Assertion: Oceans are a low productivity ecosystems despite occupying about 70% of the earth surface.
Reason: In aquatic ecosystems, productivity is limited by light which decreases with increasing water depth.

Options

A. a) Both assertion and reason are correct and reason is the correct explanation for Assertion.
B. b) Both assertion and reason are correct but reason is not the correct explanation for assertion.
C. c) Assertion is correct but reason is incorrect.
D. d) Both assertion and reason are incorrect.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that oceans are low productivity ecosystems despite covering about 70% of the Earth's surface. This means that even though oceans are vast, they do not produce a lot of organic matter compared to their size.

2. **Analyzing the Reason**: The reason provided is that in aquatic ecosystems, productivity is limited by light, which decreases as water depth increases. This implies that the deeper you go into the ocean, the less light is available for photosynthesis, which is essential for the growth of primary producers like phytoplankton.

3. **Evaluating Ocean Productivity**: According to the information, the total annual net primary productivity of the biosphere is approximately 170 billion tons of organic matter, with oceans contributing only about 32% of this total, which is 55 billion tons. This indicates that the ocean's productivity is relatively low compared to its size.

4. **Linking Assertion and Reason**: The reason explains why the assertion is true. Since light is a crucial factor for photosynthesis, and its availability decreases with depth, this directly impacts the productivity of the oceans. Therefore, the low productivity of oceans can be attributed to the limited light available for photosynthetic organisms.

5. **Conclusion**: Both the assertion and the reason are correct, and the reason provides a valid explanation for the assertion. Thus, we conclude that the oceans are indeed low productivity ecosystems due to the limitation of light with increasing water depth.

### Final Answer:
Both the assertion and the reason are correct, and the reason is the correct explanation for the assertion.

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