Algal blooms impart a distinct colour to water due to

Options

A. their pigments
B. excretion of coioured substances
C. formation of coloured chemicals in wa- ter facilitated by physiological degrada- tion of algae
D. absorption of light by algal cell wall

Answer

### Step-by-Step Solution:

1. **Understanding Algal Blooms**: Algal blooms are rapid increases in the population of algae in water bodies, often due to favorable environmental conditions such as nutrient enrichment, warm temperatures, and sunlight.

2. **Identifying the Organisms**: The primary organisms involved in algal blooms are phytoplankton, which are microscopic plants that float in water. They are photosynthetic and can grow rapidly under suitable conditions.

3. **Role of Pigments**: Phytoplankton contain pigments that are crucial for photosynthesis. The main pigment is chlorophyll, which is responsible for the green color of many algae.

4. **Accessory Pigments**: In addition to chlorophyll, phytoplankton also have accessory pigments (like carotenoids) that allow them to absorb different wavelengths of light. This ability to absorb various wavelengths can lead to different colors in the water.

5. **Color Imparted to Water**: The dense concentration of phytoplankton during a bloom can change the color of the water. Depending on the type of phytoplankton, the water can appear green, red, or brown.

6. **Evaluating Options**:
- **Option 1: Their pigments** - This is correct, as the pigments in phytoplankton are responsible for the distinct colors observed during algal blooms.
- **Option 2: Excretion of colored substances** - This is incorrect; while some algae may release substances, it is not the primary reason for the color change.
- **Option 3: Formation of colored chemicals** - This is also incorrect; the color change is not due to the formation of chemicals through degradation.
- **Option 4: Absorption of light by algal cell wall** - This is incorrect; the color change is primarily due to pigments, not the cell wall.

7. **Conclusion**: The correct answer is that algal blooms impart a distinct color to water due to their pigments.