of fisheries organisms. If, as some people claim, however, such structures slow the conversion of 

 vegetated marsh to open water, then such structures may be beneficial to fisheries in the long run. 

 But this is a big, and important, "if." Therefore, we believe the statement just quoted from Herke 

 (1979) should now be rephrased by substituting "marsh loss" in place of "nursery production (not 

 standing crops)." Although such structures may help to save the marsh under some circumstances, 

 we know of no published scientific study showing that they do so over the long term. The basis 

 for such claims seems to be personal observations unsupported by controlled experimentation or 

 critical analysis. 



On the other hand, there are a number of published indications that water-level control in the 

 marsh may actually hasten loss of vegetation. Concerning semi-impoundments controlled by fixed- 

 crest weirs, dams, and earthen plugs, Cowan et al. (1988) stated, "In some cases, land loss rates 

 within the impoundments may increase due to hydrologic isolation in an otherwise tidally influenced 

 area. Marshes in these impoundments tend to break up and die back to form open water." Some 

 possible reasons for this follow. 



There is general agreement that the primary reason coastal Louisiana is losing land is because 

 sediment deposition is not keeping pace with land subsidence and the rise in relative sea level (e.g., 

 DeLaune et al. 1983; Salinas et al. 1986; Coalition To Restore Coastal Louisiana 1987; Walker et 

 al. 1987). Moreover, the rate of eustatic (true) sea level rise is predicted to increase substantially 

 during the next century because of the "greenhouse effect" (Titus 1986). The only way to counter 

 this rise in relative sea level is to increase the rate at which sediment from the Mississippi River 

 is introduced into the marsh. Semi-impoundment will have no effect on eustatic sea level rise, but 

 will likely increase the rate of relative sea level rise by restricting the inflow of sediment-bearing 

 water. 



The distribution of marsh plants is largely governed by the depth and duration of inundation of 

 their roots, and the tolerance of the plants to salinity. Chabreck et al. (1979) found that weirs 

 increased both the average depth and the duration of inundation. Semi-impoundment levees also 

 affect the marsh water-level regime since even canal spoil banks that result in unintentional partial 

 impoundment do so. Swenson and Turner (1987) found that, on the average, an area affected by 

 such a spoil bank (1) was flooded 141 h more per month than the adjacent control area; (2) had 

 fewer but longer flooding events; (3) had fewer but longer drying events; and (4) had reduced 

 water exchange, both above- and belowground. They stated that the ecological significance of the 

 monthly hydrologic averages may be overshadowed by the few but relatively stressful long periods 

 of inundation. They also stated that the soil chemistry changes one would expect under these 

 longer periods of flooding would be ecologically significant, particularly if sulphates were reduced 

 to toxic sulfides. Wiregrass {Spartina patens) is the predominant plant in Louisiana brackish marsh, 

 and smooth cordgrass {Spartina alterniflora) predominates in the saline marsh. The effect of 

 prolonged inundation on wiregrass has not been definitely determined. In field tests, however, as 

 the degree of soil drainage increased, height of smooth cordgrass increased in a significant linear 

 fashion (Mendelssohn and Seneca 1980); continuous flooding resulted in a decline in growth and 

 ultimately in dieback (Mendelssohn et al. 1981). 



Wicker et al. (1983) evaluated wetland management techniques on the Rockefeller State Wildlife 

 Refuge and Game Preserve. The Price Lake unit of the preserve contains about 3,000 ha of 

 brackish to saline marsh and shallow, open-water bodies; it is surrounded by levees, and water level 

 is controlled by two standard fixed-crest weirs. They found that open water bodies are increasing 

 in this unit, and stated, The factors responsible for the increasing marsh breakup are difficult to 

 delineate, but levee enclosure and weirs may possibly contribute to the problem by maintaining 

 higher-than-normal water levels." The preceding paragraph gives support to this hypothesis. 



Permit applications for marsh management practices that will involve the use of levees and 

 water-control structures frequently cite the need to prevent saltwater intrusion, which the applicants 

 believe to be a major cause of Louisiana's coastal land loss. Although saltwater does kill plants 



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