( ( Krill migrations ) ( show ( ( seasonal ( and ( annual changes ) ) ) ( ( related mainly ) ( to ( ( food availability ) ( expressed ( as ( ( the concentration ) ( of ( chlorophyll a. ) ) ) ) ) ) ) ) ) ) )
(ROOT (S (NP (JJ Krill) (NNS migrations)) (VP (VBP show) (NP (NP (JJ seasonal) (CC and) (JJ annual) (NNS changes)) (VP (VBN related) (ADVP (RB mainly)) (PP (TO to) (NP (NP (NN food) (NN availability)) (VP (VBN expressed) (PP (IN as) (NP (NP (DT the) (NN concentration)) (PP (IN of) (NP (JJ chlorophyll) (NN a.)))))))))))))
Krill migrations show seasonal and annual changes related mainly to food availability expressed as the concentration of chlorophyll a.
(ROOT (S (NP (NP (DT A) (NN change)) (PP (IN of) (NP (NP (NN season)) (CC and) (NP (JJR less) (NN food))))) (ADVP (RB usually)) (VP (VB cause) (S (NP (JJ certain) (NNS animals)) (VP (TO to) (VP (VB migrate))))) (. .)))
A change of season and less food usually cause certain animals to migrate.