mortality occurred only at “1.0” ppm. Pronounced seasonal variation in 
toxicity was evident (Figure 11-1). 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 preliminary 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. 
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 measurements) were: 1.25 ppm ±0.34 (25), winter; 0.94 ppm ±0.44 
(31) summer. 
139 
