Select the correct statement about productivity from the following.

Options

A. Gross primary productivity is the available biomass for the consumption to heterotrophs.
B. Secondary productivity depends on the photosynthetic capacity of the plant species inhabiting a particular area.
C. Productivity is expressed in terms of `kg^(-2)yr^(-1)` or `(kcal m^(-2))yr^(-1)` to compare the productivity of different ecosystems.
D. Of an approximate NPP of 170 billion tonnes, the productivity of the oceans are only 55 billion tons despite accupying 70% of the earth.

Answer

To solve the question regarding productivity, we need to analyze each of the provided options and determine which statement is correct. Here's a step-by-step breakdown:

### Step 1: Understand Gross Primary Productivity (GPP)
- **Definition**: Gross Primary Productivity (GPP) refers to the total amount of organic matter (biomass) produced by photosynthesis in a given area over a specific time period.
- **Key Point**: GPP is not the biomass available for consumption by heterotrophs; that is known as Net Primary Productivity (NPP).

**Hint**: Remember that GPP is the total production, while NPP is what remains after accounting for the energy used by plants for their own respiration.

### Step 2: Evaluate Option A
- **Statement**: "Gross primary productivity is available biomass for the consumption of heterotrophs."
- **Analysis**: This statement is incorrect because it confuses GPP with NPP.

**Hint**: Differentiate between GPP (total production) and NPP (available for consumption).

### Step 3: Understand Secondary Productivity
- **Definition**: Secondary productivity refers to the generation of biomass by heterotrophs (consumers) that eat the producers.
- **Key Point**: Secondary productivity is influenced by the biomass produced by primary producers, not directly by their photosynthetic capacity.

**Hint**: Focus on the role of consumers in secondary productivity.

### Step 4: Evaluate Option B
- **Statement**: "Secondary productivity depends on the photosynthetic capacity of a plant species inhabiting a particular area."
- **Analysis**: This statement is incorrect because secondary productivity is dependent on the biomass produced by primary producers, not their photosynthetic capacity.

**Hint**: Remember that secondary productivity is about consumer biomass, not plant photosynthesis.

### Step 5: Understand the Units of Productivity
- **Definition**: Productivity can be expressed in terms of grams per square meter per year (g/m²/yr) or kilocalories per square meter per year (kcal/m²/yr).
- **Key Point**: While kg/m²/yr can be used, it is more common to express productivity in grams.

**Hint**: Be aware of the standard units used in ecological studies.

### Step 6: Evaluate Option C
- **Statement**: "Productivity is expressed in the term of kg per square year per year or kilocalorie meter square per year to compare the productivity of different ecosystems."
- **Analysis**: This statement is partially correct but misleading. While productivity can be expressed in kg, it is typically expressed in grams.

**Hint**: Check the common conventions in expressing productivity.

### Step 7: Understand Net Primary Productivity (NPP)
- **Definition**: Net Primary Productivity (NPP) is the amount of biomass available for consumption by heterotrophs after accounting for the energy used by plants in respiration.
- **Key Point**: NPP is a crucial measure for understanding ecosystem productivity.

**Hint**: Remember that NPP is what supports the food web.

### Step 8: Evaluate Option D
- **Statement**: "An approximate NPP of 170 billion tons exists, with ocean productivity at 55 billion tons despite occupying 70% of the Earth."
- **Analysis**: This statement is correct. It highlights the disparity between land and ocean productivity.

**Hint**: Look for statements that provide accurate data and context.

### Conclusion
After evaluating all options, the correct statement about productivity is **Option D**.

### Final Answer
**Option D** is the correct statement regarding productivity.