COMPARISONS OF SALINITY, HYDROLOGY, AND VEGETATION 



CHARACTERISTICS BETWEEN FREE-FLOWING 



AND SEMI-IMPOUNDED INTERMEDIATE-TO-BRACKISH 



TIDAL MARSH SYSTEMS 



John F. Meeder 



Ecosystem Research Unit 



National Audubon Society 



Route 6, Box 610 



Abbeville, LA 70510 



ABSTRACT 



The National Audubon Society began a long-term ecosystem research study of the Paul J. Rainey 

 Wildlife Sanctuary's coastal marshes in November 1984. The purpose of the study is to identify 

 and quantify marsh processes, to identify and quantify the effects of marsh management, and to 

 create a data base for evaluation of ongoing processes and future changes within the system. 



Results for the first 2 years of data are presented for Toms Bayou, which is free-flowing, and 

 Big Island Bayou, which has been semi-impounded since 1967 by a fixed-crest weir set 9 inches 

 below marsh level. Data on climatic conditions, water levels, salinity, vegetation composition, and 

 standing crop were collected at two stations in each system: one located in the marshes of the 

 upstream (U) one-third, and one in the downstream (D) one-quarter. In addition, data were 

 obtained from a tidal recorder in Fearman Bayou, which is tidally connected to both systems. 



Both downstream sites have higher plant diversities, higher maximum standing crops than their 

 upstream counterparts and are dominated by Spartina patens (Ait.) Muhl. Upstream sites are 

 characterized by greater relative abundance of Scirpus olneyi. Belowground biomass averaged 

 5,010 g/m 2 at the four stations, with 50% of the biomass found in the upper 10 cm of soil. 



The 2-year hydroperiod in the upstream marshes is 401 days in Big Island Bayou compared to 

 132 days in Toms Bayou, and the number of inundations is greater (32 versus 20 flooding events). 

 However, the reduced tidal height in the Big Island system prevents frequent flooding over the 

 well-developed natural levee in downstream marshes, reducing the 2-year hydroperiod to 299 days 

 in Toms and 85 days in Big Island. During droughts, marsh salinities became more elevated in the 

 semi-impounded Big Island system because of inefficient tidal exchange, and concentrations of salts 

 by evapotranspiration (up to 12 mm water/day). Salinities in the system were also elevated for a 

 longer period after Hurricane Danny because of inefficient runoff. Year 1 downstream marsh 

 salinities in Big Island averaged 2.4 ppt higher than at Toms for these reasons. Marsh salinities 

 were everywhere higher than adjacent bayou salinities with the exception of measurements taken 

 immediately after rainfall events. 



The data indicate that the vegetation parameters in the marsh semi-impounded by fixed-crest 

 weirs were not statistically different from those measured in the free-flowing marshes. However, 

 the semi-impounded marshes were more prone to salinity crises and the effects of extended 

 hydroperiod and wetter soils. Big Island marshes also had lower maximum standing crops than 

 Toms Bayou marshes. 



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