nourishing as was once thought. For 

 example, while mummichogs will eat 

 detritus, they cannot gain weight on a 

 detrital diet that is not supplemented 

 with protein (Prinslow et al. 1974). 

 Marsh ki 11 i fishes, especially mummichogs, 

 feed on detritus though much of it may get 

 into their stomachs by accident when they 

 are really seeking animals in marsh 

 sediments. 



Detritivores accelerate the 

 decomposition rate of Spartina litter by 

 grinding the particles (thus creating more 

 surface by digesting the particles to a 

 small extent) and by stimulating the 

 growth of decomposers by cropping them. 

 Such feeding activities stir up the 

 particle accumulations, increase the 

 available nutrients and oxygen, and 

 perhaps remove anti-microbial substances 

 from particle surfaces. Although the 

 relative importance of these various 

 mechanisms is not clearly understood, the 

 exclusion of macrofauna from some 

 decomposition experiments tripled the 

 amount of litter that normally remained 

 after 1 year (Valiela et al. 1984). 



5.2.2. Belowground 



As described in Section 5.1.1., much 

 of the production in salt marshes goes 

 into the belowground parts of the plants, 

 the roots and rhizomes. The marsh surface 

 accumulates only a small percent of the 

 total plant production because most of it 

 decomposes in place or is washed away by 

 the tides. The belowground parts cannot 

 be washed out and, therefore, they 

 decompose within the marsh sediments. 

 Some of this underground decomposition 

 occurs through the same aerobic processes 

 as aboveground decomposition. But since 

 most of the sediment is anoxic, the major 

 portion of underground decomposition 

 occurs by anoxic means. 



Anoxic processes common to marine 

 systems use nitrate (denitrif ication) and 

 sulfate (sulfate reduction) as electron 

 acceptors in place of oxygen. These 

 anoxic processes yield less energy to the 

 microbes that perform them than oxygen- 

 consuming processes do to aerobic 

 microbes. There is slightly less energy 

 produced in the case of denitrif ication 

 but substantially less in the case of 



sulfate reduction. A vertical cross 

 section of marsh sediments might reveal 

 the oxygen-using organisms at the surface, 

 denitrif iers below them, and finally the 

 sulfate reducers in deeper layers. As 

 long as oxygen is present, organisms that 

 can use oxygen outcompete the others 

 simply because they can obtain energy from 

 organic matter more efficiently and thus 

 grow faster. At the depth where all of 

 the oxygen has been used, the denitrifiers 

 are most efficient, and at the depth where 

 the nitrate is also exhausted, the sulfate 

 reducers come into their own. 



Carbon dioxide is another potential 

 electron acceptor. Its use by microbes 

 produces reduced carbon or methane. But, 

 because of the abundance of sulfate in 

 seawater and the small potential energy 

 available from methane production compared 

 with sulfate reduction, this path of 

 decomposition is of minimal importance in 

 salt marshes. 



Decomposers that use nitrate and 

 sulfate as electron acceptors can usually 

 use only a limited number of organic 

 molecules (e.g., acetate and simple 

 organic acids) as substrates. These 

 compounds are made by microbial 

 fermentation that breaks apart the more 

 complex organic molecules in Spartina 

 roots and rhizomes. 



Anoxic decomposition is slower than 

 aerobic decomposition. The underground 

 leaching phase is similar to that 

 aboveground, but the subsequent phases are 

 slower. After 2.5 years, less than 20% of 

 the original litter remained in 

 belowground field experiments in Great 

 Sippewissett Salt Marsh (Valiela et al., 

 unpubl. data). These researchers also 

 found that belowground litter enriched in 

 nitrogen decayed more rapidly than 

 unenriched litter in control experiments. 

 This indicates that nitrogen limitation 

 plays a role in anoxic decomposition in 

 salt marshes. There are other differences 

 from aboveground decomposition: lignin 

 decomposes poorly in anoxic conditions, 

 and fungi are not active in the absence of 

 oxygen. 



The actual amounts of decomposition 

 that proceed via these various paths are 

 not very well known. The greater part of 



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