Nitrogen tj 



Figure 4-9. A model of the marsh nitrogen (N) cycle showing the major stores of N and interrelated processes 

 (Hopkinson and Day 1977). 



the total nitrogen supply (Valiela and Teal 1972). 

 Normally, the growth rate of marsh grasses seems to 

 be limited by the rate at which organic nitrogen in 

 the sediment can be oxidized into ammonia (3). As 

 this is a relatively slow process, the addition of in- 

 organic nitrate or ammonia to the marsh often stimu- 

 lates growth. 



Aside from the nutritional value of specific in- 

 organic elements to marsh plants, the total salt con- 

 centration in the root zone exerts a strong influence 

 on plant growth. All of the major salt and brackish 

 marsh plants appear to be inhibited by high salt con- 

 centrations. Even though they thrive in a moderately 

 saline environment, growth is more vigorous for the 

 same species in soils with lower salt content when 

 competition from other species is eUminated. Thus 

 the dominant species in the salt and brackish marsh 

 habitats arc not there because they are stmiulated by 

 the salt solution in which they grow, but because 

 they tolerate moderately sahne conditions better than 

 fresh marsh species. This has been demonstrated by a 

 number of individuals (Phleger 1971, Seneca 1972, 



Parrondo et al. 1977) whose results are sliown in 

 figure 4-10. Fresli marsh vegetation is particularly 

 susceptible to salt intrusion since these species seem 

 to be intolerant of even low salt levels. 



The role of severe storms in the control of vegeta- 

 tion has often been ignored because of the difficulty 

 of documentation. Hurricanes that strike the Chenier 

 Plain are sufficiently intense to cause considerable 

 short- and long-temi changes in wetlands. The imme- 

 diate effects have been difficult to ascertain. Day et al. 

 (1977) reported that Hurricane Carmen in 1974 de- 

 foliated swamp forests in its path two months earlier 

 than nomial leaf fall. A large amount of organic 

 carbon, nitrogen, and phosphorus was flushed from 

 the swamp to the lower estuary (fresh, brackish, and 

 salt marshes) by the accompanying torrential rains. 

 Part of tliis material undoubtedly resulted from the 

 early defoliation, but much visual evidence points to 

 thorougli Hushing of stored detritus from the swamp 

 floor which would not wash out under normal weather 

 conditions. Short-temi effects of Hurricane CamUle 

 on species composition in fresh and brackish marshes 



162 



