relative abundance of nutrients. He further attributed the difference in nutrient distribution to 

 tidal flooding frequency; the more frequently flooded areas had more limiting nutrients as well as 

 sediments transported to them. I propose that this same process is working on a much larger, 

 tidally influenced, watershed scale. I further propose that this streamside effect on a watershed 

 scale may be partially responsible for the marsh loss observed between watersheds, especially in the 

 downstream areas between Toms and Big Island Bayous. Natural levees along tidally dominated 

 watersheds become less pronounced upstream, reflecting the decreasing accumulation rate of 

 sediments both away from the bayou and in the upstream direction. The accumulation rate is 

 determined by suspended sediment loads and frequency of flooding. Support for this concept is 

 twofold: 1) sediments forming natural levees in both systems are primarily mineral soils; these same 

 minerals are much less abundant marshward and in the upstream direction. In the upstream 

 marshes and in the marsh interior between Toms and Big Island Bayous very little mineral is found 

 (less than 5% by dry weight); and 2) the tide nodal point in Toms Bayou is approximately at the 

 point where natural levees disappear. The tide nodal point is the area along the tidal reach of a 

 drainage system where the incoming tidal waters usually stop. Waters upstream of the nodal point, 

 therefore, are not flushed as frequently as further downstream, but move back and forth with the 

 tide. Flushing above the tide nodal point usually occurs during spring tides, rainfall events, and 

 strong southeasterly fronts. 



CONCLUSIONS 



The conclusions of this study based upon data analysis and field observations are summarized: 



1. Fixed-crest weirs are inadequate management tools because they: 



a. prevent storm-water runoff (21% to 56% increased hydroperiod), 



b. increase salinities in downstream sites, 



c. increase hydroperiods in upstream sites, and 



d. reduce maximum standing crops. 



2. Downstream sites: 



a. Have higher plant diversities (Spartina patens dominates), and 



b. higher maximum standing crops, 



c. TBD flushed best with lowest salinity and highest standing crop. 



3. In 1985 mean salinity at BID was high because of storm tides. 



4. BIU had longest hydroperiod, lowest maximum standing crop. 



5. TBU has higher standing crop and more Scirpus olneyi than does semi-impounded BIU. 



6. Extended hydroperiod decreases maximum standing crop values in similar marsh types. 



7. Increased salinities decrease both total and Scirpus olneyi standing crops in similar marsh types. 



ACKNOWLEDGMENT 



I would like to thank Ecosystem Research Unit Director Dr. Michael Duever and Paul J. 

 Rainey Wildlife Sanctuary Manager Lonnie Lege for their valuable discussions on the marsh. 



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