Introduction:

The energy which is utilized in every ecosystem eventually comes from sun in form solar energy. Such energy is generally trapped and converted to chemical energy through the procedure of photosynthesis by photoautotrophic organisms. It is such energy which is converted to other forms of energy available in such ecosystems and is transferred from one organism to another through energy flow. Procedure of converting solar energy and storing chemical energy (in form of food) by green plants is known as primary production while such procedure in heterotrophs is known as secondary production.

In ecology, productivity or production refers to rate of generation of biomass in the ecosystem. It is generally expressed in units of mass per unit surface (or volume) per unit time, for example grams per square metre per day. Mass unit may associate to dry matter or to mass of carbon produced. Productivity of autotrophs like plants is known as primary productivity, while that of heterotrophs like animals is known as secondary productivity Freshwater systems gain most of their energy from photosynthesis performed by aquatic plants and algae. This autochthonous procedure involves combination of water, carbon dioxide, and solar energy to generate carbohydrates and dissolved oxygen. Within the lake or pond, potential rate of photosynthesis usually decreases with depth because of light attenuation. Photosynthesis, though, is frequently low at top few millimeters of surface, likely because of inhibition by ultraviolet light. Exact depth and photosynthetic rate measurements depend on:

1) Total biomass of photosynthesizing cells,

2) Amount of light attenuating materials and

3) Abundance and frequency range of light absorbing pigments (i.e. chlorophylls) inside of photosynthesizing cells. Energy generated by these primary producers is significant for community as it is transferred to higher trophic levels using consumption.

Algae, comprising both phytoplankton and periphyton are principle photosynthesizers in ponds and lakes. Phytoplankton is found drifting in water column of the pelagic zone. Several species have higher density than water that must make them sink and end up in benthos. To combat this, phytoplankton have developed density changing mechanisms, by creating vacuoles and gas vesicles or by changing the shapes to induce drag, slowing their descent. Very sophisticated adaptation used by the small number of species is the tail-like flagella which can adjust vertical position and permit movement in any direction. Phytoplankton can also keep their presence in water column by being circulated in Langmuir rotations. Periphytic algae, on other hand, are joined to the substrate. In lakes and ponds, they can cover all benthic surfaces. Both kinds of plankton are significant as food sources and as oxygen providers. Freshwater biota is joined in complex web of trophic relationships. These organisms can be considered to loosely be related with specific trophic groups (like herbivores, primary producers, secondary carnivores, primary carnivores, etc.). Scientists have developed numerous theories to understand mechanisms which control abundance and diversity in these groups. Very generally, top-down procedures dictate that abundance of prey taxa is dependent on actions of consumers from higher trophic levels. Usually, these procedures operate only between two trophic levels, with no effect on others. In some cases, though, aquatic systems experience a trophic cascade; for instance, this might take place if primary producers experience less grazing by herbivores as these herbivores is suppressed by carnivores. Bottom-up proceudres are functioning when abundance or diversity of members of higher trophic levels is dependent on availability or quality of resources from lower levels. At last, combined regulating theory, bottom-up: top-down, combines predicted influences of consumers and resource availability. It forecasts that trophic levels close to lowest trophic levels will be most influenced by bottom-up forces, where as top-down effects must be strongest at top levels

The huge majority of bacteria in lakes and ponds attain their energy by decomposing vegetation and animal matter. In pelagic zone, dead fish and occasional allochthonous input of litterfall are examples of coarse particulate organic matter (CPOM>1 mm). Bacteria degrade these in fine particulate organic matter (FPOM<1 mm) and then further in usable nutrients. Small organisms like plankton are also classified as FPOM. Very low concentrations of nutrients are released during decomposition as bacteria are using them to build their own biomass. Bacteria, though, are consumed by protozoa, that are in turn consumed by zooplankton, and then further up trophic levels. Nutrients, including those which have carbon and phosphorus, are reintroduced in water column at any number of points along this food chain through excretion or organism death, making them available again for bacteria.

Measurement of productivity:

In aquatic systems, main production is typically estimated using one of four major methods:

1. Variations in oxygen concentration inside the sealed bottle (developed by Gaarder and Gran in 1927)

2. Incorporation of inorganic carbon^-14 (¹⁴C in form of sodium bicarbonate) in organic matter.

3. Stable isotopes of Oxygen (¹⁶O, ¹⁸O and ¹⁷O)

4. Fluorescence kinetics

Method developed by Gaarder and Gran utilizes variations in concentration of oxygen under various experimental conditions to deduce gross primary production. Usually, three identical transparent vessels are filled with sample water and stoppered. First is analyzed instantly and utilized to find out initial oxygen concentration; generally this is done by doing a Winkler titration. Other two vessels are incubated, one each in under light and darkened. After the fixed period of time, experiment ends, and the oxygen concentration in both vessels is estimated. As photosynthesis has not occurred in dark vessel, it gives a measure of ecosystem respiration. Light vessel allows both photosynthesis and respiration, so gives the measure of net photosynthesis (that is oxygen production through photosynthesis subtract oxygen consumption by respiration). Gross primary production is then attained by adding oxygen consumption in dark vessel to net oxygen production in light vessel.

The method of using ¹⁴C incorporation (added as labelled Na₂CO₃) to infer primary production is most frequently used today as it is sensitive, and can be utilized in all ocean environments. As ¹⁴C is radioactive (using beta decay), it is relatively straightforward to estimate incorporation in organic material using devices like scintillation counters.

Depending on incubation time chosen, net or gross primary production can be measured. Gross primary production is best evaluated using relatively short incubation times (1 hour or less), as the loss of incorporated ¹⁴C (by respiration and organic material excretion / exudation) will be more restricted. Net primary production is fraction of gross production remaining after these loss procedures have consumed some of the fixed carbon.

Loss procedures can range between 10-60% of incorporated ¹⁴C according to incubation period, ambient environmental situation (particularly temperature) and experimental species used. Aside from those caused by physiology of experimental subject itself, potential losses because of the activity of consumers also require to be considered. This is mainly true in experiments making use of natural assemblages of microscopic autotrophs, where it is not feasible to isolate them from their consumers.

Usually, Wetlands are the most productive natural ecosystems due to the proximity of water and soil. Because of their productivity, wetlands are frequently converted in dry land with dykes and drains and utilized for agricultural purposes. Their closeness to lakes and rivers means that they are frequently developed for human settlement.

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