Ecosystem

Step 1: Definition.
Decomposers are organisms that break down dead organic matter into simpler substances, recycling nutrients back to the ecosystem.
Step 2: Identifying decomposers.
Fungi and bacteria are the primary decomposers in ecosystems, converting organic matter into inorganic nutrients.
Step 3: Analyzing options.
(A) Fungi and algae: Incorrect — Algae are autotrophs, not decomposers.
(B) Fungi and virus: Incorrect — Viruses are obligate parasites, not decomposers.
(C) Fungi and bacteria: Correct — Major decomposers.
(D) Fungi, bacteria and virus: Incorrect — Virus is not a decomposer.
Step 4: Conclusion.
Hence, Fungi and bacteria are decomposers.

Step 1: Primary productivity.
It is the rate at which energy is captured by producers (plants, algae) through photosynthesis per unit area per unit time.
- Gross Primary Productivity (GPP): Total energy fixed.
- Net Primary Productivity (NPP): Energy available to consumers (GPP – respiration losses).
Step 2: Secondary productivity.
It is the rate at which energy is assimilated at the consumer level (herbivores, carnivores). It represents biomass gained by consumers per unit area per unit time.
Step 3: Conclusion.
Primary productivity → producers; Secondary productivity → consumers.

Phosphorus Cycle.
The phosphorus cycle is the process by which phosphorus moves through the environment, including the soil, water, and living organisms. Unlike other biogeochemical cycles, the phosphorus cycle does not involve the atmosphere. The main stages are:
1. Weathering of rocks: Phosphorus is released from rocks through weathering.
2. Absorption by plants: Phosphorus is absorbed by plants in the form of phosphate.
3. Consumption by animals: Herbivores consume plants, and carnivores eat herbivores, thus transferring phosphorus.
4. Decomposition: When organisms die, decomposers break down organic matter, returning phosphorus to the soil.
5. Leaching: Phosphorus may be washed into water bodies, where it can contribute to aquatic life.

Conclusion.
The phosphorus cycle is vital for ecosystem productivity.

Step 1: Understanding the process.
Leaching is the process by which water percolates through the soil and carries soluble inorganic nutrients downward, away from the root zone.
Step 2: Analyzing the options.
(A) Fragmentation: Incorrect — It is the breakdown of detritus into smaller pieces by detritivores.
(B) Leaching: Correct — The downward movement of soluble nutrients is called leaching.
(C) Catabolism: Incorrect — It is the breakdown of organic molecules inside cells.
(D) Humification: Incorrect — It refers to the formation of humus from decomposed organic matter.
Step 3: Conclusion.
The correct term for downward movement of soluble nutrients is Leaching.

Step 1: Understanding 'Terror of Bengal'.
Water hyacinth (\textit{Eichhornia crassipes}) is an invasive aquatic weed. It clogs water bodies, depletes oxygen, and hampers aquatic biodiversity.
Step 2: Reason for the name.
It spreads rapidly in Bengal’s water bodies, causing serious ecological and economic problems, hence called the "Terror of Bengal".
Step 3: Conclusion.
Thus, the correct plant is Water hyacinth.