Question 17 from UPSC CSE 2025 - Environme nt & Ecology

Question

With reference to the planet Earth, consider the following statements:
I. Rain forests produce more oxygen than that produced by oceans.
II. Marine phytoplankton and photosynthetic bacteria produce about 50% of the world's oxygen.
III. Well-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.
Which of the statements given above is/are correct?

IASCODEX · Accepted Answer

Correct Option: B. II only

Strategy: Focus on comparative and quantitative cues. Statement I claims rainforests produce more oxygen than oceans—contradicted by well-known scientific consensus that oceans, via marine phytoplankton, contribute about half or more of Earth's oxygen. Statement II directly matches established data: marine phytoplankton and photosynthetic bacteria contribute ~50% of global O2. Statement III's dramatic 'several folds higher' oxygen in surface water compared to air can be checked with rough scientific values, clearly showing air contains vastly more oxygen than even well-oxygenated water. Retain only Statement II as correct.

Key Info: Rainforests are important for oxygen production and carbon sequestration, but they contribute a smaller percentage to global oxygen when compared to oceans. Oceans cover over 70% of Earth's surface and are home to vast amounts of phytoplankton, microscopic organisms capable of photosynthesis. These include algae and cyanobacteria (notably Prochlorococcus), which are estimated to supply approximately 50% (or slightly more) of the world's oxygen output, making marine sources collectively the most significant. Rainforests contribute less than 30% to global oxygen turnover.

Phytoplankton are the primary producers in the ocean's food web and play a crucial role in the carbon cycle, converting carbon dioxide to organic compounds and releasing oxygen. The oxygen produced is both used in aquatic respiration and released into the atmosphere.

Dissolved oxygen in water is typically measured in milligrams per liter (mg/L), with well-oxygenated surface waters averaging around 6-8 mg O2/L, depending on temperature and salinity. By comparison, the atmosphere contains approximately 210,000 mg O2/L (about 21% of air by volume), making atmospheric air much richer in oxygen content than any natural body of water. The relatively low solubility of gases in water ensures that even saturated surface water contains only a small fraction of oxygen compared to air. Oxygen levels in water decrease with depth and are affected by mixing, temperature, biological consumption, and photosynthetic activity. The oxygen minimum layer in the ocean typically occurs at intermediate depths, far below the surface.

IASCODEX · Answer

Option A: I and II

IASCODEX · Answer

Option C: I and III

IASCODEX · Answer

Option D: None of the above statements is correct