If an algae dies the oxygen of the water will be depleted. The lack of oxygen will make it more difficult for aquatic organisms to survive. The lack of oxygen can cause algae blooms.
Some algae blooms can smell bad, block sunlight, and even release toxins in some cases. It is a problem for aquatic environments as the excess of nitrogen can cause algal blooms. Some of these algal blooms produce toxins that can be harmful to life.
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