competed for space on the slides but also preyed upon the diatoms 

 and protozoans, accounting for some of the lower species diversity 

 values and numbers of individuals at Station 3C compared to the 

 control station. 



IN SITU BIOASSAYS 



Echinoids showed greatest sensitivity to the effluent and died 

 rapidly at Station 3 (the closest biological station to the discharge). 

 Table IX shows the average number of days echinoids survived at the 

 three test stations from September, 1970 to June, 1971. The concen- 

 tration of effluent decreased from an average of 3.8 percent in Dec- 

 ember, 1970 to 2.1 percent In February, 1971. Even so, echinoids 

 placed at Station 3 in February died in only three days. In June, 

 1971, removal of the badly corroded copper-nickel trays from the 

 desalination plant reduced the copper discharge. Following the lower- 

 ing of copper content in the effluent, echinoid survival increased 

 markedly (Table IX) . 



In Figure 50, the number of days survival of echinoids are plotted 

 against maximum concentration of effluent during the total period of 

 exposure. Levels of only 1.5 percent effluent were apparently toxic 

 to the echinoids. Gorgonians survived brief exposure to four or 

 five percent effluent and stone crabs tolerated six to seven percent 

 peaks of effluent concentration. Numerous mortalities are indicated 

 on Figure 50 during periods of supposedly low effluent concentrations. 

 As the study progressed, more and more 'unexplained' deaths occurred. 

 It became evident that the transient peaks of contaminants were criti- 

 cal and that the sampling technique used for the effluent was not ade- 

 quate to register these peaks. Average percentage of effluent or 

 effluent exposure indices showed little significant correlation (P>.20). 



Continuous monitoring of the effluent during cleaning operations 

 showed copper and nickel levels increased markedly in the effluent 

 for about twenty-four hours after the plant resumed operation (see 

 discussion in Section VI Copper and Nickel). Because of the low 

 salinity of the effluent when the plant first began operation, the 

 discharge readily mixed with the ambient water and did not stratify. 

 As a result, shallow water stations, as well as deep water stations, 

 received high concentrations of copper every time the plant began 

 operating again after a maintenance period. 



High copper levels were frequently associated with mortalities of the 

 echinoids. On January 11th, 1971, for example, the copper concentra- 

 tion at Stations 7A and 7B increased 100 percent over the December, 

 1970 average. The following day one echinoid at 7A and two at 7B were 

 dead. All of the echinoids at Stations 3A and 3C died the same day. 



119 



