Sentence:  The increase in oxygen uptake rates from 1.2 to 2.6 mg / l / hr which followed an increase in rotor speed was believed to <WSD>be</WSD> related to resuspension of solids which had settled at the lower rotor speeds . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)