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28 chapter  Molluscan Ecology and Habitats david w. hicks The 595 km (370 mi) Texas coastline (Fig. 3.1) supports a remarkable diversity of features, including wave-swept sandy beaches, jettied tidal inlets, shallow-water bays and lagoons, and intertidal bay shores and sand flats. While these physical features are common to the entire Texas coast, environmental gradients, both north to south and shore to sea, result in great biotic diversity. Latitudinal gradients of increasing temperature and decreasing rainfall toward the south have direct effects on water temperature and salinity in the bays and lagoons and hence, the ecology of their biota (Britton and Morton 1989; McAlister and McAlister 1993). From Sabine Pass to Galveston Island in the north, the climate is humid with a marked surplus of moisture (Table 3.1). Extending southward from Galveston to Port Lavaca, the climate is characterized as wet subhumid, with balanced moisture supply and loss. The next zone to the south, extending from Port Lavaca to Corpus Christi, is a dry subhumid belt. The transition between this dry subhumid region and the semiarid southernmost Texas coastline is gradual. This southernmost portion of theTexas coast is characterized by a climate that is permanently moisture deficient, almost a desert (Hedgpeth 1953). These climatic conditions produce bay systems with relatively greater freshwater inflows (lower salinities), higher water turbidity, and vast areas of well-developed marshes to the north. Oyster reefs are characteristic of these low-salinity northern bays and often occur immediately adjacent to coastal marshes. To the south of Mustang Island, fringing marshes are largely replaced by wind tidal flats and intertidal mangroves due to lower precipitation and greater evaporation rates. In addition , the highly saline, shallow lagoons in the south support vast seagrass meadows.The diversity of climates along theTexas coastline produces unique environmental challenges in each bay system and heavily influences their biotic communities. Physical Description of the Texas Coast The present-day bay and lagoon–barrier island system is relatively young, existing in its present state for only about the past 3000 years.To put this into perspective, consider that when the Egyptian pyramids at Giza were new, prominent coastal features such as Padre Island were just beginning to form. The Texas barrier-island chain has been described as the longest in the world. These islands have a minimum width of 100 m (328 ft) Figure 3.1. Map of the Texas coast showing the bay systems and associated rivers. Molluscan Ecology and Habitats 29 or specifically, the Sabine-Neches Estuary (Fig. 3.2). The Sabine-Neches Estuary is part of the larger Chenier Coastal Plain Ecosystem of Louisiana and northeastern Texas (Gosselink et al. 1979). It connects to the Gulf of Mexico through a long, narrow, jettied tidal inlet at Sabine Pass. At 243 km2 (94 mi2 ), the Sabine-Neches Estuary is the smallest of theTexas bay systems in terms of open water. However, precipitation and subsequent freshwater inflow give it the most extensive network of intertidal marshes anywhere in the state, extending from the western shores of Sabine Lake south to the Gulf at Texas Point and westward to MacFadden Beach. Due to the shoreward and a maximum width of nearly 7 km (4.5 mi).Their southern ends are typically narrower than their northern ends as a result of differential sediment deposition by longshore currents. Texas coastal embayments are classified as either coastal plain estuaries or bar-built estuaries.The coastal plain estuary is essentially the footprint of a former river delta that became flooded when sea levels rose approximately 4500 years ago. Coastal plain estuaries typically receive drainage from one or more rivers; thus, salinities are generally lower (i.e., positive or normal-type estuaries). The majority of Texas bays are coastal plain estuaries (e.g., Sabine Lake and Galveston, Trinity, Matagorda, Lavaca, San Antonio, Copano, and Corpus Christi bays). Bar-built estuaries are embayments paralleling the coastline between barrier islands and the mainland (e.g., East and West Galveston, East Matagorda, Espiritu Santo, Aransas, and Redfish bays and the Laguna Madre). The origin of bar-built estuaries is related to the formation of the barrier-island system. In contrast to coastal plain estuaries, bar-built estuaries have few, if any, rivers draining into them, making them generally more saline than coastal plain estuaries and in some cases, hypersaline (i.e., negative estuaries). Indeed, the Laguna Madre of Texas and its southern counterpart, Laguna Madre de Tamaulipas, together form 1...

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