Assertion: In a marine aquatic ecosystem, the biomass of phytoplanktons (producers) at any given time, is lower than the biomass of zooplanktons (primary consumers).
Reason: Phytoplanktons are consumed almost as rapidly as they are formed and thus have shorter life spans.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that in a marine aquatic ecosystem, the biomass of phytoplankton (producers) is lower than that of zooplankton (primary consumers) at any given time.
- **Explanation**: Phytoplankton are the primary producers in marine ecosystems, and they form the base of the aquatic food web.

2. **Understanding the Reason**: The reason provided is that phytoplankton are consumed almost as rapidly as they are formed, leading to shorter life spans.
- **Explanation**: Phytoplankton reproduce quickly but are also preyed upon by zooplankton, which means their population is kept in check, resulting in lower biomass compared to their consumers.

3. **Evaluating the Assertion**: The assertion is correct because it reflects the dynamics of the marine food web, where producers (phytoplankton) are typically less in biomass than the primary consumers (zooplankton) due to their rapid consumption.

4. **Evaluating the Reason**: The reason is also correct as it explains why phytoplankton have lower biomass. Their rapid consumption by zooplankton leads to shorter life spans and thus a lower overall biomass at any given time.

5. **Conclusion**: Since both the assertion and the reason are correct, and the reason accurately explains the assertion, the correct answer is that both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

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

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