Churchill, Cok, and Riner (1978), van Breedveld (1975), Eleuterius 

 (1974), and Phillips (unpublished research, 1974-1979) found that NAPH 

 did not aid in the growth of vegetative plants. Eleuterius stated that 

 the hormone treatment may have killed some plants. 



IX. SELECTED ENVIRONMENTAL CONDITIONS 



1. Depth , 



The depth distribution of seagrasses depends on a complex of 

 interrelated factors: waves, currents, substrate, turbidity, and light 

 penetration. In the temperate zone eelgrass occurs from low tide to 

 about 33 feet (10 meters) deep (Phillips, 1974). Cottam and Munro (1954) 

 observed eelgrass down to 100 feet (30 meters) where the water was clear. 

 In the tropics shoalgrass grows in the intertidal to 45 feet (14 meters) 

 deep (personal observations at St. Croix, Virgin Islands), while turtle 

 grass and manatee grass grow from the low tide line to 35 and 55 feet 

 (11 and 17 meters) deep, respectively (personal observations). Phillips 

 (I960) observed turtle grass down to 100 feet in extremely clear water 

 in the Bahamas. In very turbid waters seagrasses are restricted to less 

 than a 3- foot depth (Thayer, Wolfe, and Williams, 1975). 



2. Light . 



Backman and Barilotti (1976) demonstrated that eelgrass flowering 

 and density in a southern California lagoon were inversely related to 

 light intensity and penetration through the water column. Using canopies 

 over growths in shallow water, they reduced down-welling illuminance by 

 63 percent, simulating light conditions extant at the lower limit of 

 eelgrass growth. After 18 days, mean shoot densities under the canopies 

 decreased relative to that of adjacent unshaded growth. After 9 months, 

 shoot densities declined to 5 percent of the adjacent control unshaded 

 growth. Flowering was also reduced under the shading canopies. Some 

 shading could occur with increased water turbidity caused by an increased 

 silt load from sewage effluents, nearby dredging, or even oilspills. Long- 

 term shading could reduce seagrass density, increase erosion of bottom sedi- 

 ments, and affect seagrasses in adjoining areas. 



3. Temperature . 



All seagrass species appear to have upper and lower temperature 

 tolerance levels (McMillan, 1978; Thayer, Wolfe, and Williams, 1975). 

 These levels vary with the local area (McMillan, 1979; Phillips, unpub- 

 lished research, 1974-1979). Eelgrass at the northern and southern 

 extremes of distribution on both the Atlantic and Pacific coasts appears 

 to tolerate a much broader temperature range than that in the middle of 

 the range . 



In Alaska, Biebl and McRoy (1971) found that tidepool eelgrass showed 

 increased photosynthesis up to 95° Fahrenheit (35° Celsius) while photo- 

 synthesis in subtidal plants declined above 86° Fahrenheit (30° Celsius) . 

 The tidepool plants were also more cold-resistant. 



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