Excess nitrates and phosphates are water soluble and enter rivers/lakes/waterway due to leaching.Algae in the water use the increased level of ions for growth. This is called Algal bloom.Build up of algae on surface of water prevents light reaching the plants at the bottom of the lake, therefore these plants die because they can't photosynthesise. Saprobionts decompose the dead plant matter and population of saprobionts increases. Saprobionts respire aerobically and therefore use up O2 in the water, causing depletion of dissolved O2. O2 is now the limiting factor.Depletion of O2 causes other organisms such as fish to die
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