Equivalent conjectures on blowing-ups of P2

We provide a characterization of asymptotical speciality of a nef and big divisor D on an algebraic surface in terms of the arithmetic genus of curves in D-perpendicular to. As a consequence we prove that the SHGH conjecture for linear systems on the blowing-up of the projective plane at points in very general position is equivalent to the fact that each nef class is non-special. Finally we prove that if r < 2(n) then any nef divisor of the blowing-up of the n-dimensional projective space at r points in very general position is asymptotically non-special.