detritus originates as upstream run- 

 off from extensive sawgrass marshes 

 and wet prairies. Autochthonous 



sources include mangrove debris, 

 black needlerush ( Juncus ) and cord- 

 grass ( Spartina ) debris, debris from 

 less abundant plant species, and 

 animal detritus. Of all the poten- 

 tial sources, mangrove debris is by 

 far the most important. Heald 



(1971) estimates that 85% of the 

 "debris" produced in wetlands sur- 

 rounding the North River estuary 

 originates from one species, the red 

 mangrove, while Juncus and Spartina 

 contribute little to the total 

 available debris. 



As the mangrove debris awaits 

 its fate of either sedimentation or 

 washout into the open waters by 

 tidal or freshwater flushing, it is 

 subjected to a variable intensity of 

 degradation forces (Heald 1971). In 

 general, leaves degrade faster if 

 they are in predominantly fresh 

 water than they do on dry land. 

 They degrade even faster if they are 

 in brackish or salt water. This 

 increase in rate is apparently due 

 to increased grazing by small marine 

 crustaceans, particularly amphi- 

 pods. 



This pattern of detrital de- 

 gradation also coincides with the 

 quality of the mangrove forest 

 structure: the best developed for- 

 est structures tend to be found 

 where soil salinities are well mod- 

 erated by adequate freshwater and/or 

 tidal flushing. Marginal environ- 

 ments for forest development occur 

 in association with either uniformly 

 high or low annual salinities, ex- 

 cessive siltation, arid climates, in 

 sedimentary carbonate environments, 

 or where tidal amplitude is small 

 (Snedaker and Brown 1982). 



Using the six mangrove forest 

 type categories of Lugo and Snedaker 

 (1974), Snedaker and Brown (1982) 

 present an index of mangrove ecosys- 



tem dynamics based on leaf litter 

 production rates (Table 30). This 

 index has proven to be the most 

 reliable indicator of mangrove eco- 

 system dynamics. 



Table 30. Leaf litter production 

 rates of mangrove eco- 

 systems (adapted from 

 Snedaker and Brown 

 1982). 



In general, grazing of freshly 

 fallen leaves is delayed by the 

 heavy cuticular wax of the mangrove 

 leaves. As this disintegrates, sub- 

 sequent grazing by microcrustaceans 

 increases, ostensibly because of a 

 higher nutritive content of bacteri- 

 al and fungal food sources. Needle- 

 rush and sawgrass debris are seldom 

 grazed upon after abcission and thus 

 degrade at a slower rate. 



Heald (1971) documents a micro- 

 floral succession on red mangrove 

 leaves leading to the increased 

 availability and usefulness of the 

 detritus to macroconsumers. Figure 

 52 summarizes the principle physical 

 and biochemical features of this 

 successional process i.e., a rela- 

 tive enrichment of the leaf with 

 animal protein at the expense of 

 plant protein, as particle size 

 decreases. 



With regard to water quality, 

 Snedaker and Brown (1982) find 

 mangroves are extremely tolerant 

 to a wide variety of conditions. 



152 



