summer heating of shallow coastal water, 

 Moore (1963a) found that the ratio of mean 

 temperature range (30° to 50° N) to mean 

 tropical range (20° N to 20° S) to be 

 2.5:1 for oceanic waters, but increased to 

 4.2:1 for shallow coastal waters. 



Because of thermal tolerance reduc- 

 tion in the tropics, the biological result 

 is a loss of cold tolerance; that is, the 

 range of thermal tolerance of tropical 

 organisms is about half that of temperate 

 counterparts, whereas the upper tolerance 

 limit is similar (Zieman and Wood 1975). 



Turtle grass thrives best in tempera- 

 tures of 20° to 30°C (68° to 86°F) in 

 south Florida (Phillips 1960). Zieman 

 (1975a, 1975b) found that the optimum 

 temperature for net photosynthesis of 

 turtle grass in Biscayne Bay was 28° to 

 30°C (82° to 86°F) and that growth rates 

 declined sharply on either side of this 

 range (Figure 6). Turtle grass can toler- 

 ate short term emersion in high tempera- 

 tures (33° to 35°C or 91° to 95°F), but 

 growth rapidly falls off if these tempera- 

 tures are sustained (Zieman 1975a, 1975b). 



In a study of the ecology of tidal 

 flats in Puerto Rico, Glynn (1968) observ- 

 ed that the leaves of turtle grass were 

 killed by temperatures of 35° to 40°C (95° 

 to 104°F), but that the rhizomes of the 

 plants were apparently unaffected. On 

 shallow banks and grass plots, tempera- 

 tures rise rapidly during low spring 

 tides; high temperatures, coupled with 

 desiccation, kill vast quantities of 

 leaves that are later sloughed off. The 

 process occurs sporadically throughout the 

 year and seems to pose no long-term prob- 

 lem for the plants. Wood and Zieman (1969) 

 warn, however, that prolonged heating of 

 substrate could destroy the root and rhi- 

 zome system. In this case, recovery could 

 take several years even if the stress were 

 removed. 



The most severe mortalities of organ- 

 isms in the waters of south Florida are 

 usually caused by severe cold rather than 

 heat, as extreme cold water temperatures 

 are more irregular and much wider spaced 

 phenomena than extreme high temperatures. 

 McMillan (1979) tested the chill tolerance 

 of populations of turtle grass, manatee 



20 25 30 



TEMPERATURE 



35 



Figure 6. Response of Thalassia production to temperature in south Florida, 



13 



