All those tree leaves from the willow (Salix) tree will be in the pond decomposing contributing to algae. Remember this when you are dealing with or killing lots of algae each year. Roots will penetrate from all sides of the island into the pond then transpiring lots of water into the air. Also note in the 1st video below she mentions shrub Salix (willow) as an invader in the later Reed-Swamp Stage of pond succession. In a brand new pond, you are introducing the Salix (willow) tree at the start or beginning.
You should eventually aerate to keep the pond as good as it can naturally be and slow pond succession, although you won't have to aerate right away. Pond health is best if aerated soon after the pond fills, but significant ecological nor biological damage is not done if not aerated the first year or two. The longer one delays aeration the faster the pond moves through the aging process (aquatic or pond succession see example videos below).
http://www.limusicindex.com/video/UsiEcapJ3KU/pond-succession.htmlhttp://www.limusicindex.com/video/fYC5v5d0-jA/pond-succession-l9-6.html 
