measured by Hoar in the marshes with weirs, are indicative of significant waterlogging, or stress, 

 on the plants (Burdick and Mendelssohn 1987). In other wetland studies with increased and 

 decreased waterlogging, the plants respond with less or more plant growth, respectively (see Table 

 3). The implication is that the increased flooding caused by a weir 6 inches below marsh level may 

 be a significant physiological stress on the plants, perhaps enough of a stress to result in plant 

 death. 



Soil salinity was slightly higher behind weirs in one study, but unchanged in another. The soil 

 organic content of marshes with weirs was apparently unchanged by weirs. 



Chabreck et al. (1979) reported a significant decrease in sedimentation rate over several years 

 for ponds in marshes with weirs, as compared to ponds without weirs. In fact, he noted that weirs 

 may "have actually functioned to reduce the "rate" of sediment deposition. Lower net 

 sedimentation rates in areas with weirs indicates fewer flooding events and lower transport of 

 suspended sediments into marshes. They do not support the idea that tidal scouring is a significant 

 contributor to marsh breakup. On the contrary, tides bring suspended sediments into a marsh, 

 sustaining both a healthy soil Eh profile and positive accretion rate. 



Vegetation and Marsh Degradation 



Weirs result in more submerged aquatic vegetation in low salinity marshes. The same is not 

 true for emergent vegetation, which was unchanged by weirs at Marsh Island, but was lower at 

 three other sites (Table 2). At the Biloxi Marsh Public Shooting Ground, the rate of formation 

 of new ponds from 1955 to 1978 was 3 times higher inside the marsh management area compared 

 to the surrounding marsh (Table 4). High vegetation losses in marshes with weirs were also 

 apparent in the low salinity marsh studied by Larrick (1975; 3-12 ppt; Table 5). These results are 

 also consistent with the results of Chabreck and Hoffpauir (1962), who presented data indicating 

 that emergent vegetation gives way to submergent vegetation when flooding increased (Table 2). 

 Gosselink (1984) reported similar results for impounded marshes in the Chenier Plain data. 



The negative effect of weirs on emergent vegetation is consistent with results from experimental 

 studies in other coastal marshes (Table 3) and soil Eh data (Figure 3) and the decrease in 

 sedimentation rate behind weirs at Marsh Island (Chabreck and Hoffpauir 1962). Our 

 interpretation is that the weirs result in increased flood duration, fewer flooding events, and 

 subsequent detrimental stresses on emergent vegetation resulting from waterlogging and from a 

 decreased ability of plants to adjust their vertical position in a sinking landscape (caused by 

 subsidence and sea-level rise). 



Animals 



The only difference in the number of animals in marshes with and without weirs was the higher 

 number of overwintering waterfowl in marshes with weirs. This is attributable to the increased 

 amount of submerged vegetation and higher water levels in winter. Otherwise there were no 

 statistically significant differences in resident species of mammals, non-game bird species, or 

 waterfowl. Hunter usage in areas with and without weirs was the same, indicating that weirs did 

 not encourage utilization of ponds by animals or hunters. 



Herke (1979) has summarized the various influences of weirs on coastal Louisiana fish and 

 crustaceans. Weirs present a physical and ethological barrier to fish migration; adults may 

 congregate near the weir. Subsequent research has substantiated his concerns that coastal 

 migratory species and resident estuarine species are adversely affected. The effects of weirs on 



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