underground production is probably 

 decomposed through the fermentation and 

 sulfate reduction pathways (Howes et al . 

 1984). Sulfate reducers are not efficient 

 at converting nutrients into microbial 

 cells and the carbon to nitrogen ratio of 

 anoxic litter is about 45:1 after 1 year 

 compared with 20:1 for aerobically 

 decomposing litter. Thus, in the former 

 case, about half of the nitrogen has been 

 lost to the sediment pore water or 

 mineralized. This lack of nitrogen 

 conversion into microbial biomass may be 

 one of the reasons for the generally high 

 nitrogen levels in marsh sediments and for 

 the eutrophic nature of salt marshes since 

 the water oozing out of the mud is high in 

 nitrogen. Less than 3 g C/m 2 /yr is 

 accounted for by the denitrifiers in Great 

 Sippewissett Salt Marsh (Kaplan et al . 

 1979), because there is not a large supply 

 of nitrate available to the denitrifiers. 

 Net methane loss to the atmosphere is 

 less than 4 g C/m 2 /yr (Howes et al . 1985), 

 which is less than 1% of total 

 decomposition. Methane loss has been 

 increased 2.5 times by poisoning sulfate 

 reducers with molybdate (Howes et al., 

 unpubl. data). This indicates that more 

 methane is produced in the marsh than is 

 lost to the air, but it is consumed by 

 sulfate reducers. 



Recent measurements from the Great 

 Sippewissett Salt Marsh indicate that 

 decomposition via respiration with oxygen 

 accounts for approximately half of 

 estimated underground production (Howes et 

 al. 1984). But since a substantial part 

 of the salt marsh production is too far 

 underground to be reached by oxygen, a 

 major fraction of this is decomposed 

 through the sulfate reduction pathway. 



abundance of the grasses on the marsh also 

 seems to be determined by nitrogen 

 availability. Salt marsh algae are more 

 productive when their nitrogen supply is 

 increased in the spring; in summer, 

 increased nitrogen supplies enhance the 

 growth of the marsh grass and algal 

 production is reduced due to shading by 

 the grass canopy. 



Marsh herbivores are also nitrogen 

 limited: the more nitrogen in their food, 

 the higher their production. For example, 

 insects are more abundant in parts of the 

 marsh where the grass has a higher 

 nitrogen content. Those parts of the 

 marsh are also much more attractive to 

 geese and voles. 



The food quality of salt marsh 

 detritus is also affected by nitrogen 

 availability. Salt marsh detritus is not 

 a very nutritious food for animals. Its 

 carbon to nitrogen ratio (C/N) varies from 

 20:1 to 60:1 while phytoplankton, protein, 

 and bacteria have values that range from 

 4.5:1 to 6:1. Since animals require a C/N 

 ratio of about 17:1 for minimal 

 maintenance, nitrogen content is of great 

 importance in the detritus cycle in the 

 marsh. When the amount of nitrogen in 

 detritus was experimentally doubled, there 

 was a four- to fivefold increase in the 

 abundance of detritivores on the marsh 

 surface (J.M. Teal, unpubl. data), but no 

 change in the abundance of animals living 

 in the bottoms of the marsh creeks (Wiltse 

 et al. 1984). There is some evidence that 

 the fish in the marsh grow faster in the 

 fertilized parts of the marsh than they do 

 in control areas, but the response is not 

 as clear as it is with either the plants 

 or marsh surface detritivores (Connor 

 1980). 



5.3 NUTRIENT CYCLING 



5. 3. 1. Nitrogen 



The nitrogen cycle is of great 

 importance to the ecology of the marsh. 

 Nitrogen clearly controls a wide variety 

 of marsh processes. The level of 

 available nitrogen and its uptake by the 

 plants determines the productivity of the 

 marsh. The more nitrogen that is 

 available, the greater the percentage of 

 grasses that set seed. The relative 



Nitrogen enrichment affects the 

 spacing of grass stems. In the more 

 productive parts of the marsh, the stems 

 are thicker but farther apart than 

 elsewhere. Hartman et al. (1982) report 

 that in the highly productive creek banks, 

 about 40% of the surface area lies between 

 the grass stems, while in the less 

 productive low marsh the space between the 

 stems is only half as great. The marsh 

 fishes are much more successful in hunting 

 among these widely spaced stems than they 

 are among the closely spaced stems in the 



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