diatoms (Riera et al. 2000). From 6''C and 6''N 

 values, it appears organic matter inputs earned by river 

 inflow could also contribute significandy to the feeding 

 of migratory brown shrimp. In the marsh habitats of 

 Rincon Bayou and the Nueces Delta, shrimp isotopic 

 ratios changed rapidly, indicating high tissue turnover 

 rates and rapid growth. Therefore, re-introduction of 

 fresh water to the marsh results in conditions within 

 nursery areas {e.g., higher benthic biomass) favorable 

 for feeding and growth of juvenile brown shrimp. 

 Further discussion of the approach and results of the 

 stable isotope analyses conducted by Riera et al. (2000) 

 has been included as Appendix E of this Concluding 

 Report. 



Effects of Diversions as Disturbances 



Rapid changes in salinity due to flooding events can be 

 classified as a disturbance. The frequency and timing 

 of these freshwater inflow events into Rincon Bayou 

 were very important. The effects of disturbance 

 frequency and altered flow on macro-benthic 

 community structure and colonization in Rincon 

 Bayou were also independentiy studied during the 

 demonstration period (Ritter and Montagna 2000). 

 Abundance and biomass decreased with increasing 

 disturbance frequency, indicating post-disturbance 

 community persistence is important in regulating 

 community structure. There was higher abundance 

 and biomass in defaunated sediments relative to 

 background sediments, indicating disturbance plays an 

 important role in community production of early 

 succession communities. The collection date was the 

 most important factor determining community 

 structure, thus natural variabilit)' overwhelmed effects 

 of both the flow and disturbance frequency 

 manipulations. The temporal changes were driven by a 

 Strehlospio benedicti recruitment event (resulting in 

 abundances as high as 1.3 10' m"^ captured June 20, 

 1997 (one day before the beginning of Event 16) and 

 during the subsequent freshwater event. After the 

 event, S. benedicti abundance declined rapidly, and 

 freshwater species invaded, leading to the progression 

 of three distinct community states {i.e., community 

 structure and species dominance changed three times) 

 over the 14-week period of the study. The 



overwhelming significance of "temporality" {i.e., 

 short-term temporal change in community structure) 

 was the unexplained temporal component of 

 commumty variation in experimental manipulations. 

 Temporality is simply a smaller temporal scale than 

 seasonality'. The importance of short term changes 

 relative to flood events indicated that the 

 demonstration project was responsible for high 

 productivity during the summers of 1997 through 

 1999. Further discussion of the approach and results 

 of the stable isotope analyses conducted by Ritter and 

 Montagna (2000) has been included as Appendix F of 

 this Concluding Report. 



SUMMARY 



Benthos in Rincon Bayou are normally under great 

 stress due to high salinity concentrations, especially 

 during summer. The benthos responded positively to 

 inflow events by increased biomass, abundances and 

 diversity. Direct and indirect mechanisms were 

 responsible for the benthic response. The direct 

 mechanism included increased physiological tolerance 

 to oligohaline and estuarine salinity concentrations 

 relative to hyper-saHne conditions. Inflow events at 

 various times during the year, but especially in the 

 spring and fall, were likely cues for benthic 

 reproduction and settiement of planktonic lan'ae. 

 Trophic interactions and habitat utilization represented 

 indirect mechanisms to which benthos responded. 

 Increased microalgal food and marsh habitat 

 availability stimulated secondary production. As the 

 frequency and magnitude of freshwater inflow events 

 increased due to the demonstration project, the 

 opportunity for positive responses also increased. 

 Without the demonstration project features, Rincon 

 Bayou would experience considerably less inflow, and 

 revert to a reverse estuary with consequentially less 

 biodiversity and productivity. 



5-24 



Benthic Communities 



