the soil surface, exposing them to salinity levels 2 to 

 100 times that of the subsoil (Ungar 1978). 



While adult plants have been reported to tolerate 

 salinity levels 10 to 100 times greater than seedlings 

 (Mayer and Poljakoff-Mayber 1963), high soil salinity 

 levels can negatively affect adult plant growth as well. 

 Plant sunaval does not necessarily mean plant growth, 

 as a plant may continue to sun-ive at a particular 

 salinity level without increasing in si2e or actively 

 reproducing. Several primary mechanisms have been 

 suggested to explain the negative effects of 

 hypersalinity on halophytes. These include ion toxicity 

 of internal cells, interference with the uptake of 

 essential nutrient ions, lowered external water potential 

 and energy constraints (e.g., a large amount of energy is 

 required to actively salt ions) (Greenway and Munns 

 1983; Yeo 1983). 



Most halophytes can survive over a range of salinity 

 concentrations, but no species has been reported to 

 have maximal growth rates at salinity levels at or above 

 seawater concentration (35 parts-per-thousand (ppt)) 

 (Ungar 1991). For example, Spartina foliosa, a California 

 salt marsh plant, was found to have 50% less dry mass 

 production in sea water than in fresh water, with only 

 39% of the plants sur%dving in the saltwater treatment 

 (Phleger 1971). Barbour (1970) reported growth 

 reductions in Salicomia virginica and Distichlis spicata at 

 salinity values ranging from 5 to 22 ppt. Adams (1963) 

 noted that in North Carolina salt marsh plants could 

 not tolerate soil salinity levels over 70 ppt. Allison 

 (1992) found a reduction in species number in a 

 California salt marsh after periods of low freshwater 

 availability suggesting that only a few stress -tolerant 

 species could survive the high saUnity. 



In many instances, short-term freshwater flooding of 

 hjrpersaUne marshes leads to an increase in primary 

 productivity. Zedler (1983) found biomass oi Spartina 

 foliosa to increase 40% in the Tijuana Estuary, 

 CaUfomia, after two months of flooding rains. Covin 

 and Zedler (1988) noted a 60% increase in the stem 

 density oi S. foliosa after summer reservoir discharges 

 and sewage spills along the Mexico border. They also 

 found near extinctions of Salicomia higelovii and Suaeda 



esteroa during a drought period in 1984 that led to 

 hypersaline conditions. 



In the Nueces Delta, hypersalinity occurs as a result of 

 both natural and human-induced conditions. The 

 region is semiarid, having low annual rainfall (70 

 centimeters (cm) or 28 inches per year) and hot, dry 

 summers. A net annual water deficit is common, as 

 evaporation (1 52 cm or 60 inches per year) often 

 exceeds precipitation (Longley 1994). These 

 conditions produce hj^persaline soils that are diluted 

 only through direct precipitation or by flooding of the 

 Nueces River. The natural salinity stress is accentuated 

 in the Nueces Delta because considerable harnessing of 

 river water for municipal, agricultural and industrial 

 purposes has reduced the opportunity for freshwater 

 flooding events into the marshlands (Irlbeck and Ward 

 2000). In years prior to the demonstration project, the 

 river breached its banks and flooded the marsh only 

 during infrequent flooding events. 



OBJECTIVES 



1) To determine the effects of the demonstration 

 project on the open water and pore water salinity 

 and nitrogen levels; and 



2) To determine the project effects on the 

 distribution and abundance of emergent marsh 

 vegetation at three different stations over four 

 growing seasons in the upper Nueces Delta. 



MATERIALS AND METHODS 



Monitoring Stations 



The emergent vegetation and related physio-chemical 

 parameters (i.e., salinity and nitrogen levels) were 

 quantified at three sampling stations in the upper 

 Nueces Delta, including one reference station and two 

 treatment stations (Figure 6-1). Station I (Reference 

 Station) was located west of the tidal flats area of the 

 upper delta, about 0.9 km from the outfall of the 

 Rincon Overflow Channel. This location was selected 

 to limit the amount of influence by fresh water 

 diverted by the demonstration project, but also to 



6-2 



Vegetation Communities 



