  If $u$ is a smooth solution of the Navier--Stokes equations on ${\mathbb
R}^3$ with first blowup time $T$, we prove lower bounds for $u$ in the Sobolev
spaces $\dot H^{3/2}$, $\dot H^{5/2}$, and the Besov space $\dot
B^{5/2}_{2,1}$, with optimal rates of blowup: we prove the strong lower bounds
$\|u(t)\|_{\dot H^{3/2}}\ge c(T-t)^{-1/2}$ and $\|u(t)\|_{\dot
B^{5/2}_{2,1}}\ge c(T-t)^{-1}$, but in $\dot H^{5/2}$ we only obtain the weaker
result $\limsup_{t\to T^-}(T-t)\|u(t)\|_{\dot H^{5/2}}\ge c$. The proofs
involve new inequalities for the nonlinear term in Sobolev and Besov spaces,
both of which are obtained using a dyadic decomposition of $u$.
