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Are there other factors that affect the amount of dissolved oxygen? 

Organic material.  Organic material comes from parts of trees and plants that fall into a body of water.  Organic material also includes decaying algae, dead aquatic plants, dead fish or other organisms, and human and animal wastes.  Organic material does not directly remove the DO, but it creates conditions where large amounts of bacteria accumulate.  These bacteria consume large amounts of DO, driving the overall oxygen level down.

Nitrates and Phosphates.  If you've ever spread fertilizer on a lawn, you'd probably know that nitrates and phosphates are the main component.  When these chemicals are in high concentrations in water, they do the same thing.  They fertilize.  This causes algae and aquatic plants to thrive.  As a result, two things happen:

1. A rate of plant growth occurs that cannot be sustained.  Plants grow so dense that eventually they choke each other off.  Large amounts of plant material accumulates, creating organic matter and large amounts of oxygen-demanding bacteria.

2. Algae grow and create an unstable DO level.  A large algae population will create oxygen during the day (photosynthesis), but only consumes it during the night (no light for photosynthesis).  This results in a very low DO level just before sunrise, creating stressful conditions for some aquatic organisms.

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