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?

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:

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