al. (1980). Heald and Odum showed that 

 decomposition of red mangrove leaves 

 proceeds most rapidly under marine condi- 

 tions, somewhat more slowly in freshwater, 

 and very slowly on dry substrates. For 

 example, using the litter bag method, they 

 found that only 9% of the original dry 

 weight remained after 4 months in sea 

 water. By comparison, 39% and 54% re- 

 mained at the end of comparable periods in 

 brackish water and freshwater. Under dry 

 conditions, 65% remained. Higher decompo- 

 sition rates in sea water were related to 

 increased activity of shredder organisms, 

 such as crabs and amphipods. 



Heald (1969) and Odum (1970) also 

 found increases in nitrogen, protein, and 

 caloric content as mangrove leaves pro- 

 gressively decayed. The nitrogen content 

 of leaves decaying under brackish condi- 

 tions (on an AFDW basis) increased from 

 1.5% (5.6% protein) to 3.3% (20.6% 

 protein) over a 6-month period. Subse- 

 quent information (Odum et al. 1979b) 

 suggested that the protein increase may 

 not have been this great since some of the 

 nitrogen increase probably included non- 

 protein nitrogen compounds such as amino 

 sugars. Fell and Master (1973), Fell et 

 al. (1980), Fell and Newell (1980), and 

 Fell et al. (1980) have provided more 

 detailed information on red mangrove leaf 

 decomposition, the role of fungi in decom- 

 position (see section 4), and nitrogen 

 changes and nitrogen immobilization during 

 decomposition. Fell et al . (1980) 

 have shown that as much as 50% of weight 

 loss of the leaf during decomposition is 

 in the form of dissolved organic matter 

 (DOM). 



Heald et al. (1979), Lugo et al. 

 (1980) and Twilley (1980) discovered that 

 black mangrove leaves decompose more ra- 

 pidly than red mangrove leaves and ap- 

 parently produce a higher percentage of 

 DOM. Pool et al. (1975) have shown that 

 mangrove litter decomposes and is exported 

 most rapidly from frequently flooded 

 riverine and overwash forests. These 

 communities have little accumulation of 

 litter on the forest floor. Communities 

 which are not as well-flushed by the 

 tides, such as the basin and hammock 



forests, have slower rates of decomposi- 

 tion and lower export rates. 



3.5 CARBON EXPORT 



Research from Florida mangrove swamps 

 forms a small portion of the larger con- 

 troversy concerned with the extent to 

 which coastal wetlands export particulate 

 organic carbon (reviewed by Odum et al. 

 1979a). Available evidence from Florida, 

 Puerto Rico and Australia (Table 4) sug- 

 gests that mangrove swamps tend to be net 

 exporters. The values in Table 4 should 

 be regarded as preliminary, however, since 

 all five studies are based upon simplistic 

 assumptions and methodology. 



Golley et al. (1962) based their 

 annual estimate of particulate carbon 

 export from a Puerto Rican forest upon a 

 few weeks of measurements. Odum and 

 Heald's estimates were derived from two or 

 three measurements a month. All investi- 

 gators have ignored the importance of bed 

 load transport and the impact of extreme 

 events. All investigators except Lugo et 

 al. (1980) have failed to measure DOC 

 flux. 



It seems relatively clear that man- 

 grove forests do export organic carbon to 

 nearby bodies of water. The magnitude of 

 this export has probably been underesti- 

 mated due to ignoring bedload, extreme 

 events, and DOC. 



The value of this carbon input to 

 secondary consumers in receiving waters is 

 not clear. As shown in section 3.6, food 

 webs based primarily upon mangrove carbon 

 do exist. The relative importance of 

 mangrove carbon to Florida coastal ecosys- 

 tems remains speculative. We suspect that 

 mangrove-based food webs are dominant in 

 small bays, creeks and rivers within large 

 mangrove ecosystems such as the North 

 River system studied by Heald (1969) and 

 Odum (1970). In intermediate-sized bodies 

 of water, such as Rookery Bay near Naples, 

 Florida, mangroves are probably important 

 but not dominant sources of organic car- 

 bon. Lugo et al. (1980) estimate that 

 mangroves supply 32% of the organic carbon 



34 



