1974 and spring 1975. Survival rates 

 after 6 mo ranged from 0% to 52.5%. 

 Areas of low survival included (1) those 

 with strong tidal currents, (2) those 

 with wave action from boats on the in- 

 tercoastal waterway causing high turbid- 

 ity as well as physical impact to the 

 seedlings, and (3) a submerged dredge 

 island which was eroding with shifting 

 and unconsolidated sediments. Areas most 

 amenable to seedling growth included low 

 energy peaty bottoms or sandy consoli- 

 dated bottoms, especially in areas where 

 a pioneer seagrass species such as Halo- 

 dule or Syrinqodium had already begun to 

 recolonize after the impact (Thorhaug 

 and Hixon 1975). 



ECONOMIC ANALYSIS 



The following discussion is from 

 Thorhaug and Austin (1977). The defined 

 objective of planting seagrass is to 

 achieve a given "cover" that will reduce 

 erosion, siltation, and turbidity, and 

 to improve the habitat for marine orga- 

 nisms. Therefore, the cost analysis is 

 in terms of dollar costs to achieve a 

 given cover for a given size area (bot- 

 tom)in a given time period. The costs 

 of the three propagation phases (collec- 

 tion, nursery, and planting) are related 

 to the number of seeds to be handled. 

 The first objective is to determine the 

 number of seeds required for the pro- 

 ject. Five pieces of information are 

 required to estimate the required number 

 of seeds: 



1. natural mortality rate of the seeds 

 planted 



2. natural growth rate (lateral expan- 

 sion rate) of an individual plant 



3. the desired "cover" to be achieved 



4. time period permitted to achieve 

 the desired cover 



5. size of the area (bottom) to be 

 planted 



The first two variables are deter- 

 mined by environmental conditions at the 

 planting site (e.g., depth, turbidity, 

 temperature, wave energy level, and type 

 of bottom). The third, fourth, and fifth 

 variables are policy decisions. Presum- 

 ably the desired cover would be similar 

 to cover indigenous to the area as de- 

 termined by what existed in another area 



with similar conditions or from knowl- 

 edge about what previously existed at 

 the planting site. The time permitted 

 to achieve the cover is an arbitrary 

 policy decision, but it has a signifi- 

 cant influence on the number of seeds 

 that must be planted. 



The monetary costs of restoration 

 depend on three types of variables. The 

 first set of variables are environmental 

 parameters determining the natural mor- 

 tality and growth rates of the seeds. 

 The second type variables are policy de- 

 cisions relating to the size of the area 

 to be planted and the time period per- 

 mitted to achieve a desired cover of 

 grass. The third type variables relate 

 to the dollar cost of collecting, nurs- 

 ery work, and planting the number of 

 seeds dictated by the first two types of 

 variables. 



DISCUSSION 



Tropical and subtropical estuaries 

 are different from that of northern es- 

 tuaries where one can dump a slug of 

 heavy metals or heated effluent on a 

 phytoplankton-based food chain for a few 

 days. In northern latitudes, if one 

 stops dumping the heavy metals or the 

 heated effluent, the photoplankton will 

 renew itself and within a short time the 

 food chain can be reestablished. In con- 

 trast, there are situations in Biscayne 

 Bay, Florida, where the entire food 

 chain has been completely disrupted by 

 man's activities for decades after the 

 disruption ceased. 



I would also like to point out that 

 the plants involved are the tropical and 

 semitropical grasses. I have previously 

 said that the semitropical-tropical re- 

 gions are more fragile ecosystems than 

 the temperate (Thorhaugh 1976). This 

 fragility is unfortunate because most of 

 the activity in terms of managing estu- 

 aries and nearshore waters has occurred 

 in the temperature zone. The principles 

 gained from northern studies do not al- 

 ways relate to the tropics because the 

 tropics probably are the place where 

 life began. If these tropical organisms, 

 which are geologically very old, were 

 able to migrate out of the tropics, they 

 probably would have; but they are there 

 now because they are less flexible than 

 more northern ones. 



113 



