ponds vs. 5.3 ppt in unmanaged ponds) was small enough to suggest that the increase in aquatic 

 plants resulted from a reduction in tidal action on the floors of the managed ponds. 



Larrick and Chabreck (1978) investigated aquatic vegetation at three locations in coastal marshes 

 that were intermediate to brackish on Marsh Island, saline at the Wisner Wildlife Management 

 Area in Lafourche Parish, and brackish to saline in Jefferson and Plaquemines Parishes 40 km 

 north of Grand Isle. The amount of aquatic vegetation was lowest in the saline marsh, where 

 aquatic vegetation was found at 1.7% of the sample stations in the managed ponds and at none 

 of the stations in the unmanaged ponds. In the brackish to saline marsh, much of which had been 

 modified by dredging, aquatic vegetation occurred at 18% of the stations in managed ponds and 

 only 0.5% of the stations in the unmanaged ponds. In the intermediate to brackish marsh, aquatic 

 vegetation occurred at 74% of the stations in managed ponds and 43% of the stations in 

 unmanaged ponds. 



Larrick (1975) recorded salinity and turbidity in the same ponds at the time vegetation samples 

 were taken. Although salinities varied among areas, he reported similar salinities in managed and 

 unmanaged ponds within areas. In the brackish to saline area turbidity was slightly higher in 

 managed ponds (30.3 ppm vs. 21.6 ppm), but in the saline marsh area and the brackish to 

 intermediate area the turbidities were slightly lower in the managed ponds (31.0 ppm vs. 36.5 ppm 

 and 40.0 ppm vs. 51.2 ppm, respectively). 



Herke (1971) reported that weirs reduced the rate at which salinity changed, so that the extreme 

 low and extreme high salinities were moderated. This resulted in higher salinities behind weirs 

 when salinities were decreasing, and lower salinities behind weirs when salinities were increasing. 

 When salinities were stable, there was usually no difference in salinity between managed and 

 unmanaged ponds. 



Effects on Emergent Vegetation 



Chabreck and Hoffpauir (1962) recorded species composition for 5 years following construction 

 of weirs on Marsh Island. Although drastic differences were noted among years, the marsh affected 

 by weirs remained similar to unmanaged marsh throughout the study. Chabreck (1968) sampled 

 these same eight transects 5 years later and found differences between the managed marsh and 

 unmanaged marsh. In managed high marsh, initially characterized by marshhay cordgrass (Spartina 

 patens) and black rush (Juncus roemerianus), dwarf spikerush replaced much of the black rush. 

 Black rush is associated with higher, well-drained marshes and has little direct value to wildlife. 

 Spikerush, associated with lower, moister areas, is a valuable food for waterfowl and nutria (Martin 

 and Uhler 1939; Chabreck et al. 1981). 



Larrick (1975) examined marsh vegetation at the same three areas where Larrick and Chabreck 

 (1978) later studied aquatic vegetation. The plant species composition differed greatly among 

 marsh types because of differences in salinity. The plant communities in all three areas were very 

 similar in managed and unmanaged marshes. The increase in spikerush and decline in black rush 

 noted by Chabreck (1968) in managed brackish to intermediate marsh was not noted by Larrick 

 (1975). In the saline and brackish to saline areas, unmanaged marsh had significantly more 

 vegetative cover than managed marsh, 64.6% vs. 42.4% and 80.2% vs. 75.3%, respectively. In the 

 brackish to intermediate marsh area, the managed marsh had significantly more vegetative cover 

 than unmanaged marsh (89.7% vs 86.8%). In all three areas the free-soil-water salinities were 

 lower in managed marsh than in unmanaged marsh. 



In the upper Barataria Basin, Hoar, (1975) studied the effects of weirs on several soil 

 characteristics. Of particular interest were his data on soil redox potential, dissolved hydrogen 



144 



