of seagrassos vvas bicarbonate ion, which 

 could contribute to the calcium carhonate 

 flock frequently observed on seagrass 

 leaves (Zieran and Wetzel 1980). At normal 

 seawater pH, CO. concentrations were so 

 low that the high photosynthetic potential 

 uds linited by bicarbonate uptake (Beer 

 and Waisel 1979). Increasing the [iropor- 

 tion of CO; by lowering pH greatly in- 

 creased photosynthetic rates in C ymodocea 

 nodo sa, a large seagrass with high poten- 

 tial production. 



Much recent controversy has concerned 

 whether the nietabolic pathway of seagrass 

 photosyntliesis utilizes the conventional 

 Calvin cycle (called C3 as the initial 

 fixed sugars are 3 carbon chains) or the 

 C,, B-carhoxylative pathway. C^ plants 

 refix CO', efficiently and little respired 

 CO is lost in the light (Hough 1974; 

 Moffler et al . 1981). C^ plants are dif- 

 ficult to saturate with light and have 

 high temperature optimums. This photosyn- 

 thetic system vjould seem to be of benefit 

 in regions of high temperature and lioht 

 intensities, as well as marine waters 

 (Hatch et al . 1971). Seagrasses, hovjever, 

 are exposed to lower relative tempera- 

 tures, light levels, and oxygen concentra- 

 tions than are terrestrial counterparts; 

 and as the diffusion capacity of CO2 from 

 leaves is much slower, metabolic CO is 

 available for refixation regardless of the 

 photosynthetic pathway. After much lit- 

 erary controversy, recent evidence has 

 shown that most seagrasses, including tur- 

 tle grass, manatee grass, and shoal grass 

 are C3 plants (Andrews and Abel i979; 

 Benedict et al . 1980). 



What makes the photosynthetic pathway 

 0^ interest to those other than the plant 

 physiologist is that during photosynthesis 

 plants do not use the ^*'C and "' ^'C isotopes 

 in the ratios found in nature, but tend to 

 differentiate in favor of the ^^C isotope 

 which is lighter and more mobile. All 

 plants and photosynthetic cycles are not 

 alike, hov;ever, and those using the con- 

 ventional C. Calvin cycle are relatively 

 poor in the ^ "C isotope, while C^ plants 

 have high ratios of I'C/^'^C. The ratios 

 of i3c/i C (called 6 1'C or del i-C) gener- 

 ally varies between -24 to -36 ppt for C4 

 plants (Bender 1971). Seagrasses have rel- 

 atively high^^^C values. McMillan et al . 

 (1980) surveyed 47 species of seagrasses 



fro;i 1? genera and found that 45 species 

 were within the range of -3 to -19 ppt, 

 with only two species of Hal ophila being 

 lower. The mean values and range for the 

 local species are shown in Table 6. Turtle 

 grass shows a mean value of -10.4 ppt and 

 a total range from -8.3 to -12.5. This 

 va'^iation included samples from Florida, 

 Texas, the Virgin Islands, and Mexico. 

 The inean values and ranges for shoal grass 

 and Halophila from the Gulf of ^'exico and 

 Caribbean are also very similar with mean 

 values ranging from -10.2 to -12.6 ppt, 

 respectively. Manatee grass is the only 

 local seagrass of significantly different 

 value with a more dilated mean of -5 ppt 

 and a range of -3.0 to -9.5 ppt. In 

 general, tropical species had higher f^^^ 

 values than species from temperate re- 

 gions. There also appears to be little 

 seasonal difference in ,-,-^'C values, at 

 least for Zostcra I'arina (Thayer et al . 

 1978a). 



The.s^^C ratio has attracted much at- 

 tention recently because of its utility as 

 a natural food chain tracer (Fry and Park- 

 er 1979). The seagrasses possess a unioue 

 iS^t ratio for marine plants, and thus or- 

 ganisms that consume significant portions 

 of seagrass in their diet will reflect 

 this reduced ratio. The carbon in animals 

 has been shown to be generally isotopical- 

 ly similar to the carbon in their diet to 

 within +/-2 ppt (DeNiro and Epstein 1978; 

 Fry et al . I078). Careful utilization of 

 this method can distinguish between carbon 

 originating froin seagrasses (-3 to -15 

 ppt), marine algae (12 to -20 ppt), par- 

 ticulate organic carbon and phytoplankton 

 (-18 to -25 ppt), and manqrove (-24 to 

 -27) (Fry and Parker 1979). In Texas, or- 

 ganic matter from sediments of bays that 

 have seagrasses display a significantly 

 reduced 6^ "C ratio v,'hen compared with adja- 

 cent bays lacking seagrass meadows (Fry 

 et al . 1977). The same trends were re- 

 ported for the animals collected from 

 these bays (Fry 19S1). The {>'C value for 

 one species of worm, Diopatra cuprea , 

 shifted from an average of -13.3 to -IS. 4 

 ppt between seagrass- and phytoplankton- 

 dominated systems (Fry and Parker 1979). 

 The average values for fish and shrimp 

 show a similar trend in that the 6^^C 

 ratios are reduced in organisms from the 

 seagrass meadows. 



27 



