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!

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 www.google.com and type in dissolved oxygen lake. I hope this helps you get started.

http://waterontheweb.org/under/waterquality/oxygen.html

Thank you so much, DrBob! I appreciate it!

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

That makes a lot of sense. Thank you!

To determine whether the dissolved oxygen concentration of water samples taken from a lake at 7 am would be higher or lower than samples taken at 5 pm, several factors need to be considered.

1. Photosynthesis: Photosynthesis, performed by plants and algae, is a process that produces oxygen during daylight hours. Since 7 am falls within the early morning, photosynthesis may have occurred during the previous day and resulted in higher oxygen concentrations.

2. Respiration: Respiration by aquatic organisms can deplete oxygen levels in water. During the night, when photosynthesis does not occur, respiration continues, which can lead to lower oxygen concentrations by the morning.

3. Temperature: Temperature plays a role in determining the solubility of gases in water. As temperature increases, the solubility of oxygen decreases. If the temperature is higher at 5 pm compared to 7 am, it may contribute to lower dissolved oxygen concentrations in the evening samples.

Considering these factors, it is difficult to make a definitive prediction without specific knowledge of the lake's characteristics. However, there are a few general observations we can make:

1. If the lake has abundant photosynthetic activity (e.g., healthy plant and algae populations), it is more likely that the 7 am sample will have higher dissolved oxygen concentrations due to photosynthesis during the previous day.

2. If the lake has low photosynthetic activity or high respiration rates, the 7 am sample could potentially have lower dissolved oxygen concentrations due to respiration continuing during the early morning.

3. If the temperature is significantly higher at 5 pm compared to 7 am, the 5 pm sample may have lower dissolved oxygen concentrations due to the reduced solubility of oxygen at higher temperatures.

Ultimately, to determine whether the dissolved oxygen concentration is higher or lower, it is recommended to collect water samples at both times (7 am and 5 pm) and measure the dissolved oxygen levels using a dissolved oxygen meter or by performing chemical tests in a laboratory. Regular monitoring is important to understand the specific dynamics of the lake's oxygen concentrations.