Q.Eutrophication leads to the death of fishes due to

Removing forests removes the very biomass that stores (sinks) atmospheric carbon.

Forests are one of the largest natural carbon sinks: through photosynthesis, trees fix atmospheric CO2 into organic carbon stored in wood, leaves and soil as part of the global carbon cycle. Deforestation removes this standing biomass — the carbon stored in felled trees is released back to the atmosphere (often through burning or decomposition), and the capacity of that land to keep absorbing further CO2 …

BOD measures the oxygen consumed by microbes decomposing organic waste in water, indicating the degree of organic pollution.

BOD (Biochemical Oxygen Demand), also called Biological Oxygen Demand, is a standard measure of the degree of pollution in water (particularly organic pollution). It is defined as the amount of oxygen that would be consumed if all the organic matter present in one litre of water were oxidised by bacteria (aerobic decomposers) over a specific period (usually 5 days at 20°C). The greater the amount of biodegradable organic matter (sewage) in a water sample, the greater the number of decomposer microbes that gr …

A healthy aquatic ecosystem needs a comparatively high dissolved-oxygen (DO) content; a low DO level is a marker of water pollution and harms aquatic organisms.

Dissolved oxygen (DO) is the amount of oxygen gas dissolved in water, essential for the respiration of fish and other aquatic organisms, as well as for aerobic decomposition of organic matter by microbes. A healthy, unpolluted aquatic ecosystem maintains a relatively high level of dissolved oxygen, supporting a rich diversity of aquatic life.

…

CO₂, methane, and nitrous oxide trap heat in the atmosphere (greenhouse gases); sulphur dioxide does not act this way — it is chiefly an air pollutant causing acid rain.

Greenhouse gases absorb and re-radiate infrared (heat) radiation, warming the atmosphere. The major greenhouse gases are carbon dioxide, methane, nitrous oxide, water vapour, and chlorofluorocarbons (CFCs). Sulphur dioxide (SO₂), by contrast, is a major air pollutant released by burning fossil fuels — it reacts with water …

Eutrophication over-enriches water with nutrients, triggering algal blooms whose decomposition depletes dissolved oxygen and kills fish.

Eutrophication is the excessive nutrient enrichment (mainly nitrates and phosphates, often from fertiliser runoff and sewage) of a water body. The nutrients trigger explosive growth of algae and cyanobacteria ("algal bloom"), which forms a mat on the surface and blocks sunlight from reaching submerged plants. When the algae die, huge numbers of decomposer bacteria break down the organic matter, and their aerobi …