the rate of biological breakdown of the 

 seagrass carbon. Physical decomposition 

 rate is an approximate indication of the 

 rate at which the plant material becomes 

 available to the various groups of detri- 

 tivores and how rapidly it will be sub- 

 jected to microbial degradation. 



Evidence indicates that turtle grass 

 detritus is physically decomposed at a 

 rate faster than the marsh grass, Spartina 

 al terniflora , and mangrove leaves. Zieman 

 (1975b) found a 50% loss of original dry 

 weight for turtle grass leaves after 4 

 weeks using sample bags of l-mm mesh size 

 (Figure 23). 



Seagrass leaves are often transported 

 away from the beds. Large quantities are 

 found among the mangroves, in wrack lines 

 along beaches, floating in large mats, and 

 collected in depressions on unvegetated 

 areas of the bottom. Studies have shown 

 that the differences in the physical and 

 biological conditions in these environ- 

 ments resulted in different rates of phys- 

 ical decomposition (Zieman 1975b). Turtle 

 grass leaves exposed to alternate wet- 

 ting and drying or wave action breakdown 



rapidly, although this may inhibit micro- 

 bial growth (Josselyn and Mathieson 1980). 



Biological factors also affect the 

 rate of physical decompositon. Animals 

 grazing on the microflora of detritus dis- 

 rupt and shred the plant substrate, accel- 

 erating its physical breakdown. Fenchel 

 (1970) found that the feeding activities 

 of the amphipod Parahyel la whelpyi dramat- 

 ically decreased the particle size of 

 turtle grass detritus. 



Microbial Colonization and Activities 



Feeding studies performed with vari- 

 ous omnivores and detritivores have shown 

 that the nutritional value of macrophyte 

 detritus is limited by the quantity and 

 quality of microbial biomass associated 

 with it. (See Cammen 1980 for other stud- 

 ies of detrital consumption.) The micro- 

 organisms' roles in enhancing the food 

 value of seagrass detritus can be divided 

 into two functions. First, they enzymati- 

 cally convert the fibrous components of 

 the plant material that is not assimilable 

 by many detritivores into microbial bio- 

 mass which can be assimilated. Second, 



■ ~ ~ ^ S portino 



Distichlif 



Jun<ui 



Sportino 



"'•^. Rhiiophoro 



Tholossio (w) 



Burkholder t Bornside 



De la Crui 



Heold 



Zieman 



Salicof nio 



O 



1 2 3 4 5 6 7 



TIME IN MONTHS 



Figure 23. Comparative decay rates showing the rapid decomposition of seagrasses com- 

 pared with other marine and estuarine plants (references: Burkholder and Bornside 1957; 

 de la Cruz 1965; Heald 1969; Zieman 1975b). 



71 



