mortality occurred only at "1.0" ppm. Pronounced seasonal variation in 

 toxicity was evident (Figure 1 1-1). In one winter exposure, mortality at "1.0" 

 ppm reached 50 percent within approximately 30 days, with the remaining 

 snails dying during the subsequent 30 days. Similar results had been obtained 

 in a preHminary experiment initiated the preceding February. A very different 

 mortality profile was seen at "1.0" ppm in two summer exposures. In 1976, 

 approximately 40 percent of the snails were still living at the end of three 

 months (Figure 11-1). In 1977, presence or absence of sediment in the holding 

 containers was added as another variable. While summer toxicity of the WAF 

 was still relatively low, mortalities were substantially increased by the presence 

 of mud (Figure 11-2). In contrast, the mortality pattern was unaffected by 

 sediment in the winter. 



100 



a: 



o 



36 

 DAYS 



48 60 72 



OF EXPOSURE 



84 



96 



Figure 11-1. Survival of adult N. obsoletus exposed to 

 "1.0" ppm No. 2 fuel oil (WAF). 



NOTE: Control mortalities were less than five percent. Winter experiment was 

 run 10/22/76 - 12/15/76 (100 snails/treatment). Summer experiment was run 

 7/6/76 - 10/12/76 (20 snails/treatment). Mean oil hydrocarbon concentrations 

 ± s.d. (N) measured were: 1.25 ppm ±0.34 (25), winter; 0.94 ppm ±0.44 

 (31) summer. 



139 



