Answer: Option A. -> Coral reefs
:
A
The total measure of the energy stored in the producers after meeting the cost of respiration is called as the net primary productivity (NPP). NPP is thebiomass or energy avaiable for the heterotrophs in an ecosystem.In terms of NPP per unit area the most productive ecosystems in the world are estuaries, swamps and marshes, followed by the tropical rain forests, coral reefs, temperate deciduous forests and grasslands. The tundras, open oceans and deserts are the least productive. In coral reef, an underwater ecosystem has the highest number of producers (zooxanthallae algae in symbiosis with coral reefs).

Answer: Option C. -> Phosphorus
:
C
The phosphorus cycle is unique among other major biogeochemical cycles in having no significant atmospheric component. The reason for this being that phosphorus, is solid at normal temperature and pressure and it does not circulate in the atmosphere. The largest reservoirs of this element are sedimentary rocks.

Answer: Option D. -> A is correct, R is incorrect
:
D
In a given ecosystem, various food chains are linked together and intersect each other to form a complex network called food web. Kite is a carnivorous bird, it feeds on fishes, rabbits, snakes, rats and so on. In a food web, it occupies a specific position because it feeds onlower carnivores and herbivores also.

Answer: Option A. -> Primary source of energy is the sun
:
A
The detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus into simple organic compounds and inorganic molecules that are released into soil, these molecules are specifically reused by plants or autotrophs. In forests like tropical rain forest the amout of detritus formed is huge when compared to other ecosystems because of the large number of treesand animal species in these forests.

Answer: Option D. -> Organic molecules
:
D
Autotrophs, that is plants, utilize carbon in the form of carbon dioxide from the atmosphere and synthesize organic compounds. The heterotrophs consume these plants and utilise these organic compounds stored in them as a source of energy. Heterotrophs do not have the ability to utilize, carbon dioxide, carbon monoxide or other forms of carbon for energy.

Answer: Option A. -> Littoral zone ----- Limnetic zone ----- Profundal zone
:
A
In aquatic ecosystems like lakes and large pondsorganisms are organised in a recognizable structure or pattern in two or three stratas or zones:
1. Alittoral zone which has rooted vegetation
2. Alimnetic zone of open water containing floating plankton
3. Adeep water profundal zone where wefind leeches, crabs, some annelid worms and some insect larvae too.

:
An ecosystem consists of a community of organisms interacting with its physical environment. For the sake of convenience, ecosystems can be broadly categorised into aquatic ecosystems and terrestrial ecosystems.

Answer: Option B. -> Both A and R are true but R does not explain A
:
B
For the initiation of ecological succession on a bare rock requires a pioneer organism like lichens which can facilitate suitable environment for the successve colonisation of other organisms.The acidic secretions from the lichens break down the the substrate and add to the accumulation of soil. Mosses then colonize the soil, adding enough nutrients in the soil for alder shrubs to take hold. Over maybe a hundred years, alders build up the soil nitrogen levels until spruces are able to thrive, eventually crowding out the alder and forming a dense spruce forest.Lichens are not only found as pioneer species but can also luxuriently grow on the barks or any other moist areas throughout the world. Since the tropics are relatively more humid, lichens grow luxuriently in the tropical regions.

Answer: Option A. -> Humification - leads to the accumulation of a dark coloured humus which undergoes microbial action at a very fast rate.
:
A
Three main types of processes occur during decomposition.
First up is fragmentation in which dead plant parts and animal remains, called detritus, are processed by small invertebrate animals like earthworms and termites, called detritivores, They physically break down the organic matter into smaller fragments. Leaching is the second step, where parts of soluble substances present in the fragmented and decomposing detritus get leached to the upper layers of soil by percolation of water into soil particles. The final step is catabolism which is mainly carried out by saprotrophic bacteria and fungi which secrete digestive enzymes into the fragmented detritus, where the complex organic compounds are broken down to simpler compounds and inorganic substances. The rate of catabolism of different complex substances is different. It is further divided into two steps - humification and mineralisation. Humification is the process by which simplified detritus gets converted into a dark coloured formless substance that is rich in cellulose, lignin, tannins, resin, etc. This substance is known as humus. Its specific character is that it is highly resistant to microbial action and so,it undergoes extremely slow decomposition. Mineralisation is a very important process, because it releases inorganic substances like carbon dioxide, water, and minerals from the remains of organic matter.

Answer: Option B. -> Atmosphere
:
B
The reservoir of the gaseous type of biogeochemical cycles for example nitrogen, carbon cycles exists in the atmosphere. Ionosphere is the layer of the earth's atmosphere which contains a high concentration of ions and free electrons and is able to reflect radio waves. Lithosphere is the rigid outer part of the earth, consisting of the crust and upper mantle. Stratosphere is the zone of ozone gas.