indication of a limitation on productivity 

 due to hydrostatic pressure and not nerely 

 light limitation (Gessner and Haniner 

 1961). 



The naxiinun depth at which seagrasses 

 are found is definitely correlated with 

 the available light reqine, provided that 

 suitable sediments are available. Off the 

 northwest coast of Cuba, Buesa (1975) re- 

 ported maximum depths for tropical sea 

 grasses as follows: turtle grass, 14 n 

 (46 ft); manatee grass, 16.5 m (54 ft); 

 Halo phil ia dec ipiens, 24.3 m (80 ft); and 

 H_. englemanni, 14.4 m (47 ft). As plant 

 species grow deeper, the quality and quan- 

 tity of light changes. In clear tropical 

 water such as that near St. Croix, Cuba, 

 and portions of southern waters, the light 

 is relatively enriched in blue wavelengths 

 with depth. By comparison, in highly tur- 

 bid conditions as in shallow bays in Texas 

 and in Florida Bay, blue light is scat- 

 tered and the enrichment is in the direc- 

 tion of the green wavelengths. In both 

 clear and turbid v;aters the longer red 

 wavelengths are absorbed in the first few 

 meters of the water column. 



Buesa (1975) studied the effects of 

 specific wavelengths on photosynthesis of 

 turtle grass and manatee grass in Cuba. 

 He found that turtle grass responded best 

 to the red portion of the spectrum (620 

 nanometers); the blue portion (400 nanome- 

 ters) was better for manatee grass. 



2.9 ZONATION 



Although seagrasses have been re- 

 corded from as deep as 42 m (138 ft), ex- 

 tensive development of seagrass beds is 

 confined to depths of 10 to 15 m (33 to 49 

 ft) or less. Principal factors determin- 

 ing seagrass distribution are light and 

 pressure at depth, and exposure at the 

 shallow end of the gradient. A general 

 pattern of seagrass distribution in clear 

 waters of south Florida and the Caribbean 

 was presented by Ferguson et al . (1980). 

 Shoal grass usually grows in the shallow- 

 est water and tolerates exposure better 

 than other species. The relatively high 

 flexibility of its leaves allows it to 

 conform to the damp sediment surface dur- 

 ing periods of exposure, thus minimizing 



the leaf surfaces available for desicca- 

 tion. Turtle grass grows in waters nearly 

 as shallow as that of shoal grass. The 

 shallowest turtle grass flats are commonly 

 exposed on spring low tides, frequently 

 with much leaf mortality. Throuqhout the 

 range of 1 to 10 m (3 to 33 ft), all of 

 the species may be found, singly or mixed. 

 Turtle grass is the unquestionable domi- 

 nant in most areas, however, freouently 

 forming extensive meadows that stretch for 

 tens of kilometers. Although the absolute 

 depth limit of the species is deeper, 

 iiature meadows of turtle qrass are not 

 found belo*-,' 10 to 12 m (33 to 39 ft). At 

 this depth manatee grass replaces turtle 

 grass and forms meadows down to 15 m (50 

 ft). Past the maximum depth for manatee 

 grass development, shoal grass will often 

 occur, but it rarely develops extensively. 

 Past the point at which the major species 

 occur, fine carpets of Halophil a extend 

 deeper than 40 m (130 ft). 



Numerous studies confirmed the pat- 

 tern described above, or some portion of 

 it. The relative abundance of four spe- 

 cies of seagrasses off northv/est Cuba, is 

 graphed in Figure 8 (Buesa 1974, 1975). 

 Halophil a decipiens was the least abundant 

 with a mean density of 0.14 q/m-. Halop- 

 hil a engelmanni showed a mean density of 

 0.25 g/m-. F'anatee grass was nearly 10 

 times denser than Halophil a with an aver- 

 age density of 3.5 c/m- down to 16.5 r (54 

 ft). Turtle grass was the most abundant 

 seagrass, accounting for nearly 97.57 of 

 the total seagrass biomass, with an aver- 

 age of 190 g/i-r down to its maximum depth 

 of 14 m (46 ft). This area is unique in 

 that there is little or no shoal grass 

 which normally is either the second or 

 third most abundant species in a region. 



In St. Croix, turtle grass had the 

 shallowest range, occurring down to 12 m 

 (39 ft) on the west side of Buck Island 

 (Wiginton and Mcf'illen 1979). Shoal grass 

 and manatee grass showed progressively 

 greater depth, occurring to IS m (59 ft) 

 and 20 m (65 ft), respectively, while 

 Halophila decipiens occurred to 42 m (138 

 ft) . All the species were found in less 

 than 1 m (3.3 ft) of water in St. Croix. 



Because of the variety of rocky and 

 sedimentary patterns in the lagoons and 



18 



