respiration (calculated from literature 

 values). In the CO; flux studies, two- 

 thirds is associated with the aufwuchs 

 coinmuni ty and the sediments. The experi- 

 mentally detennined data for consumer 

 respiration are 2,140 g/m^/yr from CO2 

 flux measurements and about 300 g/m^/yr 

 from O2 flux. The CO2 flux was determined 

 with the marsh unflooded, the Oj flux when 

 the marsh surface was submerged. About 

 140 g/m'^/yr may be lost through leaching, 

 265 g/m^ /yr are lost to deep sediments, 

 and another 5 g/m-^/yr are lost as methane. 



Over the whole community the net 

 balance unaccounted for (that is, the 

 organic C available for export) is 1,120 

 g/m^/yr. Export of all the aboveground 

 production would not equal this. Hopkin- 

 son's estimate of about 300 g exported/m^/ 

 yr is also the balance left over when all 

 other inputs and outputs are considered. 

 It is a reasonable figure in that it 

 matches the estimate of Happ et al . 

 (1977). Furthermore, the H budget (see 

 Nutrient Cycling), which is derived from 

 different assumptions and measurements, 

 also makes a value of about 300 g C 

 reasonable, assuming that the exported N 

 is all organic with a C:N ratio of 21.6 

 (Delaune et al. 1981), 



The discrepancy between 300 and 1,120 

 g/m^/yr is large. The best that can be 

 said for the C balance in deltaic salt 

 marshes at present is that there appears 

 to be a large amount of organic production 

 for which the fate is unknown. Part of it 

 is certainly exported, but we do not know 

 how much. Methodological differences 

 certainly contribute to the uncertainty. 



We know even less about C balances in 

 zones other than the salt marsh. Burial 

 of C in deep sediments does not vary much 

 from salt to fresh marshes. However, as 

 sulfate availability decreases, methane 

 production increases. The annual loss of 

 C as methane increases from 5 g/m^ in salt 

 marshes to 73 g/m^ in brackish marshes and 

 160 g/m^ in fresh marshes (Smith et al . 

 1982a). 



On the other hand, because flushing 

 energies are lower than in salt marshes 

 one would expect waterborne organic export 

 to decrease toward fresh areas. The 

 brackish marsh, in particular, is very 



poorly understood. Its production is 

 high, probably higher than the salt marsh. 

 Because flushing energy is low, export is 

 expected to be low also. This suggests 

 that respiration must be very high, but 

 decomposition studies (White et al . 1978) 

 show slower loss rates than in salt 

 marshes. 



NUTRIENT CYCLES 



In coastal marsh ecosystems, as in 

 other types, organic productivity depends 

 on the availability of inorganic nutrients 

 in the right proportions at the right 

 times. Growth limitation due to both 

 nutrient limitation and toxicity can and 

 probably do occur in marshes. However, of 

 the 12 inorganic minerals known to be 

 required by plants, only N appears to be 

 regularly limiting to marsh plant growth. 



Iron limitations have been reported 

 (Adams 1963), but subsequent studies have 

 not supported this observation (Haines and 

 Dunn 1976). In fact Fe and Mn are much 

 more likely to be in toxic concentrations 

 in i.iarsh soils because of their increased 

 availability under anaerobic conditions. 

 For example, Fe is found in marsh plant 

 tissues in concentrations up to 1,800 ppm 

 (Haines and Dunn 1976), which is well over 

 10 times the concentration in most agri- 

 cultural crops. 



Marshes are open systems, and the 

 absorption and release of nutrients can 

 have strong effects on adjacent waters. 

 Marshes have been said to reduce eutro- 

 phication by removing nutrients from 

 these water bodies and, conversely, to be a 

 source of nutrients that supplements 

 aquatic production. The evidence for 

 Mississippi delta salt marshes is that 

 they are sinks for all nutrients, that 

 they absorb inorganic N and release part 

 of it as reduced ammonia and organic 

 fonns, and that they export organic 

 C. Ecologically the most important 

 nutrients in the marsh are N, P, and S. 



Ni trogen 



Nitrogen, as mentioned earlier, has 

 been found to limit growth in most marshes 

 (see Mendelssohn et al . 1982). Nitrogen 

 chemistry in anoxic soils is extremely 

 complex and is made even more so by the 



74 



