to iiiicrozooplankton and eventually to such 

 filter feeders as mollusks and menhaden 

 may be more significant than has been 

 real ized. 



The second phase of decomposition 

 often takes a year or more, depending on 

 the environment and the plant species 

 (Valiela et al. 1982). At the end of this 

 period about 10 percent of the original 

 detrital biomass may remain as refractory 

 organic compounds. 



^ common way to investigate the loss 

 rates is by enclosing dead plant material 

 in litter bags (small nylon mesh bags with 

 2 to 5-mm holes), suspending the bags in 

 the marsh, and retrieving them at 

 intervals to examine the amount of 

 material remaining. Decomposition is not 

 the only thing measured by this technique. 

 As soon as the plant fragments become 

 small enough to escape from the bags, they 

 nay be lost by the flushing action of 

 flooding water. In addition, usually 



larvae of many invertebrates find their 

 way into the bags and prosper on the 

 detritus. Their action in fragmenting the 

 detritus is undoubtedly important in the 

 loss rate. 



A number of decomposition studies 

 carried out in the delta are summarized in 

 Appendix 2. In this Appendix and the fig- 

 ures and tables that fol low, decotnposition 

 rates have been standardized by assuming 

 an exponential decay rate (Wiegert and 

 Evans 1964). The data are reported as 

 loss rates, _r [mg dry weight (dw) lost/g 

 dw detritus/day], defined as [ln(initial 

 mass/final mass)]/time interval. 



These studies support results found 

 elsewhere: the three main factors control- 

 ling decomposition are tenperature, loca- 

 tion in the intertidal zone, and the plant 

 species. Nutrient levels and the presence 

 of macro-invertebrates that shred the 

 detritus are also important. 



Figure 52 shows that the decomposi- 

 tion rate of S^. patens detritus decreases 

 with time. This could happen for two 

 reasons, "^irst, this study was initiated 

 in June, and the rate declined as the air 

 temperature declined. Second, one would 

 expect the more easily decomposed material 

 to disappear first, leaving the more 

 refractory, slowly decomposing compounds. 



50 100 150 200 250 



DAYS FROM START OF INCUBATION {JUNE 20) 



7/10 8/7 9/15 10/15 11/7 12/6 1/18 2/25 

 DATE 



Figure 52. Disappearance of S_. patens 

 litter from litter bags in the 

 Pontchartrain-Borgne basin (data from 

 Cramer and Day 1980) . 



Both of these factors are probably re- 

 flected in this graph. The histogram 

 showing the changing rate for each succes- 

 sive interval of time indicates that the 

 initial rapid rate was declining as early 

 as August before air tenperature dropped 

 significantly. This implies a change in 

 the kind of material being decomposed. On 

 the other hand, the rate began to increase 

 again at the end of the experiment when 

 the remaining materials would be most 

 refractory; this coincided with the early 

 spring increase in the ambient tempera- 

 tures. 



Figure 53 shows mean loss rates of 

 S_. al terniflora detritus from litterbags 

 submerged but susoended off the bottom in 

 a tidal stream, on the surface of a 

 streamside marsh, and on the marsh surface 

 further inland. Decomposition was fastest 

 in flowing water, second where tidal 

 flushing was vigorous, and slowest where 

 the bags tended to be submerged most of 

 the time in stagnant water. The figure 

 also demonstrates the temperature (season- 

 al) effect. 



Finally, Table 20 summarizes the 

 species-dependency of the decomposition 

 rate. Variability is high, hut I believe 

 the means are fairly reliable indicators 

 of the relative rates of decomposition of 

 different species. S^. al terniflora is the 

 most easily broken down of the grasses, 

 but they all tend to be fairly fibrous and 



58 



