Energy enters the system as sunlight, gets fixed up the autotrophs, is passed on to the heterotrophs, and passes through the trophic levels and gets dissipated in the form of heat the most thermodynamically unavailable form of energy.

Productivity is the efficiency with which the autotrophs fix the radiant energy from sunlight. Only a small fraction of the total radiant energy received by the earth is converted into chemical energy by the producers. This energy which is fixed by the producers, either through photosynthesis and chemosynthesis over a period of time is termed primary productivity. In other words, it is the rate at which the producers convert the radiant energy to organic substances. It is also measured per unit area.

There are four successive steps in productivity.

a. Gross primary productivity (GPP)
It is the total rate of photosynthesis in unit time. It is also called as total photosynthesis. This includes the matter used up in respiration. Only about half of the incoming solar radiation is absorbed on earth's surface. About 19 % is absorbed by atmosphere and clouds and 20% get reflected by clouds. About 6 % is reflected by atmosphere and 4 % by the earth's surface. Also the 'photosynthetically useful' light constitutes only 50 % of the entire radiation. These include lights of wavelengths 400-700 nm.

The reflectance is different for different terrains of the earth. Broadleaf forests reflect 0 % of the incoming radiation where as conifers reflect only 5 %. Absorbing radiation is also different for various surfaces. Those with low leaf areas absorb very less light e.g.: deserts. Conifer forests having higher leaf area index absorb almost 95% of the light. Of the light energy absorbed by chloroplasts, only 20 % is fixed into chemical energy.

b. Net Primary Productivity (NPP)
It is the rate of storage of organic substances in the plant tissues after reducing those used in respiration. The portion which is used by the plant is called net assimilation (R )

Net assimilation is a broader term which includes the plant's metabolic needs also in addition to respiration. Average net primary production is highest in algal beds and reefs. Open Ocean is the largest contributor to the primary productivity on earth's surface.
NPP is therefore dependent on the net assimilation which is limited by the availability of nutrients, light and water. This decides the carbon allocation and therefore the leaf area and leaf photosynthetic efficiency in turn.

Leaf are index = Leaf surface area (Horizontal) / Ground surface area (Horizontal)
The photosynthetic efficiency (PE) of the leaves are calculated as
PE = (Production / Incident radiant energy) * 100

c. Net Community Productivity (NCP)
It is the rate at which organic substances are stored up which is not used by heterotrophs.
NCP = NPP- heterotrophic consumption
It is measured usually in the growing season of the year.

Standing crop biomass

The ratio of NPP to GPP determines the cost of maintaining the organism's needs. Those with higher ratios require less cost while higher costs associated with lower ratios. Algae often require less energy to sustain while trees require more. More complex organisms require higher energy. The left over after being utilized by secondary consumers constitutes Standing Crop Biomass. This is in short accumulated net primary productivity.
Primary productivity is limited by availability of sunlight and nutrients. A limiting nutrient is one which increases the primary productivity when it is present in large quantities. The nutrient when in limited quantities decreases the productivity. The limiting nutrients are mostly phosphorus, nitrogen, water, iron, temperature etc.

d. Secondary productivity

This is the rate at which the heterotrophic consumers fix the energy as biomass. It is dependent on the efficiency with which the chemical energy is transferred from one trophic level to another. These transfers usually suffer from energy loss. If the consumer requires more energy to keep going, the efficiency of conversion of primary productivity to secondary productivity will be lesser.

The Trophic efficiency is the ratio of the secondary productivity to primary productivity. Inefficient digestion contributes to trophic inefficiency.

Going up the trophic levels there is a drop in 'productivity' up to 90 %. Up the heterotrophic levels, the energy flow can be better termed as 'assimilation'. While evaluating the productivity of any ecosystem, there are other factors to be considered which influence the energy drains and energy subsidies. Energy subsidies helps to retain the energy by reducing heat loss while drains result in loss of energy.

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