Would you expect the dissolved oxygen concentration of water samples taken from a lake at 7 am to be higher or lower than samples takn at 5 pm?

I feel like there may be more dissolved oxygen in the sample taken at 7 am because photosynthesis probably took place and oxygen was produced. However, I think that the sample taken at 5 pm is equally valid choice because the oxygen probably accumulated in water. But- the temperature of the water is probably higher and so there will be less dissolved oxygen.
Please help me out. Thanks in advance!

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  1. Here is a site that may provide some help with your question. I'm not sure that is a clear cut answer. You are right that photosynthesis produces more oxygen and higher temperatures will dissolve less oxygen. The article below states that dissolved oxygen (DO) is lowest just before dawn because no photosynthesis takes place during the dark hours and DO steadily declines through the night. Much of it, I think, depends upon the depth at which the samples were taken. Deep lake temperatures don't vary that much. Other web sites I visited talk about the variation of DO with depth. Lower depths are cooler and can hold more DO BUT photosynthesis doesn't take place as much at lower depths. I suggest you go to and type in dissolved oxygen lake. I hope this helps you get started.

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  2. Thank you so much, DrBob! I appreciate it!

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  3. Fish dying in deoxygenated lakes is always just before dawn, or just after.

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  4. That makes a lot of sense. Thank you!

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