CHAPTER 6 



TROPHIC RELATIONSHIPS IN SEAGRASS SYSTEMS 



6.1 GENERAL TROPHIC STRUCTURE 



Seagrasses and associated epiphytes 

 provide food for trophically higher organ- 

 isms by (1) direct herbivory, (2) detrital 

 food webs within grass beds and (3) ex- 

 ported material that is consumed in other 

 systems either as macroplant material or 

 as detritus (Figure 22). Classically the 

 detrital food web within the grass beds 

 has been considered the primary pathway, 

 and in most cases is probably the only 

 significant trophic pathway. During the 

 past few years, new information has been 

 gathered on the relative role of the other 

 modes of utilization. The picture emerg- 

 ing is that in many locations both the 

 direct utilization pathway and the export 

 of material may be of far more importance 



than previously suspected; however, it 

 still appears that the detrital food web 

 is the primary pathway of trophic energy 

 transfer (Zieman et al. 1979; Kikuchi 

 1980; Ogden 1980). 



Studies have attempted to measure the 

 proportion of daily seagrass production 

 which is directly grazed, added to the 

 litter layer, or exported. Greenway 

 (1976) in Kingston Harbor, Jamaica, esti- 

 mated that of 42 g/m"^/wk production of 

 turtle grass, 0.3% was consumed by the 

 small bucktooth parrotfish, Sparisoma rad- 

 ians ; 48.1% was consumed by the urchin, 

 Lytechinus ariegatus ; and 42.1% deposited 

 on the bottom and available to detriti- 

 vores. The rest of the production was 

 exported from the system. This study may 



PLANT CANOPY 

 STRUCTURE 



Figure 22. Principal energetic pathways in seagrass beds, 



57 



