Theoretically, the BOD value in the pond should be higher due to the large population of microorganisms and aquatic organisms like fish and turtles. Aquatic plants may use up the dissolved oxygen as well during respiration. There might be also microorganisms from the surrounding soil due to surface runoff.
The BOD value in the swimming pool is lower due to the high amount of dissolved oxygen as there are less microorganisms that are present to utilize the dissolved oxygen. The constant maintenance of the pool water prevents the growth of microorganisms in the water.
The wrong values of the graph may be due to experimental errors:
The DO probe is not washed properly with DI water, giving inaccurate results
The DO probe is not calibrated to the correct value.
As the amount of dissolved oxygen decreases, the BOD values will increase and vice versa. This is because the demand for oxygen by the bacteria is high and they are utilizing the oxygen that is dissolved in the water. If there is no organic waste present in the water, there won't be as many bacteria present to decompose it and thus the BOD will tend to be lower and the DO level will tend to be higher.
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