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Between 1996 and 2002, we have carried out a spectral monitoring program forthe Seyfert galaxy NGC 5548. High quality spectra (S/N>50), covering thespectral range (4000-7500)AA were obtained with the 6 m and 1 m telescopes ofSAO (Russia) and with the 2.1 m telescope GHO (Mexico). We found that both theflux in the lines and the continuum gradually decreased, reaching minimumvalues 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 doublepeaked structure at the radial velocity ~+-1000km/s. The relative intensity ofthese 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 adistinct third peak appeared in the red wing of H-alpha and H-beta lineprofiles. The radial velocity of this feature decreased between 2000 and 2002from ~+2500 km/s to ~+2000 km/s. The fluxes of the various parts of the lineprofiles 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 withcontinuum variations and, apparently, are not related to reverberation effects.Changes of the integral Balmer decrement are, on average, anticorrelated withthe continuum flux variations. This is probably due to an increasing role ofcollisional excitation as the ionizing flux decreases. Our results favor theformation of the broad Balmer lines in a turbulent accretion disc with largeand moving "optically thick" inhomogeneities, capable of reprocessing thecentral source continuum.Comment: Accepted for publication in A&A; 24 pages, 14 postscript figure

Profile variability of the H$\mathsf{\alpha}$ and H$\mathsf{\beta}$ broad emission lines in NGC 5548