Zieman (1975b) reported that photosynthesis in turtle grass sharply 

 declined both above and below 82° and 86° Fahrenheit (28° to 30° Celsius) . 

 Thorhaug and Stearns (1972) reported that turtle grass growing at artifi- 

 cially elevated temperatures produced flowers but no fruits. McMillan 

 (1979) found that turtle grass from a wide latitudinal gradient formed 

 an adaptive tolerance to chilling, the broadest tolerance range in the 

 northern Gulf of Mexico and the narrowest in St. Croix. 



Wood and Zieman (1969) reported that blades of turtle grass formed 

 large necrotic and discolored areas when stressed by high temperature. 

 Persistent thermal stress resulted in the loss of leaves and eventually- 

 raised sediment temperatures by heat conduction. Higher sediment 

 temperatures increased the respiration of rhizomes and caused the 

 complete collapse of stressed populations. 



4. Salinity . 



Salinity changes do not appear to be as critical as temperature 

 changes, although seagrasses do have a tolerance range to salinity. 

 The range for eelgrass appears to be 10 to 30 parts per thousand 

 (Phillips, 1972). Phillips (1960) reported a range of 20 to 35 parts 

 per thousand for turtle grass. The range of manatee grass is nar- 

 rower, 20 to 35 parts per thousand (McMahan, 1968) . Shoalgrass in 

 the tropics has the widest tolerance range (3.5 to 60 parts per thou- 

 sand) . McMillan and Moseley (1967) found that shoalgrass has the 

 greatest resistance to high salinity, turtle grass intermediate, and 

 manatee grass the least resistant. 



A more restricted range of salinity is recommended for areas 

 designated for transplanting seagrasses than these seagrasses will actual- 

 ly tolerate (cf . , Figs. 1, 2, and 3). This restricted salinity range 

 should aid in a faster establishment on a soil type which is possibly 

 different from the source and could ameliorate possible plant-substrate 

 nutrient interactions. 



5. Nutrients . 



Nutrients in the water column are not a limiting factor for sea- 

 grasses. The major nutrient activity is in the sediment. The reducing 

 environment created in the substrate forms a sink for many heavy metals 

 (Parker, 1962; Parker, Gibbs, and Lawler, 1963; Zieman, 1975a). There 

 is no evidence that these metals affect the seagrasses. 



Seagrass meadows are extremely important in the cycling of nutrients. 

 Nitrogen, carbon, sulfur, and other nutrients are converted into more 

 usable forms for other organisms. These nutrients are absorbed by the 

 plants through the roots and pumped into the water mass. 



Patriquin and Knowles (1972) found nitrogen fixed in the rhizosphere 



of eelgrass. McRoy and Barsdate (1970) reported that eelgrass leaves 



absorb phosphorus, but the major pathway is from the roots to the 

 leaves and into the water column. 



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