**Primary Productivity:**
Primary productivity refers to the amount of biomass (organic matter) produced by green plants through photosynthesis per unit area in a given time period. It is measured in terms of the energy or biomass produced per unit area per unit time. This is an important concept in ecology and is crucial for understanding the flow of energy through ecosystems.

**Explanation:**
Primary productivity is the foundation of all ecosystems as it represents the energy that is available to support the entire food chain. Green plants, also known as producers, are the primary source of energy in ecosystems as they convert sunlight, water, and carbon dioxide into organic matter through the process of photosynthesis.

During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose (a sugar) and oxygen. Glucose is then used as a building block to produce other organic compounds such as cellulose, proteins, and lipids. These organic compounds form the biomass of plants.

The primary productivity of an ecosystem is influenced by various factors:

1. **Light Availability:** Photosynthesis requires sunlight as a source of energy. Therefore, the availability of light plays a crucial role in determining the rate of primary productivity. In areas with high light availability, such as tropical rainforests, primary productivity is generally high.

2. **Nutrient Availability:** Plants need essential nutrients, such as nitrogen, phosphorus, and potassium, to carry out photosynthesis and produce biomass. The availability of these nutrients in the soil can limit primary productivity. Fertile soils with abundant nutrients can support higher primary productivity.

3. **Temperature:** Photosynthesis is temperature-dependent, with an optimum range for most plants. Extremely high or low temperatures can reduce the efficiency of photosynthesis and limit primary productivity.

4. **Water Availability:** Water is essential for photosynthesis. In areas with limited water availability, such as deserts, primary productivity is usually low. However, in aquatic ecosystems, water availability is generally not a limiting factor.

By measuring primary productivity, scientists can estimate the energy flow through ecosystems and understand the distribution and abundance of organisms at different trophic levels. It also helps in assessing the overall health and functioning of ecosystems.

In conclusion, primary productivity is the amount of biomass produced per unit area by green plants through photosynthesis. It is a fundamental process that drives energy flow and sustains life in ecosystems.