utilized by photosynthetic processes, DIN is often 

 used to estdmate nitrogen nutrient availability. 



The concentration of DIN at most stations was 

 < 10 ^imole/l, and, on only six sampling dates, it was 

 larger than 20 |imole/l, of which five were at the 

 Reference site (Figure 4-7). In general, the Reference 

 site (Stations 63 and 64) displayed most of the largest 

 concentrations of DIN, which likely was a result of 

 unusually large inputs from local runoff The Nueces 

 River (Station 68) generally had the lowest DIN 

 concentrations. 



The largest contributor to the DIN pool was 

 ammonium, as indicated by its percentage of DIN 

 (Figure 4-8). In only a relatively few samples (about 

 6%) was ammonium less than 40% of the DIN. 

 During 1999, a relatively large number of samples 

 contained 100% ammonium. 



5 |imole/l or lower, although several samples had 

 values near 30 |jmole/l (Figure 4-9). 



Also, these concentrations represented high 

 concentrations of phosphorus relative to nitrogen. 

 For example, most values for the nitrogen to 

 phosphorous (N:P) ratio calculated from phosphate 

 and DIN concentrations were below 3.0 (Figure 4-10). 

 A typical value of the N:P ratio for organic matter is 

 about 15. During the demonstration period, only 

 5 samples had values larger than 15.0, indicating a 

 possible shortage of phosphorus in the system. In 

 contrast, there were a large number of samples with 

 values < 5.0 for N:P, indicating that phosphorus was 

 ver^' abundant and nitrogen may be limiting 

 autotrophic processes. 



Phytoplankton Pigments 



Phosphate (PO4) 



Phosphate (orthophosphate) is the primary 

 phosphorus nutrient, and it's dynamics vary widely in 

 fresh and salt waters. During the demonstration 

 period, phosphate concentrations were primarily about 



Phytoplankton pigments are useful indicators of plant 

 biomass. Chlorophyll A has been the primary pigment 

 traditionally used, but better analytical methods since 

 1985 have allowed a large number of other pigments to 

 be identified and measured accurately. Chlorophyll A 



-r-rvr I I I I I I I I I T I I I 



1995 1996 



1997 



1998 



1999 



Figure 4-7: Dissolved inorganic nitrogen concentrations at all water column stations (except Station 62) for each 

 sampling date. 



4-10 ♦ Water Column Productivity 



