  Let $K$ be a null-homologous knot in a three-manifold $Y$. We give a
description of the Heegaard Floer homology of integer surgeries on $Y$ along
$K$ in terms of the filtered homotopy type of the knot invariant for $K$. As an
illustration, we calculate the Heegaard Floer homology groups of non-trivial
circle bundles over Riemann surfaces.
