The assimilation index for most stations had values of 

 20 to 100 mgC/mVday/^gChl (Figure 4-13). Many of 

 the highest values obsen^ed before appreciable inflow 

 passed through the Nueces Overflow Channel (i.e., 

 prior to October 1996) were measured at the Nueces 

 River station (68). Later during the demonstration 

 period, the upper Rincon Bayou stations (65 and 66) 

 were much higher compared to earlier years, and most 

 station samples gave higher results during the final 

 year. This apparent increase at most of the sampling 

 stations during the final year might be due to increased 

 freshwater diversions. 



Nutrient Amendment Bioassays 



The nutrient amendment studies were undertaken to 

 determine whether phytoplankton growth widiin the 

 Nueces Delta could be stimulated by the addition of 

 nitrogen, phosphorus or trace metal nutrients. The 

 biomass of the phytoplankton as measured by 

 chlorophyll was used to indicate biological response. 

 The amount of change may be influenced by the 

 incubation conditions, but the relative responses can 

 provide valuable clues to the degree of limitation of an 

 addition or set of additions. Two nutrient amendment 



experiments were performed in March and April 1997 

 during what was a relatively dry period in the delta. A 

 final amendment experiment was undertaken in August 

 1997, after a large amount of summer-time freshwater 

 inflow (Events 16 and 17). 



Amendment Series 1: March 7, 1997 



Station 68 — The two phosphate amendment additions 

 responded equivalent to the control for the four day 

 period (Figure 4- 14a). All other additions showed 

 increased chlorophyll concentrations compared to the 

 controls. Trace metals, nitrate, nitrate plus 

 phosphorus and silicon, and nitrate plus phosphorus 

 had approximately twice the response compared to 

 ammonium and ammonium plus phosphorus. 



Station 63 — AH amended samples showed a 

 chlorophyll response for days 1 and 2 that was 

 equivalent to the control with no additions 

 (Figure 4- 14b). On day 3, both concentrations of 

 phosphate had no effect compared to the control, but 

 all other additions enhanced chlorophyll production. 



Station 66 — The two phosphate amendment additions 

 showed responses equivalent to the control for the 



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250 



200 



150 



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r T I 1 I I f r I I I 



1995 



1 1 I I T 



1996 



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

 1997 1998 1999 



Figure 4-13: Assimilation index at all water column stations (except Station 62) for each sampling date. 

 4-14 •♦* Water Column Productivit)! 



