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Posted by Mercedes McKay on Tuesday, 1 December 1998, at 12:07 p.m., in response to Re: Wierd CO2 Readings, posted by Sophia on Monday, 23 November 1998, at 2:53 p.m.
Hello,
Sorry it's taken a while to get back to you...I've been talking to others here to see if they had any ideas about your results. Everyone here agrees that some of your results seem very inconsistent. For example, your dissolved oxygen levels seem very high. The "rotten egg" smell that you observed usually is an indication that there is decay of organic matter in the water by bacteria and other microorganisms. These microorganisms require oxygen to break down the organic material and thus use up the dissolved oxygen in the water. You also noticed very few fish and a lot of macroinvertebrates that are tolerant of pollution. This is also usually a sign that there are low levels of dissolved oxygen in the water.
Another thing that seems inconsistent is the pH level and the carbon dioxide level. You have a fairly neutral pH level and I would expect to see some carbon dioxide at that pH, even if a low value. Carbon dioxide becomes more soluble in water at lower pH levels, and I would expect that only at very high pH levels to find a carbon dioxide reading close to 0.
Is there any possibility that the values for dissolved oxygen and carbon dioxide got mixed up? It would make a lot more sense given how you've described your lake if the dissolved oxygen level was close to zero and the carbon dioxide levels were higher.
When scientists encounter unexpected values such as you have, they first check out their equipment (as you have already done) and then re-test any areas that seem questionable to them to verify their results. Even though you have done two samples already, the best way to proceed would be to re-test the water for both dissolved oxygen and carbon dioxide levels to confirm that the tests were performed accurately and that there weren't any other factors that might affect the results.
Unfortunately, yours is not an easy question to answer! I also realize that it may be difficult for you to re-test the lake that you already sampled. It may be useful for you to examine other schools' data and see if you can find relationships between carbon dioxide and dissolved oxygen levels or carbon dioxide and pH levels; it may help you to see that your data are somewhat inconsistent with what would normally be expected. Of course, your sampling is also complicated by the fact that an oil spill recently occurred there. The effects of the oil spill are difficult to predict.
Lastly, you may want to check with other experts in the field to see if they can provide additional information. There are some on-line experts listed in the "Useful Links" section of the web site which you might try. Good luck!
Mercedes