Water Chemistry 



Dilution water for this experiment was well water 

 with these properties: lead, 0.05 mg/liter; zinc, 0.07 

 mg/liter; copper, 0.08 nig/liter; hardness, 247-263 mg/ 

 liter as CaCO.,; total alkalinity, 151-161 mg/liter as 

 CaCOa. Heavy metal concentrations in the well water 

 were measured with an atomic absorption unit once dur- 

 ing the experiment. Hardness and total alkalinity were 

 measured by standard methods (Taras et al. 1971). The 

 temperature, pH, and dissolved oxygen of the test 

 chambers were measured with meters daily during the 

 experiment (Table 1). 



Table 1. — Test conditions during the potassium bioassay 

 with .Asiatic clams. (Means arc given in the table, with ranges 

 in parentheses; a single composite sample was analyzed for 

 potassium; therefore, no range is presented.) 



Test Potassium 

 Chamber (mg/liter) 



Dissolved „ 

 u ^ lemperature 



pH Oxygen ,^:,.:..., 



, ■ ;,? > (Celsius) 

 (mg/hter) ^ 



9&10 458 7.91 8.8 16.9 



(7.85-7.95) (8.5-9.1) (16.5-17.1) 



1 & 2 246 7.89 8.7 16.9 



(7.85-7.90) (8.3-9.0) (16.5-17.1 j 



.3&4 149 7.89 8.8 16.9 



(7.85-7.90) (8.3-9.1) (16.5-17.1) 



5&6 98 7.88 8.7 16.9 



(7.85-7.90) (8.3-9.0) (16.5-17.1) 



7 & 8 44 7.88 8.8 16.9 



(7.85-7.90) (8.5-9.2) (16.5-17.1) 



11&12 7.25" 7.80 8.6 16.9 



(7.75-7.85) (8.3-9.0) (16.5-17.1) 



» Dilution water with no added potassium. Dilution ivater had no 

 noticeable effects on clams during the acclimation and test periods. 



A concentrated stock solution of reagent-grade po- 

 tassium chloride was prepared in distilled deionized 

 water to supply the diluter. Fresh stock solution was pre- 

 pared daily during the test. The diluter was adjusted to 

 provide concentrations of 458, 246, 149. 98, and 44 mg 

 of potassium per liter of water in the test chambers and 

 to provide dilution water containing no added potassium 

 to two chambers which served as controls. A composite 

 water sample of each toxicant level, consisting of aliquots 

 removed daily, was acidified with concentrated hydro- 

 chloric acid to a pH of 1 and subsequently was anal^Tied 

 at the Illinois Natural History Sur\ey chemical labora- 

 tories by flame-emission spectrophotometiy. The dilution 

 water was similarly sampled and analyzed, and it con- 

 tained 7.52 mg of pota.ssium per liter of water (Table 1 ) . 



Response Criteria 



111 this study we investigated 10 responses, of which 

 three ])i<)\ided readily determined, reliable endpoints. 

 One response is referred to as foot immobilization. This 

 is a condition in which the clam's foot is extended but 

 does not respond to gentle prodding. The shell either 

 closed tightly on the foot or gaped slightly. The second 

 response was gaping. Obscnations of these two sublethal 



responses were made after 5, 12, 24, 36, 48. 72, and 96 

 hours of exposure to potassium. These sublethal responses 

 are considered significant for reasons described in the 

 discussion. 



The third response was death. A preHminary investi- 

 gation was carried out to determine the criteria for 

 death. A lack of response to prodding of the extended 

 foot, a lack of response to prodding of the body when the 

 shell gaped slightly, and a lack of a closing response when 

 closed shells were gently twisted open with a knife blade 

 were all shown to be insufficient criteria for death. It was 

 found that some clams not responding to these stimuli 

 recovered after being placed in clean water. To deter- 

 mine preci-sely which clams were dead, a method sug- 

 gested by Sinclair (1963:9) was used. Four clam cages, 

 each containing 10 clams, were used in each test cham- 

 ber. One clam cage was remo\ed from each test chamber 

 after 24, 48, 72, and 96 hours of pota.ssium exposure. 

 After removal, the clams in each cage were rinsed with 

 dilution water, and the cage was put back into the accli- 

 mating tank. They were inspected 1 week later at which 

 time the dead clams could easily be distinguished by 

 their widely gaping shells and decaying body tissue. We 

 also periodically obsened the clams removed after 96 

 hours for signs of recovers- from foot immobilization. 



Response and Toxicity Curves 



-Mthough the basic method of Sprague (1973:8-23) 

 is usually used to determine lethal thresholds, it was 

 equally valuable in determining response thresholds. 

 Instead of determining LC50"s (the concentrations lethal 

 to 50 percent of the test organisms) at each observation 

 time. EC50's (the concentrations producing an effect on 

 50 percent of the test organisms) were determined by 

 observing the percentage of clams exhibiting the sub- 

 lethal responses. The EC50's were plotted on graphs with 

 the time required for 50 percent of the clams to exhibit 

 foot immobilization or gaping on the vertical axis and the 

 concentration of pota.ssium on the horizontal axis. The 

 resulting curves were the foot-immobilization response 

 curve (Fig. 1) and the gaping response curve (Fig. 2). 



Mortality in the 96-hour exposure group was used to 

 determine a 96-hoin' LC50 for potassium. .\t shorter 

 ex]5osure times the data were insufficient for such an 

 analysis (i.e., the mortality rate was too low). The maxi- 

 mum likelihood estimation of Finney (1971:50-66) was 

 used to fit the line to the points. The 95-percent con- 

 fidence limits ;uid slope function of the line were deter- 

 mined by methods given bv Litchfield & Wilcoxon 

 (1949:10'l-106\ 



RESULTS AND DISCUSSION 



Table 1 gives the test conditions and potassium con- 

 centrations throughout the course of the bioassay. Fig. 1 

 indicates that pota.ssium acts rather quickly to produce 

 ftxit inmiobili/ation. with the vertical a.symptote being 

 reached within 48 hours. The res])onse threshold is the 

 ]ioint on the graph where the vertical asymptote inter- 



