ACT Science: 3-Variable Graph Interpretation (Temperature, Salinity & Dissolved Oxygen)

Exact Question Scenario

A limnology research team investigates how water temperature ($^\circ\text{C}$) and salinity ($\text{ppt}$) influence equilibrium dissolved oxygen ($\text{DO}$, in $\text{mg/L}$).

• X-axis: Water Temperature ($0^\circ\text{C}$ to $30^\circ\text{C}$)
• Y-axis: Dissolved Oxygen ($\text{mg/L}$)
• Series: $0\text{ ppt}$ (Solid line), $15\text{ ppt}$ (Dashed line), $35\text{ ppt}$ (Dotted line)

Question: According to Figure 1, at a constant water temperature of $20^\circ\text{C}$, if the salinity increases from $0\text{ ppt}$ to $35\text{ ppt}$, the dissolved oxygen concentration will:

- A) Increase by approximately $3.2\text{ mg/L}$

- B) Decrease by approximately $1.8\text{ mg/L}$

- C) Decrease by approximately $5.4\text{ mg/L}$

- D) Remain unchanged at $9.1\text{ mg/L}$

Step-by-Step Solution

1. Find $20^\circ\text{C}$ on horizontal axis.
2. Read $0\text{ ppt}$ curve: $\text{DO} \approx 9.1\text{ mg/L}$.
3. Read $35\text{ ppt}$ curve: $\text{DO} \approx 7.3\text{ mg/L}$.
4. Delta: $7.3 - 9.1 = -1.8\text{ mg/L}$.
5. Correct Answer: B