Q.Explain, step by step, how eutrophication develops in a lake or pond -- from the entry of excess nutrients to the eventual death of fish and other aquatic organisms.

Eutrophication develops through a fairly predictable sequence. First, sewage, detergents and fertiliser runoff carry large amounts of nitrogen and phosphorus compounds into a lake or pond. These nutrients act as fertiliser for the water's algae, triggering an explosive algal bloom that can cover much of the surface. The dense algal mat blocks sunlight from reaching submerged plants below, which then die back for lack of light. As the bloom itself, and the shaded-out plants, eventually die, their remains are broken down by aerobic decomposer bacteria, whose population explodes to match the sudden glut of dead organic matter. This burst of bacterial decomposition consumes dissolved oxygen from the water far faster than it can be replenished, so the water becomes progressively hypoxic (oxygen-depleted). Fish and o …

Walk through the sequence as a chain of cause-and-effect steps in order, ending with why oxygen depletion specifically is wha …

Assuming the algae themselves directly poison the fish -- the actual killing mechanism is oxygen depletion caused by bacterial decomposition o …