lists conparativG biomass values from sev- 

 eral stations in Pine Channel in the Flor- 

 ida Keys v/here the three major species co- 

 exist. Shoal grass and manatee grass have 

 less wel 1 -developed root and rhizome sys- 

 tems and consequently will generally have 

 much more of their total biomass in leaves 

 than does turtle grass. Samples for these 

 two species where the leaf component is 

 50% to 60% of total weight are not uncom- 

 mon. Maximum values for the species also 

 vary widely. Biomass measurements for 

 dense stands of shoal grass are typically 

 several hundred grams per square meter; 

 manatee grass reaches maximum development 

 at 1,200 to 1,500 g/m- , while maximum val- 

 ues for turtle grass are over 8,000 g/m . 



3. 



PRODUCriVITY 



Seagrasses have the potential for 

 extremely high primary productivity. Re- 

 corded values for seagrass productivity 

 vary enormously depending on species, den- 

 sity, season, and measurement techniques. 

 Most studies use turtle grass with only a 

 few scattered values for shoal grass and 

 manatee grass. 



For south Florida, turtle grass pro- 

 ductivity values of 0.9 to 15 g C/m^/day 

 have been reported (Table 5). The highest 

 reported values (e.g. Odum 1963) represent 

 community metabolism and reflect the pro- 

 ducts of the seagrasses, epiphytic algae, 

 and benthic algae. Measurements of sea- 

 grass production indicate that the net 

 aboveground production is commonly 1 to 

 4 g C/m /day, although the maximum rates 

 can be several times these values (Zieman 

 and Wetzel 1980). The importance of the 

 high sustained level of production of sea- 

 grasses is especially apparent when com- 

 pared with the production values of the 

 contiguous offshore waters. 



3.3 PRODUCTIVITY MEASUREMENT 



From, the earliest seagrass studies, 

 researchers have continually noted the 

 high productivity of seagrasses, and their 

 ultimate value as food for trophically 

 higher organisms. As a result, much study 

 has been devoted to methods for determin- 

 ing the productivity of seagrass beds. 



Three basic methods have been used to 

 study seagrass productivity: marking, 

 ''■^C, and 0: production. (See Zieman and 

 V'etzel 1980 for a recent review of produc- 

 tivity measurement techniques.) 



Many assumptions dre made when using 

 the oxygen production method, and all can 

 lead to large and variable errors, pri- 

 marily because leaves of aquatic vascular 

 plants can store gases produced during 

 photosynthesis for an indefinite period. 

 The largest potential error, however, is 

 related to the storage of metabol ical ly 

 produced oxygen. To use the oxygen produc- 

 tion technique, one assumes that oxygen 

 produced in photosynthesis diffuses rap- 

 idly into the surrounding water where it 

 can be readily measured. With seagrasses, 

 as with other submerged macrophytes, how- 

 ever, this gas cannot diffuse outward at 

 the rate at which it is produced and so it 

 accumulates in the interstitial lacunae of 

 the leaves (Hartman and Brown 1966). Re- 

 cent work with freshwater macrophytes has 

 suggested that under well-stirred condi- 

 tions only a short period is required for 

 equilibration (V.'estlake 1978; Kelly et al . 

 1980); however, this has not been verified 

 for seagrasses. As the gas accumulates, 

 seagrass leaves swell up to 2507' of their 

 original volume (Zieman 1975b). Some of 

 the oxygen produced is used metabol ically, 

 while the remainder either diffuses out 

 slowly or, if production is sufficient, 

 will burst from the leaves in a stream of 

 bubhles. 



f''easurement of seagrass productivity 

 by radioactive carbon uptake has the ad- 

 vantage of high sensitivity, brief incuba- 

 tion periods, and the ability to partition 

 out the productivity associated with the 

 different morphological parts of the 

 plants as well as productivity of the 

 attendant epiphytes and macroalgae. Al- 

 though this measurement technique requires 

 sophisticated and expensive laboratory and 

 field equipment, and mav have errors asso- 

 ciated with CO storage, it apparently 

 yields a value near to net productivity 

 and produces values comparable to mark and 

 recovery techniques. The application of 

 the I'^C technique to seagrasses is dis- 

 cussed in detail by Penhale (1975), Bit- 

 taker and Iverson (1"76), and Capone 

 et al. (1979). 



22 



