Union of Concerned Scientists

About Laguna Madre
The Laguna Madre, the only lagoon in the United States saltier than the ocean, is part of the South Texas Coastal Plain, which extends inland from the western Gulf of Mexico as a gently sloping prairie of short grasses, mesquite trees, thorny brush and prickly pear cactus. Much of the plain is used as rangeland, with some cropland and improved pasture. The Nueces and Rio Grande river systems encircle this semi-arid region, providing the only year-round sources of surface fresh water for the cities and estuarine systems of South Texas.

The Laguna Madre formed between the mainland and Padre Island, the longest barrier island in the United States. Tidal currents in the lagoon are weak, circulation is sluggish, and residence times of water masses are long. During exceptionally dry periods, high salinity may cause fish and other animals to leave or die. The lagoon also suffers natural, periodic large fish kills due to severe freezes.

Biodiversity
The South Texas Coastal Plain supports unique ecosystems and wildlife, including barrier island dunes and beaches, Tamaulipan brushlands, Laguna Madre seagrasses, intertidal wind flats, and the Baffin Bay and Rio Grande delta. These habitat contain remarkable biological diversity:

• Padre Island's beaches serve as nesting grounds for endangered Kemp's ridley sea turtles and threatened green sea turtles.
  
  
  
• Tamaulipan brushland is home to more than 600 vertebrate animal species and 1,100 plant species, including an oily oak tree found nowhere else.
  
  
• The banks of the Arroyo Colorado and nearby uplands shelter unusual animals like ocelot, jaguarundi, indigo snakes, horned lizards, chachalaca, green jays, kiskadee flycatchers, and other unusual animals.
  
  
• The Laguna Madre accounts for three quarters of Texas's seagrass habitat, which supports many rare and endangered species and provides vital nursery grounds for fish and shellfish.

Human Pressures
The most important human impacts on this region over the past 30 years have been:

• water diversion and flood-control projects;
• brushland clearing;
• pollution;
• continued dredging of the intracoastal waterway; and
• pressures from population growth.

The lower Laguna Madre, for instance, has lost about 60 square miles of seagrass cover due to reduced water clarity since the 1960s. Extensive agriculture has fragmented and reduced the areas of native terrestrial ecosystems. And both the northern and southern ends of Padre Island have been developing rapidly as resort and residential real estate. In addition, the large number of people now living in "colonias" without sewage treatment contributes to the contamination of ground and surface waters and poses a human health problem. This is worsened by untreated wastewater from Mexican municipalities released into the Rio Grande.

In addition to human impacts, parts of the Rio Grande watershed have experienced severe drought since 1993, and low flow has exacerbated water quantity and quality problems. Also, a lengthy, unprecedented brown tide (a type of toxic algal bloom) has persisted in the Laguna Madre since 1990.

Future changes
Global warming will compound these human pressures on Laguna Madre, in some cases improving, in others cases worsening the situation. For example, if future climate change brings a prolonged and more intense wet season in this region, the reliability of rainfall and soil moisture could improve. In wet periods, the land can retain rainfall and runoff, so wildlife and native plants increase their productivity, and the lagoon's salinity is moderated.

If rainfall decreases in the future, however, a relatively small reduction in moisture could lead to increased desertification. Moreover, all types of coastal wetlands in Texas would decline with less freshwater delivery to the estuaries, thus worsening wetland losses already occurring. Over the long term, such coastal wetland losses would diminish estuarine-dependent fisheries.

Warmer winters are especially important from an ecological point of view. A northward shift of the freeze line would bring dramatic effects to the Coastal Bend and upper Laguna Madre, allowing southerly plant and animal communities to expand northward and, due to fewer disturbances from frost, mature and develop different ecosystems over time.