CHAPTER 5. SALT MARSH PROCESSES 



5.1 PRODUCTIVITY 



5.1.1. Higher Plants 



For years the salt marsh has been 

 considered one of the most productive 

 natural systems on earth (Teal 1962; Odum 

 1971). Production values range from 

 nearly 4,000 g/m 2 per year in the 

 streamside marshes of Georgia (Odum and 

 Fanning 1973) to only a little more than 

 one-tenth of that in the short Spartina 

 alternif lora marshes of Rhode Island 

 (Nixon and Oviatt 1973) and Massachusetts 

 (Ruber et al . 1981). There is a 

 latitudinal variation in salt marsh 

 productivity, with the highest values 

 occurring in the southern States. Levels 

 decrease by one-half to two-thirds in the 

 north, presumably due to the shorter 

 growing season and lower solar input at 

 the higher latitudes (Turner 1976). In 

 the salt marshes of the eastern United 

 States there appears to be about a 

 threefold variation in production over the 

 latitudes at which Spartina al term' flora 

 marshes grow, and also about a threefold 

 variation in production within any one 

 marsh. 



Part of the variation in Spartina 

 productivity within a marsh is related to 

 sediment salinity (Nestler 1977; Smart and 

 Barko 1980). Spartina can, in fact, grow 

 well in almost freshwater sites if 

 normally occurring freshwater plants are 

 removed. If such plants are present, they 

 outcompete (grow better than) Spartina and 

 crowd it out. Spartina does well in more 

 saline locations because it has mechanisms 

 for coping with salt stress (as discussed 

 in Chapter 3). However, an increase in 

 respiration is necessary for the plants to 

 maintain the higher osmotic gradient 

 required at high salinities (Figure 7); 

 this lowers production. Increased 

 respiration uses up some of the plant's 



resources and may also reduce oxygen 

 availability to the roots which could, in 

 turn, inhibit nutrient uptake. 



Soil density is another factor which 

 can affect Spartina productivity. DeLaune 

 et al. (1979) found that in Louisiana, 

 Spartina is more productive in soils of 

 high density. This high density is the 

 result of great amounts of mineral matter 

 and accompanying high nutrient levels. In 

 addition, the higher density soils in 

 Louisiana are also those without much 

 peat. As a result, they are more 

 permeable to water movements and attendant 

 flushing actions. 



A substantial portion of the 

 production of Spartina alternif lora has 

 been measured in the belowground parts of 

 the plants: the roots and rhizomes (Table 

 3). These data indicate there is 

 typically more production underground than 

 aboveground in the most productive parts 

 of the marsh, and considerably more 

 underground production in the less 

 productive areas. All salt marsh 

 production (i.e., photosynthesis) takes 

 place in the leaves. However, in the less 

 productive parts of the marsh, a great 

 deal of the organic matter produced is 

 translocated underground and used to 

 construct roots and rhizomes. The grasses 

 seem to behave as if they first produce 

 enough underground parts to acquire 

 necessary nutrients and then put any 

 excess into the photosynthetic machinery, 

 i.e., leaves. In the richer parts of the 

 marsh (the creek banks or tall grass 

 marsh), nearly equal amounts of biomass 

 are produced above and below the sediment 

 surface. This distribution of biomass has 

 considerable significance for what 

 eventually happens to salt marsh primary 

 production, a point to which we will 

 return. 



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