Can I use 16ga wire to complete a 150W landscape lighting circuit?
Generally its not OK.   You don't say whether the power pack is 12 or 24 
VAC.


110W / 12 V = 9.16 A
110W / 24 V = 4.53 A


AWG 14 wire is rated for 15A

AWG 16 wire is rated for  8A

So, if you are using a 12 V power pack, 16ga is too small.

It would be technically OK, current-wise, for 120VAC, but I don't know of any 16ga wire rated for distribution outdoors.