Sediment 



Elevation 



(cm) 



Leaf 

 Density 

 (leaves / 

 iOOcm^) 



Leaf 

 Length 

 (cm) 



Sediment 

 Depth 

 (cm) 



Distance Across Bed (nn) 



Figure 7. Response of a Thalassia bed to increasing sediment depth. Note increasing 

 blade length and density with increasing depth of sediment. The increase in elevation 

 in the center of the bed is due to the trapping action of the denser blades. 



Particles of carbonate are locally 

 produced in seagrass beds and removed from 

 the surrounding water. Older leaves are 

 usually colonized by encrusting coralline 

 algae such as Melobesia or Fosliella . It 

 has been estimated that these encrusting 

 algae produce from 40 to 180 g/m/yr of 

 calcium carbonate sediment in Jamaica 

 (Land 1970) and upwards to 2,800 g/m-^/yr 

 in Barbados (Patriquin 1972a). 



The high production of seagrasses can 

 affect the production of inorganic partic- 

 ulates also. Cloud (1962) estimated that 

 75% of aragonitic mud in a region of the 

 Barbados was due to direct precipitation 

 of carbonate when the seagrasses had 

 removed CO from the water during periods 

 of extremely high primary productivity. 

 Zieman (1975b) also noted the ability 

 of seagrasses under calm conditions to 



overcome the carbonate buffer capacity of 

 seawater and drive the pH up to 9.4. 



The microbial ly mediated chemical 

 processes in marine sediments provide a 

 major source of nutrients for seagrass 

 growth (Capone and Taylor 1980). Bacte- 

 rial processes convert organic nitrogen 

 compounds to ammonia (Capone and Taylor 

 1980; Smith et al . 1981b), primarily in 

 the anoxic sediment which usually exists 

 only a few millimeters beneath the sedi- 

 ment surface. The ammonia that is not 

 rapidly utilized will diffuse upward to 

 the aerobic zone where it can either 

 escape to the water column or be converted 

 to nitrate by nitrifying bacteria in the 

 presence of oxygen. Endobacteria were 

 found in the roots of the seagrass Zostera 

 marina (Smith et al . 1981a), and were 

 associated with nitrogen fixation (Smith 



16 



