et al . 1081b). The amount of nitrate is 

 usually low or absent in sediments as it 

 is either rapidly .Tictabol ized or converted 

 to dinitronen (N ) via denitrifying bac- 

 teria. 



Sulfur bacteria are primarily respon- 

 sible for maintaining conditions necessary 

 for the remineral ization of nutrients in 

 the sediment. By reducing sulfate to sul- 

 fide, these bacteria maintain the environ- 

 mental conditions (Eh and pH) at a level 

 whore the nitrogen mineralization proceeds 

 at a rate greater than its utilization by 

 the microbial community. This produces 

 the available nutrient fractions. 



2.6 CURRENT VELOCITY 



Little work has been done to deter- 

 mine the response of seagrass communities 

 to different current velocities (Fonseca 

 et al . in press a, b). Seagrass production 

 and bionass are strongly influenced by 

 current velocity (Conover 1968). Roth 

 turtle grass and Zostera showed naximun 

 standing crops where current velocities 

 averaged 0.5 m/sec. In south Florida the 

 densest stands of turtle grass and manatee 

 grass v.'i th bright, long leaves are observ- 

 ed in the tidal channels separating the 

 mangrove islands. Inferential evidence 

 suggests that the rapid currents break 

 down diffusion gradients and make more COj 

 and inorganic nutrients available to the 

 plants (Conover 1'^6g). In a cruise of 

 the Alpha Helix to Nicaragua in 1977, sam- 

 ples taken from a mangrove-lined tidal 

 channel showed a leaf standing crop of 

 2G2 q dry weight (dw)/m- and a total bio- 

 mass of 4,570 gdw/m'. By comparison, sam- 

 ples from a quiescent lagoon environment 

 were 185 and 1,033 g/m (McRoy, Zieman and 

 Ogden, personal communication). 



Where currents are strong and persis- 

 tent, crescentic features known as blow- 

 outs are often formed. These are cusp- 

 shaped holes that actually migrate through 

 grassbeds in the directions of the main 

 current flow, eroding at one edge and col- 

 onizing at the other. Their significance 

 is discussed in the section on succession. 



2.7 OKYGEN 



Most seagrass meadows have sufficient 

 oxygen in the water column for survival of 

 the associated plants and animals. Often 

 the shallow beds can bo heard to hiss from 

 the escaping , bubbles in the late after- 

 noon. Dense beds in shallow water with 

 restricted circulation can show extremely 

 reduced 0-^ levels or even anoxia late at 

 night on a slack tide. This can be a 

 greater problem if there is a heavy load 

 of suspended organic sediment that would 

 also consume oxygen. Generally the wind 

 required to generate the turbulence neces- 

 sary to suspend large quantities of sedi- 

 ment offsets this effect by aerating the 

 v/ater. 



Low Oj levels can also slow plant 

 respiration; internal concentrations of 0, 

 decrease rapidly and CO. increases. Respi- 

 ration then is limited by the ability of 

 oxygen to diffuse from the water. Plants, 

 however, are less affected by low oxygen 

 levels than animals. Although Kikuchi 

 (1980) recorded a marked decrease in oxy- 

 gen in Japanese Zostera beds coincident 

 with blade die-off and increased microbial 

 activity, apparently it was not lethal. 

 Productivity studies in Puerto Pico (Odum 

 et al . 1960), Florida and Texas (Odum and 

 Wilson 1962) showed nighttime oxygen val- 

 ues that were typically 4 to 7 mg 0/1; 

 the lowest reported value of 2 to 3 mg 

 0, /I occurred on a calm, extremely low 

 tide in August. 



2.8 SOLAR RADIATION 



When one considers the overriding 

 importance of, solar energy as the main 

 forcing function on any ecosystem, it is 

 amazing how infrequently values are re- 

 ported in the scientific literature. His- 

 torically there has been a consensus (even 

 without adequate measurement) that sea- 

 grasses require high light intensity for 

 photosynthesis (Zieman and Wetzel 1980). 

 This is based on the observation that ex- 

 tensive seagrass beds are not found deeper 

 than 10 m (33 ft). These observations are 

 complicated by evidence that there is also 



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