leaves plus their epiphytes averaged 1.4% 

 N. Bjorndal found that grazed turtle 

 grass leaves averaged 0.35% N (AFDW) 

 higher than ungrazed leaves, and Thayer 

 and Engel (MS, in preparation) found a 

 nitrogen content of 1.55% (DW) in the 

 esophagus of Chelonia . Zienan and Iverson 

 (in preparation) found that there was a 

 decrease in nitrogen content with age and 

 epiphytization of seagrass leaves. The 

 basal portion of turtle grass leaves fron 

 St. Croix contained 1.6% to 2.0% N on a 

 dry weight basis, while the brown tips of 

 these leaves contained 0.6% to 1.1% N, 

 and the epiphytized tips ranged fron 0.5% 

 to 1.7% N. Thus the current evidence 

 would indicate that the green seagrass 

 leaves contain more nitrogen than either 

 the senescent leaves or the leaf-epiphyte 

 copiplex. By successively recropping 

 leaves from a plot, the turtle main- 

 tains a diet that is consistently higher 

 in nitrogen and lower in fiber content 

 than whole leaves (Bjorndal 1980). 



Grazing on seagrasses produces 

 another effect on sea turtles. In the 

 Gulf of California (Felger and Moser 1973) 

 and Nicaragua (Mortimer, as reported by 

 Bjorndal 1980), witnesses reported that 

 turtles that had been feeding on sea- 

 grasses were considered to be good tast- 

 ing, while those that were caught in areas 

 where they had fed on algae were consid- 

 ered to be "stinking" turtles with a defi- 

 nite inferior taste. 



Thayer and Engel (MS. in preparation) 

 suggested that grazing on seagrasses can 

 short-circuit the time frame of decomposi- 

 tion. They showed that an intermediate- 

 sized green turtle which consumes about 

 300 g dry weight of leaves and defecates 

 about 70 g dry weight of feces daily, does 

 return nitrogen to the environment at a 

 more rapid rate than occurs for the decom- 

 position of a similar amount of leaves. 

 They point out that this very nutrient- 

 rich and high nutritional quality fecal 

 matter should be readily available to 

 detritivores. It is also pointed out that 

 this matter is probably not produced 

 entirely at the feeding site and thus 

 provides aa additional interconnection 

 between grassbeds and adjacent habitats. 



Like the turtles, the Caribbean 

 manatee (Trichechus manatus) formerly was 



common throughout the Caribbean, espec- 

 ially in the mainland areas, but is now 

 greatly reduced in range and population. 

 Manatees live in fresh or marine waters; 

 and in Florida, most manatee studies have 

 focused on the manatee's ability to con- 

 trol aquatic weeds. Manatees, which weigh 

 up to 500 kg (1,102 lb), can consume up to 

 20% of their body weight per day in aqua- 

 tic plants. 



When in marine waters, the manatee 

 apparently feeds much like its fellow 

 sirenians, the dugongs. The dugongs use 

 their rough facial bristles to dig into 

 the sediment and grasp the plants. These 

 are uprooted and shaken free of adhered 

 sediment. Husar (1975) stated that feed- 

 ing patches are typically 30 by 60 cm (12 

 by 24 inches) and that they form a conspi- 

 cuous trail in seagrass beds. This author 

 has observed manatees feeding in Thalassia 

 beds in much the sam.e manner. The patches 

 cleared were of a similar size as those 

 described for the dugongs, and rhizome 

 removal was nearly complete. The excess 

 sediments from the hole were mounded on 

 the side of the holes as if the manatee 

 had pushed much of it to the side before 

 attempting to uproot the plants. 



Manatees would seem to be more 

 limited in their feeding range because of 

 sediment properties, as they reouire a 

 sediment which is sufficiently unconsoli- 

 dated that they may either root down to 

 the rhizome or grasp the short shoot and 

 pull it out of the sediment. Areas where 

 manatee feeding and feeding scars were 

 observed were characterized by soft sedi- 

 ments and lush growth of turtle grass and 

 Hal imeda in mounded patches. Nearly all 

 areas in which sediments were more consol- 

 idated showed no signs of feeding. In the 

 areas where the manatees were observed, 

 the author found that he could readily 

 shove his fist 30 cm (12 inches) or more 

 into the sediments, while in the adjacent 

 ungrazed areas, maximum penetration was 

 only a few centimeters and it was impos- 

 sible to remove the rhizomes without a 

 shovel . 



6.3 DETRITAL PPOCESSING 



For the majority of animals that 

 derive all or part of their nutrition from 



69 



