Icy Pond

This is a lovely little poem.  You use assonance very effectively in the first half of the poem where the soft "i" sound is repeated.  In the second half of the poem, I see two separate uses of assonance.  In lines 5 and 6 there is repetition of the "an" sound.  In the same lines, the long "a" sound is also repeated.  The long "o" sounds in the final line tie together the uncommon combination of the words dozing open.  I'm curious about the transition between the first and second halves of the poem from short vowel sounds to long ones.  Was this intention?