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> spikes in algal populations, which then lower the oxygen content of water bodies

How can algae lower oxygen when oxygen is the very byproduct of their photosynthesis? They literally turn water into oxygen. Wouldn't that increase the oxygen content?

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When the algae bloom dies oxygen is used during decay

https://www.epa.gov/nutrientpollution/effects-dead-zones-and...

This is similar to how mulch works to suppress weed growth. Mulch is decaying matter and nutrients like nitrogen are temporarily 'locked up' during the decay process.

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