Questions: Ocean Chemistry: Nutrients, Dissolved Gases, and Buffering

Question 1

The ocean's natural pH is approximately 8.1. 'Ocean acidification' means the ocean's pH is doing which of the following?

• A. Dropping toward 7.0 (neutral)
• B. Dropping below 7.0 into truly acidic conditions
• C. Becoming less basic while remaining above pH 7
• D. Rising toward 9 due to increased carbonate

Answer: (C)

Ocean acidification describes a decrease in pH from ~8.1 toward lower values — the ocean is becoming less alkaline, not actually acidic. The term is misleading; the ocean is still basic. The drop from 8.2 to 8.1 since the industrial era represents a ~26% increase in hydrogen ion concentration, which has real biological consequences even without crossing pH 7.

Question 2

Waters with the highest dissolved nutrient concentrations will typically support the greatest marine biological production.

• T. True
• F. False

Answer: False

Biological production depends on multiple co-limiting factors. Light availability, iron, and mixing depth interact with nutrient concentrations. Polar waters can be nutrient-rich but light-limited in winter. Parts of the Southern Ocean and equatorial Pacific are nutrient-rich but iron-limited (high-nutrient, low-chlorophyll zones). Nutrients are necessary but not sufficient for high productivity.

Question 3

Why are dissolved nutrients (nitrate, phosphate) nearly depleted in surface ocean waters but elevated at depth?

Model answer: Phytoplankton in the sunlit surface zone take up nutrients for growth. When organisms die or are grazed, their organic remains sink as particles. Bacteria decompose this material at depth, releasing nutrients back into solution. This biological pump continuously exports nutrients downward, creating a vertical gradient — depleted at the surface, enriched at depth.

The biological pump is the mechanism: photosynthesis consumes nutrients in the photic zone, while bacterial remineralization returns them to dissolved form in deeper, darker water. Upwelling zones (like off Peru or Antarctica) are highly productive precisely because circulation returns this nutrient-rich deep water to the surface.