reached 35° to 40°C (95° to 104°F). 

 Planktonic species are probably less 

 affected by high temperatures than are 

 sessile populations since larvae can 

 readily be imported from unaffected areas. 



Time of exposure is critical in 

 assessing the effect of thermal stress. 

 Many organisms tolerate extreme short-term 

 temperature change, but do not survive 

 chronic exposure to smaller elevation in 

 temperature. For seagrasses that have 

 buried rhizome systems, the poor thermal 

 conductivity of the sediments effectively 

 serves as a buffer against short-term 

 temperature increases. As a result, the 

 seagrasses tend to be more resistant to 

 periodic acute temperature increase than 

 the algae. Continued heating, however, 

 can raise the sediment temperature to 

 levels lethal to plants (Zieman and Wood 

 1975). The animal components of the sea- 

 grass systems show the same ranges of 

 thermal tolerances as the plants. Sessile 

 forms are more affected as they are unable 

 to escape either short-term acute effects 

 or long-tem chronic stresses. 



The main source of man-induced ther- 

 mal stress in tropical estuaries probably 

 has been the use of natural waters in 

 cooling systems of power plants. Damage 

 to the communities involved has been 

 reported at various study sites. In Guam 

 characteristic fish and invertebrates of 

 the reef flat community disappeared when 

 heated effluents were discharged in the 

 area (Jones and Randall 1973). Virnstein 

 (1977) found a decrease in density and 

 diversity of benthic infauna in Tampa Bay 

 in the vicinity of a power plant, where 

 temperatures of 34° to 37°C (93° to 99°F) 

 were recorded. 



The most thorough investigations of 

 thermal pollution in tropical or semitrop- 

 ical environments have centered around the 

 Miami Turkey Point power plant of Florida 

 Power and Light (see review by Zieman and 

 Wood 1975). Zieman and Wood (1975) found 

 that turtle grass productivity decreased 

 as temperatures rose and showed the rela- 

 tionship between the pattern of turtle 

 grass leaf death and the effluent plume, 

 reporting by late September 1968, that 

 14 ha (35 acres) of grass beds had been 

 destroyed. Purkerson (1973) estimated 

 that by the fall of 1968, the barren area 



had increased to 40 ha (99 acres) with a 

 zone of lesser damaae extending to include 

 about 120 ha (297 acres). In 1971 the 

 effluents were further diluted by using 

 greater volumes of ambient- temperature bay 

 waters. The net effect, however, was to 

 expand the zone of thermal stress. One 

 station 1,372 m (4500 ft) off the canal 

 had temperatures of 32.2°C (90°F) only 2% 

 of the time in July 1970, but this in- 

 creased to 78% of the time in July 1971 

 (Purkerson 1973). 



Temperatures of 4°C or more above 

 ambient killed nearly all fauna and flora 

 present (Roessler and Zieman 1969). A 

 rise of 3°C above ambient damaged algae; 

 species numbers and diversity were de- 

 creased. The optimum temperature range 

 for maximal species diversity and numbers 

 of individuals was between 26° and 30°C 

 (79° and 86°F) (Roessler 1971). Tempera- 

 tures between 30° and 34°C (86° and 93°F) 

 excluded 50% of the invertebrates and 

 fishes, and temperatures between 35° and 

 37°C (95° and 99°F) excluded 75%. 



The effects recorded above resulted 

 from operation of two conventional power 

 generators which produced about 12 mVsec 

 of cooling water heated about 5°C (41°F). 

 Using this cooling system, the full plant, 

 which was two conventional and tv/o nuclear 

 generators, would produce 40 m-^/sec of 

 water heated 6° to 8°C above ambient. The 

 plant had begun operations in spring 1967 

 with a single conventional unit, and a 

 year later a second unit was added. Stud- 

 ies at the site began in May 1968 when the 

 area was still relatively undisturbed. 

 Except for a few hectares directly out 

 from the effluent canal, the communities 

 in the vicinity were the same as in adja- 

 cent areas to the north and south. As 

 temperatures increased throughout the sum- 

 mer, however, damage to the benthic com- 

 munity expanded rapidly. 



Because of the anticipated impact of 

 the nuclear powered units, a new 9-km 

 (5.6-mi) canal emptying to the south in 

 Card Sound v;as dredged. Fears that this 

 body of water also would be damaged per- 

 sisted, and as a final solution to the 

 problem a network of 270 km (169 mi) of 

 cooling canals 60 m (197 ft) wide was con- 

 structed. Heated water was discharged 

 into Card Sound until the completion of 



89 



