more nutrient absorptive area or greater 

 anchoring capacity in the coarser sedi- 

 ments. Alternatively, the decrease in 

 root listeria] in fine sediments could 

 result from a negative effect fron anae- 

 robiasis or microbial metabolites. 



Although seagrasses require a variety 

 of macro- and micronutrients for nutri- 

 tion, most research effort has been di- 

 rected to the source and rate of supply of 

 nitrogen. While phosphorous is in very 

 low concentration in tropical waters, it 

 is relatively abundant in the sediments, 

 and estimates on turnover time range from 

 one to two turnovers per year to once 

 every few years (M.cRoy et al . 1972; Patri- 

 quin 1972b). Nitrogen, however, is needed 

 in much greater quantities and its source 

 is more obscure (McRoy and McMillan 1977). 

 Patriquin (1972b) estimates that there was 

 only a 5- to 15-day supply of inorganic 

 nitrogen available in the sediments. This 

 estimate did not account for continuous 

 recycling, however. 



Seagrasses have three potential ni- 

 trogen sources: recycled nitrogen in the 

 sediments, nitrogen in the water column, 

 and nitrogen fixation. Nitrogen fixation 

 can occur either in the rhizosphere or 

 phyllosphere. Transfers between leaf and 

 epiphyte have also been demonstrated (Har- 

 lin 1971; McRoy and Goering 1974), Capone 

 et al . (1979) concluded that nitrogen 

 fixed in the phyllosphere contributed pri- 

 marily to the epiphytic community while 

 fixation in the rhizosphere contributed 

 mainly to macrophyte production. Indi- 

 rectly the contribution of nitrogen-fixing 

 epiphytes is important because after the 

 leaves senesce and detach, most of them 

 decay and become part of the litter; some 

 will be incorporated in the sediments. 

 Other sources of nitrogen to the sediments 

 include excretion by plants and animals, 

 pjrticulate matter trapped by the dense 

 loaves, and dead root and rhizome mate- 

 rial. Capone and Taylor (198C) agreed 

 with Patriquin (1972b) that the primary 

 source of nitrogen for leaf production is 

 recycled material from sediments, but rhi- 

 zosphere fixation can supply 2C% to 50% of 

 the plant's requirements. Orth (1977a) 

 applied commercial fertilizers directly to 

 a Z ostera bed in Chesapeake Bay. After 2 

 to 3 months the length and density of 

 leaves had increased, the amount of roots 



and rhizomes was 30'? greater than the con- 

 trols, and the standing crop of loaves had 

 increased by a factor of three to four. 

 Seagrasses seem to he extremely efficient 

 at capturing and utilizina nutrients, and 

 this is a major factor in their ability to 

 maintain high productivity even in a rela- 

 tively low nutrient environment. 



3.5 SEAGRASS PHYSIOLOGY 



Seagrasses have evolved a physiology 

 that often distinguishes them from their 

 terrestrial counterparts. Since water has 

 rates of gaseous diffusion that are sev- 

 eral orders of magnitude lower than air, 

 much of this physiological modification is 

 a response to the lowered gas coricentra- 

 tion and the slower rates of diffusion 

 when compared with the terrestrial envi- 

 ronment. It is commonly thought that be- 

 cause of the abundance of inorganic carbon 

 in seawater in the carbonate buffer sys- 

 tem, marine plants are not carbon limited. 

 Turing active photosynthesis, however, in 

 shallow grass beds when tidal currents are 

 slow, the pM may rise from the normal sea- 

 water pM of 8.2 to 8.9, at which point the 

 free CO is greatly reduced in the water. 

 PH values of °.4, a point at which biocar- 

 bonate is hardly present, have been re- 

 corded over grass beds. 



The internal structure of seagrasses 

 has been modified to minimize the problems 

 of life in an aquatic environment. Large 

 internal lacunal spaces have developed, 

 often comprising over 70% of the total 

 leaf volume, to facilitate internal gas 

 transport (Arber 1920; Sculthorpe 1967; 

 Zieman and Wetzel 1980). Much of the oxy- 

 gen produced in photosynthesis is appar- 

 ently retained in the lacunal system and 

 diffuses throughout the plant to the re- 

 gions of hiah respiratory demanci in the 

 roots and rhizomes. Similarly, because of 

 the general lack of stomata, the diffusion 

 of COq into the seagrasses is slow com- 

 pared with terrestrial counterparts. In 

 addition, the quiescent water layer next 

 to the leaves does not enhance diffusion 

 of gases. 



At normal seawater pH, bicarbonate is 

 much more abundant than CO.. Beer et al . 

 (1977) showed that the major source of 

 carbon for photosynthesis for four species 



26 



