Only biotic factors affect the magnitude of primary productivity.

Explanation

**Step-by-Step Solution:**

1. **Understanding Primary Productivity**:
- Primary productivity refers to the rate at which producers (like plants) convert solar energy into chemical energy through the process of photosynthesis. This process involves the use of sunlight, carbon dioxide, and chlorophyll.

2. **Identifying Factors Influencing Primary Productivity**:
- There are two main types of factors that affect primary productivity: biotic factors and abiotic factors.
- **Biotic Factors**: These include all living components of the ecosystem, such as plants, animals, and microorganisms. They can influence productivity through interactions like competition, predation, and symbiosis.
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