Sentence:  Variations in aerator speeds during the latter two months of this study caused increased mixing and increased <WSD>oxygen</WSD> demand . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust