The difference between the initial dissolved oxygen in a water sample and the dissolved oxygen in a duplicate sample after a stated period of time is called?

The term that describes the difference between the initial dissolved oxygen in a water sample and the dissolved oxygen in a duplicate sample after a stated period of time is known as Biochemical Oxygen Demand (BOD). BOD measures the amount of oxygen that microorganisms will consume while decomposing organic matter in water over a specified time period, typically five days.

In this context, when a water sample's initial dissolved oxygen is measured and then re-measured after a period (usually five days), the decrease in dissolved oxygen indicates the quantity of organic material present, as microorganisms use oxygen to break down this material. Therefore, BOD serves as an important indicator of water quality and the degree of pollution in a body of water.

The other terms do not fit this definition: Total Suspended Solids refers to the solids suspended in water that do not settle out, Chemical Oxygen Demand measures the total oxygen required to chemically oxidize all organic material in a sample, and Total Nitrogen quantifies the amount of nitrogen present in a water sample, all of which are different concepts from BOD.