Q.A large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping but you find an abundance of phytoplankton. Suggest a reason for the fish kill.

This is a classic case of eutrophication-related fish kill, and the clue given — abundant phytoplankton with no evidence of toxic dumping — points directly to it.

Reasoning:

1. An unusually large amount of nutrients (nitrates and phosphates), typically from agricultural fertiliser runoff, detergents, or sewage, must have entered the lake, providing excess food for phytoplankton (algae and other microscopic aquatic plants).
2. This caused an algal bloom — a sudden, dense proliferation of phytoplankton on/near the water surface, consistent with the observation of 'abundance of phytoplankton'.
3. A dense phytoplankton layer can block sunlight from reaching deeper into the water, reducing photosynthesis by submerged plants, which itself reduces oxygen production.
4. More importantly, when large numbers of phytoplankton eventually die, they are decomposed by aerobic bacteria, which consume large amounts of dissolved oxygen from the water in the process — this is essentially a spike in Biochemical Oxygen Demand (BOD).
5. In addition, phytoplankton themselves respire (consuming oxygen) especially at night when they cannot photosynthesise, which can also sharply lower dissolved oxygen levels, particularly in the early morning. …

Connect the two clues in the question — no toxic dumping, but abundant phytoplankton — to the concept of eutrophication and oxygen depletion: excess nutrients cause an algal bloom, whose death/decomposition (or night-time …

• Assuming the fish died from direct toxicity, ignoring the explicit statement that there was 'no evidence of toxic dumping'.
• Not connecting the abundance of phytoplankton to eutrophication and its downstream effect of oxygen depletion. …