The dynamics of decaying turbulence disturbed at small and intermediate scales by energy-conserving disturbances is investigated in direct numerical simulations at relatively low Reynolds number (Rλ≈20-30). Three types of disturbances are introduced. The reduction of the energy dissipation and related enstrophy growth is achieved by partial destruction of phase coherence ostensibly presented at almost all scales of turbulence. The disturbance preserving both energy and helicity spectrum has been found a most destructive one. Possible implications of the results for anisotropic turbulence is briefly discussed.