(a) Total annual flow 

 in the Nueces River 

 (at Calallen gauge) 



600000 



(b) Total annual precipitation 

 (at Corpus Christi International 

 Airport) 



120 



E 



*«— 



c 



CD 



o 



100 

 80 i 

 60 

 40 

 20 

 J-ls, 



(c) Total annual flow 

 into Rincon Bayou 

 (at Rincon gauge) 



7000 



i^ 6000 



^ 5000 

 o 



-™. 4000 - 



O 3000 



TO 2000 



° 1000 

 



^E^ 



XI 



a 



1992 1994 1996 1998 



1992 1994 1996 1998 



T T 



1992 1994 1996 1998 



Bef ore-project annual mean 

 After-project annual mean 



Figure 7-3: Total annual cumulations of selected freshwater sources affecting the Nueces Delta during the period 

 1992 through 1999. Flow in Rincon Bayou for the before-project period was estimated using dally stage data from the 

 Calallen gauge and the methodology described by Iribeck and Ward (2000). Total flow into Rincon Bayou for the after- 

 project interval of October 26, 1995, through May 15, 1996, was unavailable, but was estimated to be approximately 

 123 10^ m^ (100 acre-ft). The average annual flow In the Nueces River for the first period was 224,581 10' m' 

 (182,053 acre-ft) but only 127,678 10' m' (103,500 acre-ft) for the second. The average annual precipitation for the before- 

 project period was 94.2 cm (37.1 in) but only 74.7 cm (29.4 In) for the after-project period. Finally, the annual mean 

 freshwater flow into Rincon Bayou for the first period (1 ,931 10' m', or 1 ,565 acre-ft) was nearly equal to that for the period 

 after (1 ,854 10' m', or 1 ,503 acre-ft). 



Note; 1 acre-fl = 1.2336 10' m' 



In summary, the effects of the demonstration project 

 on the long-term salinity gradient in Rincon Bayou 

 were measurably significant. In a relatively short period 

 of time (only 4.2 years after the opening of the Nueces 

 Overflow Channel), the "reverse estuary" salinity 

 gradient in the upper delta before the demonstration 

 project reverted to a more natural form, with average 

 salinity concentrations in upper Rincon Bayou 

 becoming the lowest in Nueces Delta. 



BIOLOGICAL RESPONSES 



The influence of fresh water on the salinity 

 concentrations of the water and soils of the upper 

 Nueces Delta appeared to be the most important 

 parameter affecting the biological response of estuary 

 organisms in the delta. 



Water Column Productivity 



The inflow of river water into the upper Nueces Delta 

 imported vital nutrients (nitrogen, phosphorus and 

 silicon) required for plant growth into the channels and 

 ponds of the marsh ecosystem. During the 

 demonstration period, phytoplankton in the water 

 column and on the surface of the sediments rapidly 

 responded to the inputs of riverine nutrients with 

 increased growth rates and accumulation of biomass. 

 In addition, the freshwater inflow also reduced salinity 

 concentrations and lowered the osmotic stress on these 

 organisms. The increased primary production rates 

 were especially prominent during periods when salinity 

 was less than 50 ppt. The assimilation index {i.e., the 

 relative amount of growth per cell) was also generally 

 higher during periods of low salinity, indicating that 

 inherent growth rates were also increased by project 

 diversions. 



The species composition of phytoplankton apparently 

 remained dominated by primarily small diatoms during 



Chapter Seven ^ 7-5 



