is at a minimum (Figure 23). During the 

 warm weather, most of the nitrate is 

 intercepted and denitrified upon entering 

 the marsh, probably in the anoxic 

 sediments of the creek bottoms. 



The overwhelming portion of nitrogen 

 exchange is driven by physical forces, 

 with only 15% being entirely biological in 

 nature (Table 5). The salt marsh is 

 driven by its physical setting: the 

 tides, the salty water, and the anoxic 

 sediments, which determine the character 

 of the living things that can survive 

 there. But the biological components are 

 the ones with which we are primarily 

 concerned. The living organisms determine 

 how the marsh looks and persists and in 

 what ways it is important to us. 



Another way of looking at the 

 nitrogen budget is to examine the balances 

 for the various forms of nitrogen (Table 

 6). Unfortunately, we can only lump all 

 of the dissolved organic nitrogen together 

 as one number in this table. Table 6 

 emphasizes that although the total amount 

 of DON is very large, there is relatively 



• Gross denitrif ication 



o Export of N0 3 -N by tide 



□ Import of NO3-N through ground water 



ANNUAL 

 TOTALS ( kg yr 



3016 



941 



2921 



Figure 23. A comparison of nitrate input 

 from ground water, nitrate export by 

 tides, and denitrif ication within Great 

 Sippewissett Salt Marsh (Valiela and Teal 

 1979). 



little difference between the amounts 

 entering and leaving the marsh. 

 Examination of the values controlled by 

 biological processes show some interesting 

 aspects of biological activity within the 

 marsh system. For example, the values for 

 nitrogen fixation and denitrif ication are 

 approximately equal. The measured values 

 for denitrif ication in the muddy creek 

 bottoms are about equal to the net input 

 of nitrate (Howes et al., unpubl. data). 

 There is a much smaller amount of 

 denitrif ication on Spartina -covered areas, 

 the nitrate for which is probably supplied 

 by oxidation of nitrogen compounds at the 

 surface of the mud. If one adds up all 

 the sources of nitrogen available to 

 support marsh grass growth (i.e., net 

 input of ammonia, nitrogen fixation, net 

 input of DON) and subtracts from this the 

 net losses to the sediments, the total is 

 about 1,600 kg N/yr. But the total 

 production of marsh grass requires nearly 

 9 tons of nitrogen per year. Much of this 

 is supplied by cycling within the very 

 large "pool" within the sediments, which 

 balances the various demands. The 

 cycling is mostly due to the activities 

 of microbes. Animals play a smaller 

 role through feeding and excreting 

 nitrogen within the marsh, thereby 

 stimulating the rates of microbial 

 activities. For example, marsh mussels 

 deposit as pseudofeces much of what they 

 filter out of the water and thus create 

 a substrate on which microbes are active 

 (Jordan and Valiela 1982). Fiddler 

 crabs and snails turn over the surface 

 layers of the sediments and stimulate 

 microbes. 



To sum up the role of Great 

 Sippewissett Salt Marsh as an example of 

 New England marshes, one can say that if 

 the marsh were not there: (1) more 

 inorganic nitrogen would reach coastal 

 waters, (2) the nitrogen would be in the 

 more oxidized form (N0 3 rather than NH 4 ), 

 (3) nitrogen would enter coastal waters 

 more uniformly throughout the year 

 rather than principally as a pulse in 

 autumn, and (4) there would be less 

 nitrogen exported as particulate organic 

 nitrogen (detritus and living cells), 

 the form of nitrogen that can be consumed 

 directly by coastal animals such as filter 

 feeders. 



39 



