna is limited. The key species is Herichthys cyanoguttatus, the Río Grande cichlid or Texas cichlid. This species is naturally associated with northeastern Mexico and southern Texas, especially the lower Río Bravo / Rio Grande and nearby Gulf-slope drainages. Older literature may list it as Cichlasoma cyanoguttatum. It inhabits warm rivers, pools, runs, springs, and vegetated waters and is the characteristic native cichlid of this ecoregion.

Status should be handled carefully. Herichthys cyanoguttatus is native to the lower Río Bravo / Rio Grande region, but many populations elsewhere in Texas, Florida, Louisiana, and other regions are introduced or extralimital. Other cichlids in the ecoregion, especially tilapiines such as Oreochromis niloticus, Oreochromis aureus, and Oreochromis mossambicus, should be treated as introduced if present in reservoirs, ponds, canals, aquaculture systems, irrigation waters, or modified lowland habitats.

Introduced fishes are a major conservation issue. Non-native fishes may affect native species through predation, competition, disease, habitat alteration, and changes in food webs. In the lower Río Bravo, introductions interact with other major stressors, especially dewatering, salinity increase, and habitat simplification. The original freshwater fish communities of the lower river have been strongly altered since the mid-twentieth century.

Conservation priorities include maintaining environmental flows, protecting springs and aquifers, conserving resacas and floodplain wetlands, preserving riparian vegetation, improving water quality, and reducing salinity and invasive-species impacts. Major threats include dewatering, groundwater depletion, spring drying, salinization, reservoir operation, irrigation withdrawals, channel modification, pollution, agricultural runoff, urban wastewater, invasive species, riparian loss, and climate-driven drought. For FFM, Lower Grande - Bravo should be treated as a highly stressed lower Río Bravo ecoregion whose native fish fauna includes desert, subtropical, spring, cave, river, and coastal elements, with Herichthys cyanoguttatus as its characteristic native cichlid.

This account is based in part on information from Freshwater Ecoregions of the World (FEOW), © WWF/TNC, used as a reference for educational and scientific purposes. See: http://www.feow.org.