South Texas Coastal Wetlands
Padre Island and Laguna Madre

James S. Aber
Emporia State University

Table of contents
Introduction Padre Island
Atascosa NWR Related sites

Introduction

Padre Island is among the most famous barrier islands in the world. Stretching 113 miles (182 km) from Port Isabella to Corpus Christi, it is the longest barrier island in the United States (Weise and White 1980). A segment 80 miles (129 km) long is designated as a National Seashore--see texas.pdf file. The climate of this region is semitropical, as freezing conditions rarely occur, particularly on the coast--North Padre Island falls in climatic zone 9 and South Padre Island is in zone 10 (see
map).

Padre Island is constructed primarily of sand deposited along the beach and in wind-blown dunes behind the beach. The island formed during the past several thousand years. As the last great ice sheets melted, sea level rose, and the shoreline retreated inland. The shoreline stabilized about 4500 years ago, and initially several short barriers formed. These gradually grew together to create the present long island. Wetland conditions include ephemeral ponds and marshes developed within swales between dunes and in the vegetated barrier flat between the fore-island and back-island dune tracts. These are typically fresh to brackish wetlands supported mainly by precipitation.

Padre Island is separated from the mainland by Laguna Madre, a shallow lagoon that receives little runoff from the mainland and has few connections to the open sea. Owing to minimal circulation and high evaporation rates, lagoon water is hypersaline. Subdesert conditions prevail toward the south. Sediment of the lagoon consists of mud, sand, and shell berms, much of which is stablized by marine grass. Shoalgrass (Halodule wrightii) is the most abundant marine grass, which prefers shallow water (<1 m deep) and tolerates high salinity, as its Latin name suggests.

Grassflats are highly productive environments that support abundant invertebrates (snails and clams) and are spawning grounds or nurseries for many fish and crustaceans, such as shrimp and crabs (Weise and White 1980). A delicate balance of salinity, turbidity and water depth contribute to this ecological situation, which is important for commercial and sport fishing in the Gulf.

Both Padre Island and Laguna Madre are modified by human activities. The Mansfield Channel cuts across the lagoon and island, connecting Port Mansfield directly to the Gulf of Mexico. This channel has altered the flow and exchange of water between the lagoon and open sea. Salinity in Laguna Madre has moderated, and fish can migrate between the lagoon and ocean (Weise and White 1980). Another channel is the Intercoastal Waterway, a canal running the entire length of Laguna Madre. Sediment dredged from the channel was placed in spoil piles, which have been modified by erosion and vegetation growth into a chain of small islands and shoals along the canal. Some of these spoil islands have become important bird nesting sites, and many are used for sport fishing.

South Texas wetland vegetation.

Padre Island

Cattail (Typha domingensis). This species grows in moist habitats of beaches and barrier islands (Richardson 2002). Seen here in a hollow between dunes, North Padre Island. Photograph by J.S. Aber © Oct. 2005.
Sea Oats (Uniola paniculata) on a small dune, South Padre Island. Characteristic of Gulf coast beaches from southern Texas to western Florida, it is important for stabilizing sand dunes (Richardson 2002). Photograph by J.S. Aber © Oct. 2005.
View northward along the beach and sand dunes of North Padre Island. The great length and continuity of Padre Island are evident for this high-energy coastline. Laguna Madre visible in far left background. Kite aerial photograph by S.W. Aber © Oct. 2005.
Looking inland across sand dunes of Padre Island with Laguna Madre in the background. Active "blowout" dune to right. Swales to left contain wetland vegetation--darker green and reddish green zones. Kite aerial photograph by S.W. Aber © Oct. 2005.
Great Blue Heron (Ardea herodias) skims along the seashore of North Padre Island. A predator of anything it can swallow, including fish, birds, rodents, snakes, etc. It spears larger prey with its long beak, but catches smaller items between the mandibles. Found in wetlands throughout most of North America. Photograph by S.W. Aber © Oct. 2005.

Laguna Atascosa National Wildlife Refuge

Subtropical scrub vegetation of the coastal zone, Laguna Atascosa NWR. Spanish Dagger, Palma Pita (Yucca treculeana) and Texas Prickly Pear (Opuntia engelmannii) are indicative of the hot, dry conditions (Richardson 2002). Photograph by J.S. Aber © Oct. 2005.
View northward along the mainland margin of Laguna Madre. The white line at water's edge is foam, not beach sand. Notice the irregular shoreline and numerous islets of various shapes, which are typical of a low-energy coastline. Laguna Atascosa NWR. Kite aerial photograph by S.W. Aber © Oct. 2005.
View toward the southeast over Laguna Madra with Padre Island barely visible on the horizon, Laguna Atascosa NWR. Kite aerial photograph by S.W. Aber © Oct. 2005.
Closeup view of tiny, crescent-shaped island in Laguna Madre, Laguna Atascosa NWR. Such sites are important bird santuaries, as few predators can reach these islands. Kite aerial photograph by S.W. Aber © Oct. 2005.
Roseate Spoonbill (Ajaia ajaja) feeding in shallow pool on the edge of Laguna Madre, Laguna Atascosa NWR. The largest of all ibis in North America, its showy pink feathers were much prized, and the bird was hunted nearly to extinction early in the 20th century. Only 30 individuals remained in 1939. Under protection, it has since recovered. Photograph by J.S. Aber © Oct. 2005.
Texas gopher tortoise (Gopherus berlandieri) in subtropical scrub forest of the Laguna Atascosa NWR. The male (shown here) has a distinctive extension of the gular scutes (lower shell below the neck). Mainly a herbivore, it feeds on succulent plants and cacti as well as insects and snails (Ferri 2002). Photograph by J.S. Aber © Oct. 2005.


Related sites

References


Wetlands syllabus.

© Notice: Wetland Environments is presented for the use and benefit of students enrolled at Emporia State University. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor, J.S. Aber (2016).