Table 7-1 : Summary of the effects of the demonstration project on the upper Nueces Delta. 



ATTRIBUTE 



Before 



After 



Geomorphology 

 Salinity gradient 

 Nutrient cycling 



Dead-end Flow-through with free exchange 



Higher in the upper delta than in the bay Lower in the upper delta than in the bay 

 Recycled nitrogen New and recycled nitrogen 



Primary production Low In marsh 



Secondary production Constrained by dry conditions 



Habitat utilization Constrained by dry conditions 



Higher in marsh 

 Increased by flow events 

 Increased during spring and fall 



sources for higher trophic levels, indicating that 

 cyanobacteria production was a sink, not a link, in the 

 food chain. 



With the re-introduction of fresh water, more oxidized 

 forms of nitrogen were introduced into the delta. 

 Because nitrate and nitrite are the preferred forms of 

 nitrogen by diatoms, these organisms contributed a 

 substantial proportion of the productivity. Flow events 

 resulted in moderate levels of primary production (per 

 unit volume), but the total amount of fixed carbon was 

 very high because the large volumes of water involved 

 when elevations approach a meter or so in depth. 

 Because diatoms were significandy contributing to the 

 total productivity, there was a link with higher trophic 

 levels, and the grazing food chain was stimulated. 

 Increased freshwater inflow lowered salinity 

 concentrations in the soils and waters of the marsh, 

 stimulating plant production as well. Marsh 

 productivity ultimately drives the detrital food chain 

 when the plant material dies and decomposes. 



The increased primary production, in both the water 

 column and marsh, resulted in increased secondary 

 productivity in several ways. Grazing organisms could 

 direcdy utilize the benthic and plankton diatom 

 production. Increased carbon fixation led to higher 

 amounts of detritus and higher amounts of food 

 available to detritivores. A multiplier effect was present 

 because increased marsh vegetation also increased 

 habitat quality and complexity. Marsh areas are 

 important because they provide nursery habitats and 

 for many commercially and recreationally important 

 species, {e.g., shrimp, red fish, and sea trout). Without 



freshwater flow and concomitant increases in the 

 marsh system, the habitat does not support these 

 estuarine-dependent species. Consequendy, the 

 combination of increased food and increased habitat 

 quality and area likely resulted in geometric increases in 

 secondary production. 



A CONCEPTUAL MODEL 



The data collected during the present study allowed the 

 creation of the conceptual model presented in 

 Figure 7-4 on how Rincon Bayou fimctions, and, 

 perhaps more importandy, how the upper delta 

 functions with and without freshwater inflow. Rincon 

 Bayou's connection with Nueces Bay allows the 

 exchange of materials and energy flow between the two 

 bodies of water. In addition, nekton (fish and 

 epibenthic shrimp) utilize Rincon Bayou as a nursery 

 habitat when water elevations and salinity conditions 

 are suitable. 



The water levels and salinity concentrations in Rincon 

 Bayou are governed by the interactions between tide, 

 rain, evaporation and freshwater inflow (Figure 7-4). 

 These outside forcing elements also drive nutrient 

 concentrations, primarily through tides and inflow. 

 Nutrient concentrations are also governed by 

 biogeochemical processes associated with 

 decomposition of marsh grass, excretion by organisms 

 and recycling of dead organic matter. The nutrients, 

 with sunlight, drive primary production in the water 

 column (phytoplankton), which in turn drives a grazing 



7-8 ^ Synthesis and Conclusions 



