( ( ( The ( main factor ) ) ( contributing ( to ( changes ( in ( dissolved ( oxygen levels ) ) ) ) ) ) ) ( ( is ( ( ( ( ( the build-up ) ( of ( organic wastes ) ) ) , ) or ) ( wastes ( ( ( ( from ( once-living ( ( plants and ) animals ) ) ) , ) and ) ( from ( ( the feces ) ( of animals ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ main) (NN factor)) (VP (VBG contributing) (PP (TO to) (NP (NP (NNS changes)) (PP (IN in) (NP (JJ dissolved) (NN oxygen) (NNS levels))))))) (VP (VBZ is) (NP (NP (NP (DT the) (NN build-up)) (PP (IN of) (NP (JJ organic) (NNS wastes)))) (, ,) (CC or) (NP (NP (NNS wastes)) (PP (PP (IN from) (NP (JJ once-living) (NNS plants) (CC and) (NNS animals))) (, ,) (CC and) (PP (IN from) (NP (NP (DT the) (NNS feces)) (PP (IN of) (NP (NNS animals))))))))) (. .)))
The main factor contributing to changes in dissolved oxygen levels is the build-up of organic wastes, or wastes from once-living plants and animals, and from the feces of animals.
(ROOT (S (NP (NNS Plants)) (VP (CONJP (RB not) (JJ only)) (VP (VBP contribute) (NP (NN food))) (CC but) (VP (VBD oxygen) (PP (IN for) (NP (NNS organisms))))) (. .)))