  Between 1996 and 2002, we have carried out a spectral monitoring program for
the Seyfert galaxy NGC 5548. High quality spectra (S/N>50), covering the
spectral range (4000-7500)AA were obtained with the 6 m and 1 m telescopes of
SAO (Russia) and with the 2.1 m telescope GHO (Mexico). We found that both the
flux in the lines and the continuum gradually decreased, reaching minimum
values during May-June 2002. The mean, rms, and the averaged over years,
observed and difference line profiles of H-alpha and H-beta reveal the double
peaked structure at the radial velocity ~+-1000km/s. The relative intensity of
these peaks changes with time. During 1996, the red peak was the brightest,
while in 1998 - 2002, the blue peak became the brighter one. In 2000-2002 a
distinct third peak appeared in the red wing of H-alpha and H-beta line
profiles. The radial velocity of this feature decreased between 2000 and 2002
from ~+2500 km/s to ~+2000 km/s. The fluxes of the various parts of the line
profiles are well correlated with each other and also with the continuum flux.
Shape changes of the different parts of the broad line are not correlated with
continuum variations and, apparently, are not related to reverberation effects.
Changes of the integral Balmer decrement are, on average, anticorrelated with
the continuum flux variations. This is probably due to an increasing role of
collisional excitation as the ionizing flux decreases. Our results favor the
formation of the broad Balmer lines in a turbulent accretion disc with large
and moving "optically thick" inhomogeneities, capable of reprocessing the
central source continuum.
