In which of the following ecosystem, productivity is about 55 million tonnes ?

Options

A. Forest
B. Grassland
C. Agroecosytem
D. Oceans

Answer

To determine which ecosystem has a productivity of about 55 million tonnes, we need to analyze the options provided: forests, grasslands, agro-ecosystems, and oceans.

### Step-by-Step Solution:

1. **Understand Primary Productivity**:
- Primary productivity refers to the amount of biomass produced by plants through photosynthesis in a given area over a specific time period. It is usually measured in terms of dry weight.

2. **Consider the Annual Net Primary Productivity of the Biosphere**:
- The total annual net primary productivity of the biosphere is approximately 170 billion tonnes. This figure provides a context for understanding the productivity levels of different ecosystems.

3. **Analyze Each Ecosystem**:
- **Forests**: Generally have high productivity due to optimal conditions for photosynthesis (adequate sunlight, water, and nutrients). Therefore, productivity in forests is likely to be much higher than 55 million tonnes.
- **Grasslands**: Also have good productivity, although typically less than forests. However, they still tend to exceed 55 million tonnes in biomass production.
- **Agro-ecosystems**: These are managed ecosystems (like farms) that can have high productivity due to agricultural practices, often surpassing 55 million tonnes.
- **Oceans**: While oceans cover a vast area, their productivity is limited by factors such as sunlight penetration and nutrient availability. The productivity in oceans is generally lower than in terrestrial ecosystems.

4. **Compare the Ecosystems**:
- Given the context of the total biosphere productivity and the characteristics of each ecosystem, the oceans are known to have relatively low productivity compared to forests, grasslands, and agro-ecosystems.

5. **Conclusion**:
- Based on the analysis, the ecosystem with a productivity of about 55 million tonnes is the **Oceans**. This is due to the limiting factors affecting photosynthesis in aquatic environments.

### Final Answer:
The correct option is **Oceans**.