CHAPTER 3 



PRODUCTION ECOLOGY 



The densities of seagrasses can vary 

 widely; under optinum conditions they form 

 vast meadows. The literature is becoming 

 extensive and often bewildering as density 

 values have been reported in many forms. 

 For consistency, the terms used here con- 

 form to those of Zieman and Wetzel 

 (1980): standing crop refers to above- 

 ground (above-sediment) material, whereas 

 biomass refers to the weight of all living 

 plant material, including roots and rhi- 

 zomes. Both quantities should be expressed 

 in terms of mass per unit area. These 

 measurements both have valid uses, but it 

 is sometimes difficult to determine which 

 an author is referring to, because of in- 

 complete or imprecise descriptions. His- 

 torically, standing crop has been the pri- 

 mary measure of comparison because of the 

 relative ease of sampling compared with 

 the laborious methods needed to collect 

 and then sort belowground material. 



3.1 BIOMASS 



Seagrass biomass varies widely de- 

 pending on the species involved and the 

 local conditions. The biomass of the spe- 

 cies Halophila is always small, whereas 

 turtle grass Ras been recorded at densi- 

 ties exceeding 8 kg dry weight/m (Bauers- 

 feld et al . 1969). Representative ranges 

 of seagrass biomass in south Florida and 

 in neighboring regions are given for com- 

 parison in Table 3. Because of the ex- 

 treme variations found in nature and re- 

 flected in the literature, one must be 

 careful not to place too much value on a 

 few measurements. Many of these studies 

 have been summarized by McRoy and McMillan 



(1977) and Zieman and Wetzel (1980). Be- 

 cause these studies represent a variety of 

 habitats, different sampling times and 

 seasons, wide variation in sample repli- 

 cates (if any), as well as the diverse 

 reasons for which the investigators col- 

 lected the samples, it becomes difficult 

 to draw meaningful patterns from these 

 published results. 



While the standing crop of leaves is 

 significant, the majority of the biomass 

 of seagrasses is in the sediments, especi- 

 ally for the larger species. Although the 

 relative amounts vary, turtle grass typi- 

 cally has about 15% to 22% of its biomass 

 in emergent leaves and the rest below the 

 sediment surface as roots and rhizomes. 

 The published ranges for turtle grass, 

 however, vary from 10% to 45/' for leaf 

 biomass (Zieman 1975h). In central Bis- 

 cayne Bay, Jones (1968) found a relatively 

 consistent ratio of 3:2:2 for leaves and 

 short shoots; rhizomes: roots. Studies 

 with turtle grass and Zostera have indi- 

 cated that the ratio of leaves to roots 

 increased with a shift in substrate from 

 course sand substrates to fine muds (Ken- 

 worthy 1981). This can be interpreted to 

 indicate either the positive effect of the 

 richer fine muds on more robust plant de- 

 velopment, or the need for a better devel- 

 oped nutrient absorptive (root) network in 

 the coarser sediments that tend to be low- 

 er in nutrients and organic matter. Thus, 

 substrate may be an important variable 

 when determining phonological indices. 



Structurally, turtle grass has the 

 most wel 1 -developed root and rhizome sys- 

 tem of all the local seaorasses. Table 4 



20 



