Explain eutrophication and why it can lead to the death of invertebrates and fish in rivers.

Eutrophication is due to high levels of nutrients (nitrates) in the water. This leads to increased growth of algae and other plants which leads to "algal blooms". This is where the surface of the water is covered in algae, preventing sunlight from reaching the river bed and in turn inhibiting photosynthesis there. The plants on the river bed die, leading to an increase in decomposition by bacteria which use up the only available oxygen in respiration and hence cause the death of fish and invertebrates in the river.