Ocean colour & phytoplankton — apparent water colour in sunlight Water that is rich in phytoplankton typically appears what colour to the human eye under sunlight, and why?

Introduction / Context:
Remote sensing and ocean optics relate water colour to the substances in the water column, notably phytoplankton, coloured dissolved organic matter (CDOM), and suspended sediments. Chlorophyll-a in phytoplankton strongly absorbs parts of the visible spectrum, which changes the apparent colour seen from above.

Given Data / Assumptions:

• Sun-lit conditions; observation by eye or broad-band sensors.
• Phytoplankton contain chlorophyll-a and accessory pigments.
• No extreme sediment or CDOM dominance masking the signal.

Concept / Approach:

Chlorophyll-a absorbs strongly in the blue (~0.44 μm) and red (~0.67 μm) and reflects/scatters more in the green band. Thus, as phytoplankton concentration increases, water tends to shift from deep blue (oligotrophic) toward green hues (eutrophic), all else equal. Brown hues are more typical of high suspended sediment, while red is unusual except during specific blooms or optical illusions at low sun angles.

Step-by-Step Solution:

Verification / Alternative check:

Standard ocean colour algorithms (e.g., band ratios including green) exploit this spectral behavior to estimate chlorophyll concentration from satellite data.

Why Other Options Are Wrong:

• Blue corresponds to clear, low-chlorophyll waters.
• Brown often indicates sediments or humic substances.
• Red is atypical; certain blooms can redden water but are exceptions.

Common Pitfalls:

• Confusing sediment-driven colour changes with phytoplanktonic signals.
• Ignoring the role of sun angle and sky reflection at glint angles.

Final Answer:

Green.