The excess nutrients from the fertilizer runoff from the farmland into the water stream due to rain or percolate into ground water
The nutrients in the water left by the fertilizer is what causes the algae to grow. The algae is produced when nitrogen and phosphorus is in the water.
The sunlight is unable to reach the algae, which cause the plants and algae to die off because they are unable to go through photosynthesis.
Algae and plants need oxygen to live and breathe carbon dioxide, but without the sun they are unable to take in nutrients and then die.
During algae blooms, the dissolved-oxygen concentration sometimes drops too low for the fish, and a die-off can occur.
The excess of carbon over nitrogen in organic materials being decomposed is too great, biological activity diminishes because of anoxicity. Several cycles of organisms are then required to burn most of the carbon.