In order for snails to return to Richard Creek, the pH of the creek must exceed (A)

Question:

In order for snails to return to Richard Creek, the pH of the creek must exceed
(A) 5.4
(B) 5.7
(C) 6.0
(D) 6.1

+20% +10%+ 0% -10%- -20%+ 1985 1990 Snails Crayfish Brook trout + 1995 2000

Over the period of 1980 to 2000, the average pH in Richard Creek changed drastically. An ecological survey was performed to evaluate the effect of detectable decreases in pH on the aquatic life of the creek. Four times a year, ecological surveys were performed to identify the number of snails (a primary consumer), crayfish (a secondary consumer), and brook trout (a tertiary consumer) present at five different locations. The percent change relative to 1980 is shown in Figure 1 above. Many aquatic organisms cannot live in low pH conditions. The minimum pH necessary for common aquatic organisms to sustain life is shown in Table 1.

Table 1. Minimum pH Tolerance of Common Aquatic Organisms Animal Brook trout American bullfrog Yellow perch