With the exception of five Diadema antiZlanon at the most distant 

 points of the western transects, no live echinoids were seen in 

 the flats west of the turning basin. Dead Lyteohinus variegatus 

 and Tripneustes ventvicosus tests and fragments were found in the 

 area indicating the recent presence of living specimens there. 



During the 1968 and 1969 surveys, both Lyteohinus variegatus and 

 Tripneustes ventvicosus occurred in the same area with densities 

 closely approximating the recent population to the east of the turn- 

 ing basin (Clarke, unpublished data). Laboratory and in situ bio- 

 assays showed a high sensitivity of echinoids to the copper con- 

 tained in the effluent. The high copper levels in sediments west 

 of the turning basin implicate the flow of effluent over the flats 

 as the cause of the echinoid mortality. 



In November, 1971, an aerial photo transect was made over the turn- 

 ing basin to determine if there had been any changes in the pattern 

 of turtle grassj Thalassia testudinum^ along its borders since a 

 similar photo transect was made in 1968 (Fig. 36). There were no 

 detectable changes. In fact, the stability of the turtle grass was 

 remarkable. Note, for example, the persistent shape and size of 

 sand patches just off the two prominences which form the entrance to 

 the canal (Fig. 36). The width of the barren area between the edge 

 of the turning basin and the Thalassia beds did not change appreciably 

 although in the 1971 survey it was covered with more algal growth. and 

 was thus darker in color. 



PLANKTON TOWS 



Plankton tows were taken along the desalination plant sea wall and 

 along the eastern edge of the turning basin wall at Station 9. To 

 determine effects of effluent on plankton populations, tows at 

 Station 9 were used as references to compare with tows in front of 

 the desalination plant. Shallow tows and deep tows were compared 

 with each other and the number of plankters at the desalination plant 

 expressed as percentages of comparable tows at Station 9. In October, 

 1970, the deep tow at the desalination plant had 33.8 percent the 

 number of diatom cells found in the deep tow at Station 9. The com- 

 parable percentage for the shallow tows was 45.5 percent. Theoretically, 

 the two tows at the desalination plant should have the same percentage 

 differences from the control station tows. Effluent caused a greater 

 reduction of the expected percentage in deep water (i.e. 33.8 percent 

 rather than 45.5 percent). In October, therefore, the deep tow had 

 only 74.3 percent the number of phytoplankters expected. 



When the desalination plant was shut down, deep water was more pro- 

 ductive than shallow water, averaging 132 percent more than 

 the theoretical population. When the plant was operating the deep, 

 effluent-laden, water averaged only 50.57 percent of the expected 

 phytoplankton population. 



100 



