Ocean Colour & Bio-optics – Effect of phytoplankton on spectral response Which statements about phytoplankton and water-leaving radiance in the visible spectrum are correct?

Introduction / Context:
Ocean-colour remote sensing interprets water-leaving radiance to infer chlorophyll concentration and particle content. Phytoplankton pigments and associated particulates modify absorption and scattering, especially in the blue–green region used by sensors like SeaWiFS, MODIS, and OCM.

Given Data / Assumptions:

• Case-1 waters where phytoplankton and covarying particles dominate optical properties.
• Visible bands spanning blue (~440–490 nm) and green (~550–560 nm).
• Qualitative relationships between pigment concentration, absorption, and scattering.

Concept / Approach:
Chlorophyll-a strongly absorbs blue light while particulate matter (including phytoplankton and detritus) contributes to scattering, often enhancing green reflectance relative to blue as concentration rises. Therefore statements about pigment presence, increased green backscattering, and increased blue absorption are correct; the claim that green backscattering decreases is contrary to typical behaviour in Case-1 waters.

Step-by-Step Solution:
Recognize pigment: chlorophyll-a in phytoplankton ⇒ (a) true.With more phytoplankton/particles, backscattering and thus green-region reflectance tend to rise ⇒ (b) true.Chlorophyll absorption peaks in blue, reducing blue reflectance as concentration increases ⇒ (c) true.Statement (d) claims decreased green backscatter with more phytoplankton ⇒ generally false in Case-1 waters.

Verification / Alternative check:
Empirical ocean-colour algorithms (e.g., blue/green band ratios) exploit the decrease in blue reflectance and relatively higher green reflectance with increasing chlorophyll, consistent with (a)–(c).

Why Other Options Are Wrong:

• (d) contradicts observed trends; phytoplankton and associated particles tend to raise scattering, not reduce it, in green bands.

Common Pitfalls:
Confusing absorption (which lowers reflectance) with backscattering (which raises reflectance), or applying Case-2 coastal water logic indiscriminately to open ocean scenarios.

Final Answer:
Only (a), (b), and (c) are correct