Currently the main limitations of the 

 carbon isotope method are equipment and 

 interpretation. It requires use of a mass 

 spectrometer which is extremely costly, 

 although today a number of labs will pro- 

 cess samples for a reasonable fee. The 

 interpretation can become difficult when 

 an organism has a 5^^C value in the middle 

 ranges. If the f/^'C value is at one ex- 

 treme or another, then interpretation is 

 straightforward. However, a mid-range 

 value can mean that the animal is feeding 

 on a source that has this6^^C value or 

 that it is using a mixed food source which 

 averages to this value. Recent studies 

 utilizing both isotopes of carbon and sul- 

 fur (Fry and Parker 1982) and nitrogen 

 (Macko 1981) show much promise in deter- 

 mining the origin of detrital material as 

 well as the organic matter of higher 

 organisms. Knowledge of the feeding ecol- 

 ogy and natural history of the organism is 

 needed, as is an alternate indicator. 



3.6 PLANT CONSTITUENTS 



Recognition of the high productivity 

 of seagrasses and the relatively low level 

 of direct grazing has led to questions 

 regarding their value as food sources. 

 Proximate analyses of seagrasses in south 

 Florida, particularly turtle grass, have 

 been performed by many authors (Burkholder 

 et al. 1959; Pauersfeld et al . 1969; Walsh 

 and Grow 1972; Lowe and Lawrence 1976; 

 Vicente et al . 1978; Bjorndal 1980; Dawes 

 and Lawrence 1980); their results are 

 summarized in Table 7. As noted by Dawes 

 and Lawrence (1980), differences in the 

 preparation and analysis of samples, as 

 v/ell as low numbers of samples used in 

 some studies, make data comparison dif- 

 Mcul t. 



The reported ash content of turtle 

 grass leaves ranges from 45" dry weight 

 for unwashed samples down to around 25? 

 for samples washed with fresh water. 

 Leaves washed in seawater contained 29'' 

 +/- 3.6" to 44% +/- 6.7?o ash (Dawes and 

 Lawrence 1980). 



Values for the protein content of 

 leaves vary from a low of 37 of dry weight 

 for unwashed turtle grass leaves with 



epiphytes (Dawes et al . 1979) to 29.7% for 

 leaves washed in distilled water (Walsh 

 and Grow 1972), although numbers typically 

 fall in the range of 10% to 15% of dry 

 weight. Protein values may be suspect if 

 not measured directly, but calculated by 

 extrapolating from percent nitrogen. In 

 grass beds north of Tampa Pay, Dawes and 

 Lawrence (1980) found that protein levels 

 of turtle grass and manatee grass leaves 

 varied seasonally, ranging from 8% to 11% 

 and 8% to 13%, respectively, with the 

 higher levels occurring in the summer and 

 fall. The protein content of shoal grass 

 ranged from a low of 14% in the fall up to 

 19% in the winter and summer. Tropical 

 seagrasses, particularly turtle grass, 

 have been compared to other plants as 

 sources of nutrition. The protein content 

 of turtle grass leaves roughly equaled 

 that of phytoplankton and Bermuda grass 

 (Burkholder et al . 1959) and was two to 

 three times higher than 10 species of 

 tropical foraae grasses (Vicente et al . 

 1078). Walsh and Grow (1972) compared 

 turtle grass to grain crops, citing stud- 

 ies in v/hich 114 varieties of corn con- 

 tained 9.8% to 16% protein; grain sorghum 

 contained between 8.6% and 16.5%; and 

 wheat was lowest at 8.3% to 12%. Although 

 several studies have included measurements 

 of carbohydrates (Table 7), it is imprac- 

 tical to compare much of the data because 

 various analytical methods were employed. 



Studies using neutral detergent fiber 

 (NDF) analyses found that cell wall carbo- 

 hydrates (cellulose, hemicel lulose, and 

 lignin)"made up about 45% to 60% of the 

 total dry weight of turtle grass leaves 

 (Vicente et al . 1978; Bjorndal 1980). 

 Dawes and Lawrence (1980) reported that 

 insoluble carbohydrate content in the 

 leaves of turtle grass, manatee grass, and 

 shoal grass was 34% to 46%. The rhizomes 

 of seagrasses &rG generally higher in 

 carbohydrates than ^.tq the leaves. Dawes 

 and Lawrence (1980) found that soluble 

 carbohydrates in turtle grass and manatee 

 grass rhizomes varied seasonally, indicat- 

 ing the production and storage of starch 

 in summer and fall. These authors, how- 

 ever, were working in an ^rab. north of 

 Tampa Bay, where such seasonal changes 

 would be more pronounced than in the 

 southern part of Florida and the Keys. 



29 



