The two sources of pollution that decrease the oxygen level of water are:
- Plant fertilizer - When it runs off into water bodies, it can lead to algal blooms, which consume oxygen as they decompose.
- Animal manure - Similar to plant fertilizers, it can also lead to algal blooms and increased organic matter decomposition, which depletes oxygen in the water.
Toxic metals, gasoline, and radioactive substances do not directly decrease oxygen levels in the same way.