Explain how eutrophication can result in low oxygen levels in the water in the stream.

Eutrophication occurs when excess nitrates from fertilisers seep into the stream. The nitrates cause algal bloom on the surface of the water. As a result, sunlight is blocked from reaching the plants underwater so they cannot photosynthesise. Without photosynthesis, no oxygen is produced from the plants. The plants are then decomposed by bacteria, which use up oxygen during respiration, contributing to the low levels of oxygen in the water.
So, the important steps to remember are: Nitrates enter the stream; Algal bloom occurs covering the surface of water; Light is blocked so no photosynthesis; No oxygen produced by plants; Plants die; Decomposition by bacteria uses up even more oxygen.