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In page Algal bloom:

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The reduction of phosphorus inputs is required to mitigate blooms that contain cyanobacteria.[4] In lakes that are stratified in the summer, autumn turnover can release substantial quantities of bio-available phosphorus potentially triggering algal blooms as soon as sufficient photosynthetic light is available.[5] Excess nutrients can enter watersheds through water runoff.[6] Excess carbon and nitrogen have also been suspected as causes. Presence of residual sodium carbonate acts as catalyst for the algae to bloom by providing dissolved carbon dioxide for enhanced photosynthesis in the presence of nutrients.[citation needed]