less productive parts of the marsh (Vince 

 et al. 1976). 



Nitrogen also has an effect on the 

 decomposers in the marsh. Decomposition 

 rates were found to be slightly increased 

 in areas with added nitrogen (Valiela et 

 al. 1984). The difference is small and 

 only means that in the less productive 

 parts of the marsh the detritus lasts a 

 little longer since the normal decomposi- 

 tion process eventually does away with 

 almost all of the organic matter that is 

 produced in the marsh. 



The following discussion of the marsh 

 nitrogen cycle draws mostly on data from 

 Great Sippewissett Salt Marsh (Valiela and 

 Teal 1979). This is the only salt marsh 

 anywhere for which there is a complete 

 published nitrogen budget at the present 

 time (Table 5). Except when identified as 

 measured in low marsh, the data refer to 

 the nitrogen budget for that entire marsh 

 including the regularly flooded intertidal 

 marsh and also high marsh, pannes, sand 

 flats, and creeks. Great Sippewissett 

 Salt Marsh is enclosed behind a barrier 

 beach and interacts with the bay through a 

 single channel in which many of the 

 measurements of exchange were made. The 

 exchanges between the different parts of 

 the marsh were not measured, so the 

 regularly flooded marsh cannot be 

 discussed in isolation. 



Nitrogen is supplied to the marsh by 

 both physical and biological processes 

 (Figure 21). Ground water and flood tides 

 bring nitrogen into the marsh system; ebb 

 tides remove it. If there is significant 

 river or stream flow into a marsh, this 

 can be an important source of nitrogen. 

 Bacteria and blue-green algae fix nitrogen 

 gas from the air and denitrifying bacteria 

 convert the nitrogen in nitrate back to 

 gaseous form. Plants and micro-organisms 

 build nitrogen, mostly from ammonia and 

 nitrate, into organic compounds such as 

 amino acids, proteins, and nucleotides. 

 Some of the export from the marsh is in 

 the form of organic matter (as organic 

 detritus, plankton, and animals) 

 containing these nitrogen compounds. 



By far the largest fluxes of nitrogen 

 both into and out of the marsh are those 

 carried in the tidal flows. In Great 

 Sippewissett Salt Marsh, over 70% of the 

 inputs and nearly 90% of the outputs were 

 carried by the tides (Table 5). The tidal 

 creeks carrying this water occupy about 

 34% of the total marsh area, a situation 

 similar to that in other mature marshes 

 that have completely filled their basins. 

 The largest part of the nitrogen exchange 

 is in the form of dissolved organic 

 nitrogen (DON) which did not change much 

 in concentration between inflow and 

 outflow (Table 6). Because the 

 concentration did not change measurably, 

 it is assumed that most of this organic 



Table 5. Nitrogen budget for Great Sippewissett Salt Marsh. Values are in 

 kg N/yr for the entire marsh of 48.3 ha (Valiela and Teal 1979, and unpubl. 

 data). 



36 



