CHAPTER 4 



THE SEAGRASS SYSTEM 



4.1 FUNCTIONS OF SEAGRASS ECOSYSTEMS 



In addition to being high in net pri- 

 mary production and contributing large 

 quantities of detritus to an ecosystem, 

 seagrasses perform other functions. Be- 

 cause of their roots and rhizomes, they 

 can modify their physical environment to 

 an extent not equaled by any other fully 

 submerged organism. Phillips (1978) stated 

 that, "by their presence on a landscape of 

 relatively uniform relief, seagrasses 

 create a diversity of habitats and sub- 

 strates, providing a structured habitat 

 from a structureless one." Thus seagrasses 

 also function to enhance environmental 

 stability and provide shelter. 



Seagrass ecosystems have numerous im- 

 portant functions in the nearshore marine 

 environment. Wood et al . (1969) originally 

 classified the functions of the seagrass 

 ecosystem. The following is an updated 

 version of the earlier classification 

 scheme. 



(1) High production and growth 



The ability of seagrasses to exert a 

 major influence on the marine seacape 

 is due in large part to their ex- 

 tremely rapid growth and high net 

 productivity. The leaves grow at 

 rates typically 5 mm/day, but growth 

 rates of over 10 mm/day are not 

 uncommon under favorable circum- 

 stances. 



{?.) Food and feeding pathways 



The photosynthetically fixed energy 

 from the seagrasses may follow two 



general pathways: direct grazing of 

 organisms on the living plant mate- 

 rial or utilization of detritus from 

 decaying seagrass material, primarily 

 leaves. The export of seagrass mate- 

 rial, both living and detrital, to a 

 location some distance from the sea- 

 grass bed allows for further distri- 

 bution of energy away from its orig- 

 inal source. 



(3) Shelter 



Seagrass beds serve as a nursery 

 ground, that is a place of both food 

 and shelter, for the juveniles of a 

 variety of finfish and shellfish of 

 commercial and sportfishing impor- 

 tance. 



[i] Habitat stabilization 



Seagrasses stabilize the sediments in 

 two ways: the leaves slow and retard 

 current flow to reduce water velocity 

 near the sediment-water interface, a 

 process which promotes sedimentation 

 of particles as well as inhibiting 

 resuspension of both organic and 

 inorganic material. The roots and 

 rhizomes form a complex, interlocking 

 matrix with which to bond the sedi- 

 ment and retard erosion. 



(5) Nutrient effects 



The production of detritus and the 

 promotion of sedimentation by the 

 leaves of seagrasses provide organic 

 matter for the sediments and maintain 

 an active environment for nutrient 

 recycling. Epiphytic algae on the 



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