Nueces Bay (AJCTiitledge and Stockwell 1995). 

 Therefore, the rapid uptake of nitrate due to freshwater 

 inflow very likely occurred in the upper Nueces Delta 

 shordy after numerous hydrographic events during the 

 demonstration period, even if these responses were not 

 readily observed within the (infrequent) sampling 

 intervals. 



Second, the total of water column and sediment 

 primary production in the delta had an inverse 

 relationship with salinity. Although the water column 

 contributed the largest fraction of the total ica. 75%), 

 the sediment production rates also provided a 

 significandy large amount. As salinity values in Rincon 

 Bayou declined below 60 psu, the range of primary 

 production increased. Therefore, the diversion of an 

 estimated 8,810 10' m' (7,142 acre-ft) of water into 

 Rincon Bayou during the demonstration period 

 lowered salinity concentrations, the osmotic stress on 

 individual organisms lowered and almost certainly 

 increased primary production in those waters. 



Third, although the increase of N:P was not strongly 

 correlated widi inflow events, at least four sampling 

 dates had values near or exceeding 1 5 which were likely 

 the result of new nitrogen being added to the study 

 area by river inflow. In general, the nutrient 

 concentrations in the water column, especially 

 dissolved inorganic nitrogen, were within the range to 

 allow large amounts of primary production. Most 

 (50 to 90%) of die DIN was in the form of 

 ammonium, which is readily utilized by both water 

 column and benthic phytoplankton. The shallow water 

 environment enhanced nutrient remineriaUzation, so 

 both the ability to utilize and produce nutrients were 

 relatively large. The N:P ratio indicated that nitrogen 

 was typically the nutrient in lowest concentration. 



Fourth, the species composition of phytoplankton 

 apparently remained dominated by primarily small 

 diatoms during most of the monitoring period. 

 However, several observations of blooms of other 

 phytoplankton were noted immediately after 

 freshwater inflow events. These blooms were typically 

 comprised of single celled blue-green algae (not the 

 filamentous cyanobacteria of algal mats) normally 

 present in fresh water or very low salinity 



environments. Although t)'picaUy short-lived, 

 (persisting no more than a few days), the presence of 

 these blooms did not frequentiy occur in the study area 

 prior to the demonstration project except under natural 

 freshening events that occurred every several years. 

 The more frequent presence of these blooms in the 

 upper and central Rincon Bayou was an indication that 

 the water column ecosystem was showing a more 

 typical response to freshwater inflow. 



Finally, the lack of regularly observed chlorophyll 

 biomass accumulation was Ukely a result of the rapid 

 flux of water through the monitoring area during 

 inflow events. However, the occasional obsen^ation of 

 high chlorophyll biomass during periods of moderate 

 salinity concentrations of 30-60 psu showed that 

 phytoplankton could remain in die monitoring area for 

 a sufficient time period to accumulate biomass. This 

 analysis is supported by the assimilation index, which is 

 the rate of carbon growth per unit of chlorophyll. The 

 assimilation index for sampling stations in Rincon 

 Bayou was often in the range of 50-100 mg C/m2/day 

 per unit of chlorophyll, which is a typical value for 

 estuarine systems. There were also numerous 

 assimilation values > 100 mg C/m2/day per umt of 

 chlorophyll at several sampling stations. These values 

 indicate that rapid primary production per unit of 

 phytoplankton biomass was occurring, especially 

 during 1999, which was a consistendy wet year in terms 

 of project diversions (Chapter 3). 



In general, the temporal responses of the 

 phytoplankton were too rapid to intensely observe 

 nutrient accumulation, chlorophyll increases or primary 

 production stimulation to specific inflow events. 

 Therefore, the nutrient amendment bioassays were the 

 primary data to demonstrate immediate phytoplankton 

 responses to freshwater inflow events in the Rincon 

 Bayou. Future monitoring of nutrient and water 

 column primary production should employ automated 

 sampling instrumentation that responds to inflow 

 events with increased sampling frequency. The 

 increased number of samples plus the temporal 

 correlation with other recorded variables, such as 

 salinity, would gready improve the understanding the 

 dynamics and importance of deltaic habitat for 

 phytoplankton. 



Chapter Vour ♦ 4-23 



