Ocean colour — appearance of waters poor in phytoplankton under sunlight In clear open-ocean conditions with low phytoplankton concentration, what colour does the water typically appear to the human eye?
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Adeep blue
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Bred
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Cgreen
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Dyellow
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ENone of these
Answer
Correct Answer: deep blue
Explanation
Introduction / Context:Ocean colour depends on absorption and scattering by water molecules, phytoplankton pigments (e.g., chlorophyll-a), coloured dissolved organic matter, and suspended particles. Low-chlorophyll waters exhibit characteristic spectral behaviour that translates to perceived colour.
Given Data / Assumptions:
- Sunlit viewing conditions (no extreme glint).
- Low phytoplankton concentration and low suspended sediments.
- Observer perceives bulk water colour away from shallow bottoms.
Concept / Approach:Pure water strongly absorbs in the red and reflects/scatters more in the blue, while chlorophyll increases absorption in blue and enhances green reflectance. Therefore, oligotrophic (phytoplankton-poor) waters appear deep blue; productive waters trend greener.
Step-by-Step Reasoning:
1) Consider inherent optical properties: water molecules preferentially scatter short wavelengths.2) With little chlorophyll, blue light is less absorbed, so blue upwelling radiance dominates.3) Human eye perceives the ocean as deep blue under these conditions.Verification / Alternative check:Satellite products (e.g., chlorophyll maps) show oligotrophic gyres as dark/deep blue in true-colour imagery.
Why Other Options Are Wrong:
- Red/yellow: Would require high suspended sediments or sunset lighting.
- Green: Typical of higher chlorophyll waters.
- None of these: Incorrect since “deep blue” is standard.
Common Pitfalls:Confusing shallow sandy bottoms (turquoise) with true open-ocean colour; ignoring atmospheric and sun–sensor geometry effects.
Final Answer:deep blue