grass, and shoal grass in various loca- 

 tions from Texas to St. Croix and Jamaica, 

 Populations from south Florida were inter- 

 mediate in tolerance between plants from 

 Texas and the northern Florida coast 

 and those from St. Croix and Jamaica in 

 the Caribbean. In south Florida, the 

 most chill -tolerant plants were from the 

 shallow bays, while the populations that 

 were least tolerant of cold temperatures 

 were from coral reef areas, where less 

 fluctuation and greater buffering would be 

 expected. During winter, the cold north- 

 ern winds quickly cool off the shallow 

 (0.3 to 1 m or 1 to 3.3 ft) waters of 

 Florida Pay. The deeper waters, however, 

 in the area below the Keys and the reef 

 line (up to 15 m or 50 ft) not only have a 

 much greater mass to be cooled, hut are 

 also flushed daily with warmer Gulf Stream 

 water which further tends to buffer the 

 environmental fluctuations. 



The amount of direct evidence for the 

 temperature ranges of shoal grass and man- 

 atee grass is far less than for turtle 

 grass." Phillips (1960) suggested that 

 shoal grass Generally prefers temperatures 

 of 20 "to 30°C (68° to 86°F), but that it 

 is somevvhat more eurythermal than turtle 

 grass. This fits its ecological role as a 

 pioneer or colonizing species. Shoal grass 

 is commonly found in shallower water than 

 either turtle grass or manatee grass, 

 where thermal variation would tend to be 

 greater, ^''c^'illan (1979) found that shoal 

 grass had a greater chill tolerance than 

 turtle grass, while manatee grass showed 

 less resistance to chilling. 



embayments with restricted circulation, 

 such as southwest Biscayne Bay, many 

 algal species are reduced during summer 

 high temperatures and some of the more 

 sensitive types such as Caulerpa , CI adop- 

 hora and Laurencia may be killed (Zieman 

 1975a). 



2.4 SALINITY 



While all of the common south Florida 

 seagrasses can tolerate considerable sa- 

 linity fluctuations, all have an optimum 

 range near, or just below, the concentra- 

 tion of oceanic water. The dominant sea- 

 grass, turtle grass, can survive in salin- 

 ities from 3.5 ppt (Sculthorpe 1967) to 60 

 ppt (McMillan and Moseley 1957), but can 

 tolerate these extremes for only short 

 periods. Even then, severe leaf loss is 

 common; turtle grass lost leaves when 

 salinity was reduced below 20 ppt (den 

 HartOQ 1970). The optimum salinity for 

 turtle grass ranges from 24 ppt to 35 ppt 

 (Phillips 1960; McMillan and Moseley 1^67; 

 Zieman 1975b). Turtle grass showed maximum 

 photosynthetic activity in full-strength 

 seawater and a linear decrease in activity 

 with decreasing salinity (Hammer 196ob). 

 At 5Q7o strength seawater, the photosynthe- 

 tic rate was only one-third of that in 

 full-strength seav>'ater. Following the 

 passage of a hurricane in south Florida in 

 1960, Thomas et al . (1961) considered the 

 damage to the turtle grass by freshwater 

 runoff to have been more severe than the 

 physical effects of the high winds and 

 water surne. 



Seagrasses are partially buffered 

 from temperature extremes in the overlying 

 water because of the sediinents covering 

 the roots and rhizomes. Sediments are 

 poorer conductors of heat than seawater 

 and they absorb heat more slowly. In a 

 study by Redfield (1965), changes in the 

 tei:;perdture of the water column decrease 

 exponentially with depth in sediments. 



The tolerance of local seagrass spe- 

 cies to salinity variation is similar to 

 their temperature tolerances. Shoal nrass 

 is the most broadly euryhaline, turtle 

 grass is intermediate, and manatee grass 

 and Halcphila have the narrowest tolerance 

 ranges, with Halophila being even more 

 stenohaline than manatee arass (McMillan 

 1979). 



Macroalgao associated with grass bods 

 exist totally in the water column, and 

 thus will be affected at a rate that is 

 dependent upon their individual temper- 

 ature tolerances. Most algae associated 

 with tropical seagrass beds are more 

 sensitive to thermal stress than the 

 seagrasses (Zieman l'^75a). In shallow 



2.5 SEDIMENTS 



Seagrasses qrow in a wide variety of 

 sediments from fine muds to coarse sands, 

 depending on the type of source material, 

 the prevailing physical flow regime, and 

 the density of the seagrass blades. As 



14 



