50 

 45 

 40 

 35 



E 30 

 o 



15 

 10 



5 -] 

 



e 



o 



5000 



4000 



3000 - 



2000 



1000 - 



150 



120 



Q. 



>. 



60 ™ 



1995 



1996 



1997 



1998 



1999 



H Total rainfall 



■ Total flow into Rincon Bayou 



■ Average salinity: Reference Site (Stations 63 and 64) 



- Average salinity: central Rincon Bayou Site (Stations 60 and 61] 



- Average salinity: upper Rincon Bayou Site (Stations 65 and 66) 



Figure 4-3: Average salinity at each sampling site for each sampling date. Cumulative daily rainfall and inflovt/ into 

 Rincon Bayou also plotted for each monthly period between sampling dates. 



of sunlight often determine the amount of biological 

 productivity in an estuary. Organic matter from die 

 plants may go through several padiways depending on 

 whether it was eaten, decomposed or buried in the 

 sediments. Organic matter that is consumed may be 

 excreted back into the environment or be incorporated 

 into the animal tissue. Microbial populations can also 

 absorb or breakdown organic matter and return it to 

 the environment. Both organic matter pathways, 

 through higher animals or microbes, produce 

 regenerated nutrients which enhance nutrient 

 concentrations in the water and again become available 

 for uptake by plants. The relative amounts of the 

 original nutrients and the regenerated nutrients can 

 often provide rate process estimates for turnover of 

 organic matter in estuaries. 



The relative importance of different nutrients varies in 

 freshwater and marine environments. As a result, the 



importance of phosphorus nutrients are often clearly 

 obser\^ed in freshwater segments, while nitrogen 

 nutrients are most important in saline segments. These 

 differences are caused by many physical and biological 

 processes that vary over short time and spatial scales. 



Nitrate (NO,) 



Nitrate is the most common form of nitrogen nutrient 

 in oxygenated environments. The concentrations of 

 nitrate in many rivers has increased over the past five 

 decades as a result of increased usage of agricultural 

 fertilizer and wastewater effluents. 



The nitrate content of the Nueces River and the 

 Nueces Delta stations (Figure 4-4) were relatively low 

 (< 2 ^mole/l). Less dian 1% of the data were > 

 2 |jmole/l and usually occurred during flooding events. 

 The largest concentrations of nitrate (> 20 ^xmole/I) 



Chapter Four *♦* 4-7 



