Question 8·Medium·Command of Evidence
Average Nitrate and Phosphate Concentrations in Seawater after Volcanic Eruption
| Nutrient | Seawater in lava-affected area, 5–45 m below surface | Seawater in lava-affected area, 75–125 m below surface | Seawater outside lava-affected area, 5–45 m below surface | Seawater outside lava-affected area, 75–125 m below surface |
|---|---|---|---|---|
| Nitrate (micromoles per liter) | 3.1 | 0.4 | ≤0.03 | ≤0.01 |
| Phosphate (micromoles per liter) | 0.17 | 0.09 | 0.14 | 0.06 |
After a volcanic eruption spilled lava into North Pacific Ocean waters, a dramatic increase of diatoms (a kind of phytoplankton) near the surface occurred. Scientists assumed the diatoms were thriving on nutrients such as phosphate from the lava, but analysis showed these nutrients weren’t present near the surface in forms diatoms can consume. However, there was an abundance of usable nitrate, a nutrient usually found in much deeper water and almost never found in lava. Microbial oceanographer Sonya Dyhrman and colleagues believe that as the lava plunged nearly 300 meters below the surface it dislodged pockets of this nutrient, releasing it to float upward, given that ______
For command-of-evidence questions with tables, first restate in your own words what specific claim in the passage needs support (here, that lava activity increased usable nitrate). Then scan the table focusing only on the relevant variables (nitrate vs. phosphate, lava-affected vs. outside, specific depths). Eliminate answers that are true but off-topic (wrong nutrient, wrong locations, or incomplete comparisons), and choose the option that directly connects the table’s numbers to the passage’s claim, especially when the sentence uses a phrase like “given that,” which signals you need a fact that makes the scientists’ explanation reasonable.
Hints
Clarify what the blank must do
Reread the sentence with the blank. What belief are the scientists trying to support, and what kind of information (from the table) would make that belief more convincing?
Focus on the right nutrient
Notice which nutrient the scientists say the diatoms are actually using. Is it phosphate or nitrate? Eliminate any choices that focus on the wrong nutrient.
Notice what needs to be compared
To show that lava changed the water, should the evidence compare (a) inside vs. outside the lava-affected area, or (b) just different depths within one area? Look for a choice that makes the right comparison and mentions both depth ranges.
Step-by-step Explanation
Understand what needs to be supported
Focus on the last sentence: "Dyhrman and colleagues believe that as the lava plunged nearly 300 meters below the surface it dislodged pockets of this nutrient, releasing it to float upward, given that ______."
The blank must be filled with evidence from the table that supports the idea that lava activity caused extra usable nitrate to appear in the water.
Identify the key nutrient and comparison
The paragraph says phosphate from the lava was not in a usable form, but there was an abundance of usable nitrate, a nutrient usually found much deeper. So the evidence has to be about nitrate, not phosphate.
Also, to show lava caused the nitrate, we need to compare lava-affected seawater with seawater outside the lava-affected area, not just within one area or just by depth alone.
Read the nitrate rows in the table
Look at the nitrate values:
- Lava-affected area, 5–45 m: 3.1 micromoles per liter
- Lava-affected area, 75–125 m: 0.4 micromoles per liter
- Outside lava-affected area, 5–45 m: ≤0.03 micromoles per liter
- Outside lava-affected area, 75–125 m: ≤0.01 micromoles per liter
In both depth ranges, the lava-affected area has much higher nitrate than the area outside the lava’s influence.
Match the observation to the answer choice
The scientists’ idea is that lava dislodged deep nitrate and brought it into the water column. The strongest evidence is that, at both depth ranges, nitrate concentrations are substantially higher in lava-affected seawater than in seawater outside the lava-affected area—exactly what choice B) for both depth ranges measured, the average concentrations of nitrate were substantially higher in the seawater in the lava-affected area than in the seawater outside of the lava-affected area. says.