Net production ineasurernents for most 

 seaqrasses can be obtained by marking 

 blades and measuring their grov/th over 

 time (Zieman 1974, 1975b). With this 

 method, the blades in a quadrat are marked 

 at their base, allowed to grow for several 

 weeks, and then harvested. As seagrass 

 leaves have basal growth, the increment 

 added below the marking plus the newly 

 emergent leaves represent the net above- 

 ground production. After collection, the 

 leaves of most tropical species must be 

 gently acidified to remove adhered carbon- 

 ates before drying and weighing. 



Bittaker and Iverson (1976) critical- 

 ly compared the marking method with the 

 measurement of productivity by radioactive 

 carbon uptake. When the ^"^C method was 

 corrected for inorganic losses (13°'), 

 incubation chamber light energy absorption 

 (14?.), and difference in light energy re- 

 sulting from experimental design {8%), the 

 differences in productivity wore insignif- 

 icant. These results reinforce the concept 

 that the i"C method measures a rate near 

 net productivity. In a study of turtle 

 grass productivity near Bimini, however, 

 Capone et al . (1979) found that the ^''C 

 measurements yielded values nearly double 

 that of the marking methods. 



A method developed by Patriquin 

 (1973) uses statistical estimates based on 

 the length and width of the longest 5% 

 of the leaf population of a given area. 

 Capone et al . (1979) used this method; it 

 agreed +/-15': with the staple marking 

 method. Indications arc that this method 

 is very useful for a first order estimate, 

 but more comparative studies are still 

 needed. 



Some form of oxygen measurement v/as 

 used to attain the highest production 

 values recorded in the literature for tur- 

 tle grass and Zostera . Recently Kemp 

 et al . (1981) surveyed numerous productiv- 

 ity measurements from the literature and 

 confirmed that for seagrasses and several 

 freshwater nacrophytes, the oxygon method 

 shov/ed highest productivity values; mark- 

 ing methods, the lowest; and I'+C values 

 Mere intermediate. Although those compar- 

 isons required numerous assumptions, the 

 results show the need for further study. 

 The marking method probably gives the 

 least ambiguous answers, showing net 



aboveground production quite accurately. 

 It underestimates net productivity as it 

 does not account for belowqround produc- 

 tion, excreted carbon, or herbivory. Mod- 

 ifications of the marking method for 

 Zostera marina have been used to estimate 

 root and rhizome production (Sand-Jensen 

 1975; Jacobs 1979; Kenworthy 1981) and 

 could be adopted for tropical seagrasses. 

 The generalization that emerges from these 

 various diverse studies is that seagrass 

 systems are highly productive, no matter 

 what method is used for measurement, and 

 under optimum growth conditions production 

 can be enormous. 



3.4 NUTRIENT SUPPLY 



Seagrasses along with the rhizophytic 

 green algae are unique in the marine envi- 

 ronment because they inhabit both the wa- 

 ter column and the sediments. There was 

 previously much controversy whether the 

 seagrasses took up nutrients through their 

 roots or their leaves. McRoy and Barsdate 

 (1970) showed that Zostera was capable of 

 absorbing nutrients either with the leaves 

 or roots. McRoy and Barsdate found that 

 Zostera could take up ammonia and phos- 

 phate from the sediments through their 

 roots, translocate the nutrients, and pump 

 them out the leaves into the surrounding 

 water. This process could profoundly 

 affect the productivity of nutrient-poor 

 waters. 



Sediment depth directly affects sea- 

 grass development (Figure 7). The implica- 

 tion is that the deeper sediment is re- 

 quired to allow sufficient root develop- 

 ment which would in turn increase the 

 nutrient absorptive capabilities of the 

 roots. Thus to sustain growth, the plants 

 would need greater nutrient absorptive 

 tissue in sediments that contained less 

 nutrients. While studying turtle grass 

 in Puerto Rico, Burkholder et al . (1959) 

 found a change in the leaf to root and 

 rhizome ratios of the plants as the sed- 

 iment type changed. The ratio of leaf 

 to root and rhizome of turtle grass was 

 1:3 in fine mud, 1:5 in mud, and 1:7 in 

 coarse sand. Kenworthy (l^Rl) noted a 

 similar change in Zost era in North Caro- 

 lina. The plants from sandy areas had 

 over twice the root tissue per unit leaf 

 tissue, possibly indicating the need for 



25 



