I I I I I I I I I I I I I I 

 1998 1999 



1 1 I I r 



Figure 4-25: Assimilation index at all water column stations (except Stations 62 and 68) for each sampling date. 



DISCUSSION 



The extreme environment of Rincon Bayou is readily 

 apparent when the wide ranges of temperature and 

 salinit}' are considered. The nutrient content of the 

 water column required for primary' production to occur 

 was quite variable during the demonstration period. 

 The combination of water column inventories of 

 nutrients and chlorophyll pigments with instantaneous 

 measurements of primary production provided some 

 indications of resources available and their rates of 

 utilization. However, the relatively small area and 

 volume of water contained in the Rincon Bayou 

 ecosystem reduced residence times and accelerated 

 fluxes through the marsh. The monthly sampling 

 schedule was therefore not generally sensitive enough 

 to fully analyze the effects of freshwater inflow events 

 on phytoplankton production and growth. However, 

 even witli the constraints of monthly sampling 

 intervals, it was quite apparent that positive water 

 column nutrients and primary production effects 

 resulted from the demonstration project. 



First of all, nutrient amendment experiments were 

 performed in March and April 1997 to ascertain what 

 element(s) Hmited phytoplankton production in the 



waters of Rincon Bayou. A subsequent amendment 

 experiment was undertaken in August 1997, after a 

 large amount of freshwater inflow (Events 16 and 17), 

 to determine if nitrogen was still the limiting nutrient. 

 Before freshwater inflow, ammonium additions 

 increased chlorophyll production compared to control 

 samples from several sites. Those experiments also 

 indicated that vital trace metals used in "f ' media also 

 stimulated chlorophyll production. Additions of 

 nitrate also stimulated chlorophyll production to about 

 the same extent as ammonium, but very small ambient 

 concentrations of nitrate were observed at the 

 sampling sites. In August, after a significant inflow of 

 riverine fresh water, no nutrient additions increased 

 chlorophyll production at any station in the delta, 

 indicating that nutrients were no longer limiting 

 phytoplankton production. 



These nutrient amendment bioassays were the primary 

 indicator of the relatively rapid nutrient utilization and 

 phytoplankton response. Initial responses to the single 

 and multiple nutrient and/or trace metal additions 

 were on the order of a day and continued for 2 to 

 3 days. It has been clearly demonstrated from other 

 analyses of freshwater inflows that nitrate utilization 

 increased primary productivity' in the Nueces River and 



4-22 



Water Column Productivity 



