Figure 2-7: View of salt marsh habitat in the upper 

 Nueces Delta. The vegetation transect for Station I is in 

 this vicinity. 



Photo courtesy of the Bureau of Reclamation. 



Like seagrass beds, salt marshes are also an important 

 nursery and feeding grounds for a variety of 

 invertebrates and fish. Because of the amount of dead 

 and decaying plant matter in salt marshes, their 

 contribution of detritus to the food-web of adjacent 

 habitats is important. The plant litter is uti]i2ed by 

 micro-organisms and other small estuarine animals 

 (Marples 1966) and serves as a critical link between 

 primary and secondary trophic levels (Burkholder and 

 Burkholder 1956; Odum and Wilson 1962; Teal 1962). 

 Marsh plants also provide shelter for a variety of small 

 organisms, including crustaceans, avians and mammals, 

 and serve to stabiLi2e marsh sediments. Because of 

 these functions, the marsh subsystem in the Nueces 

 Delta supports numerous estuarine organisms, 

 including shrimp, crabs and fin-fish, by providing large 

 amounts of food and structure. 



One of the primary variables affecting the distribution 

 and abundance of vegetation species in the delta marsh 

 is salt. Salt is generally imported into the delta from 

 Nueces Bay by a variety of tidal and wind forces, and 

 generally exported from the delta by infrequent 

 freshwater inundation events resulting from over- 

 banking floods in the Nueces River. Water and soil 

 salinity can be highly variable, depending upon 

 precipitation, tide level and temperature (Henley and 



Rauschbauer 1981), especially because evaporation 

 often exceeds precipitation for several months a year 

 (Longley 1994). Elevation and proximity to channels 

 and creeks are also factors that affect salinity, which 

 often results in distinct vegetation zones (Chapman 

 1960; Chapman 1974; Nixon 1982). 



Although most marsh plant species in the delta are salt- 

 tolerant (halophytic), excessive salt concentrations can 

 cause hypersaline conditions which are adverse to plant 

 survival. For example, although halophytic species can 

 survive in intermittent hypersaline environments, 

 prolonged periods of salinity stress can stunt active 

 growth and reproduction, leading to decreases in 

 abundance and productivity (Deegan et al. 1986; 

 Bertness ^/ «/. 1992). Ultimately, decreased vegetative 

 coverage can create bare areas, which are then direcdy 

 exposed to evaporation, leading to further increases in 

 salinity (Zedler 1983; Bertness 1991). Successful re- 

 colonization of bare areas requires at least a short-term 

 lowering of salt concentrations in the soils, which 

 allows re-invasion by vegetative (rhizomes) or 

 reproductive (seed germination) growth. 



Water Column 



The water column habitats of the Nueces Estuary and 

 Delta are shallow and are often only as productive as 

 their substrate (Figure 2-8). This is in contrast with 

 marine environments, such as the Gulf of Mexico, 

 where the water column habitat is very deep and more 

 productive than the bottom. Because fresh water 

 mixes with salt water in the bays, the resulting salinity 

 concentrations are often brackish (10 to 25 ppt). 

 However, when evaporation exceeds freshwater inflow 

 and flushing by the ocean, salinity concentrations can 

 become saltier than the ocean (> 35 ppt). During dry 

 periods in the delta, where supratidal pools and 

 channels may become isolated for extended periods of 

 time, salinity concentrations may exceed that of the 

 ocean by several times. The estuarine water column is 

 usually well oxygenated but can become quite turbid as 

 sediment is re-suspended by wind or human actixdties. 

 Mixing, due to the consistent high winds and shallow 

 depths, prevents significant stratification of the bay 

 water column. 



Chapter Tm ♦ 2-9 



