rooted plants, seagrasses require a suf- 

 ficient depth of sediment for proper 

 development. The sediment anchors the 

 plant against the effects of water surge 

 and currents, and provides the matrix for 

 regeneration and nutrient supply. Run- 

 ners occasionally adhere directly to a 

 rock surface, with only a thin veneer of 

 sediment surrounding the roots, but this 

 happens sporadically and is quantitatively 

 insignificant. The single most important 

 sediment characteristic for seagrass 

 growth and development is sufficient sedi- 

 ment depth. 



Depth requirements also vary with the 

 different species. Because of its shal- 

 low, surficial root system, shoal grass 

 can colonize thin sediments in an area of 

 minimal hydraulic stability (Fonseca 

 et al . 1981). Turtle grass is more robust, 

 requiring 50 cm (20 inches) of sediment to 

 achieve lush growth, although meadow for- 

 mation can begin with a lesser sediment 

 depth (Zieman 1972). In the Bahamas, 

 Scoffin (1970) found that turtle grass did 

 not appear until sediment depth reached at 

 least 7 cm (3 inches) . 



The density of turtle grass leaves 

 greatly affected the concentration of 

 fine-grained (less than 63u) particles in 

 sediments. Compared with bare sediment 

 which showed only 1% to 3°^ fine-grained 

 material, sparse to medium densities of 

 turtle grass increased the fine percentage 

 from 3% to 6% and dense turtle grass 

 increased this further to over 15%. 



The primary effects of the grass 

 blades are the increasing of sedimentation 

 rates in the beds; the concentrating of 

 the finer-sized particles, both inorganic 

 and organic; and the stabilizing of the 

 deposited sediments (Fonseca, in press a, 

 b; Kenworthy 1981). Burrell and Schubel 

 (1977) described three effects produced by 

 these mechanisms: 



(1) Direct and indirect extraction 

 and entrapment of fine water- 

 borne particles by the seagrass 

 loaves. 



(2) Formation and retention of par- 

 ticles produced within the grass 

 beds. 



(3) Binding and stabilizing of the 

 substrate by the seagrass root 

 and rhizome system. 



One of the values of the seagrass 

 system is the ability to create a rela- 

 tively low energy environment in regions 

 of higher energy and turbulence. In addi- 

 tion to the fine particle extraction due 

 to decreased turbulence, the leaves trap 

 and consolidate particles of passing sedi- 

 ment which adhere to the leaf surface or 

 become enmeshed in the tangle of epiphytes 

 of older leaves. As the older portion of 

 the leaves fragment, or as the leaves die 

 and fall to the sediment surface, the or- 

 ganic portions of the leaves decay and the 

 inorganic particles become part of the 

 sediment. The continued presence of the 

 growing leaves reduces the water velocity 

 and increases the retention of these 

 particles, yielding a net increase in 

 sediment. 



Key elements in a plant's efficiency 

 of sediment stabilization are plant spe- 

 cies and density of leaves. From observa- 

 tional data in Bermuda, researchers found 

 open sand areas had 0.17, to 0.2« fine par- 

 ticles (less than 63p). In manatee grass 

 beds this increased to 1.9/o fines, while 

 turtle grass beds had a.?% to 5.^% fine 

 material (Wood et al . 1969). In the same 

 study organic matter (% dry weight) was 

 2.57, to 2.6% in open sand areas with simi- 

 lar values in manatee grass beds; the 

 organic matter in turtle grass beds was 

 3.5% to 4.9%, demonstrating the increased 

 stabilization and retention pov/er of the 

 more robust turtle grass. 



Seagrasses not only affect mean grain 

 size of particles, but other geologically 

 important parameters such as sorting, 

 skewness, and shape (Rurrell and Schubel 

 1977). Swinchatt (1965) found that the 

 mean size of sand fraction particles, the 

 relative abundance of fines, and the stan- 

 dard dimension all increased with an 

 increase in blade density near a Florida 

 reef tract. The nuantitative effect of 

 the trapping and bonding was discussed in 

 several studies (Ginsberg and Lov/enstam 

 1958; Wood et al . 1969; Fonseca in press 

 a, b) and is shown graphically in Figure 7 

 (Zieman 1972). 



15 



