  Characterisation of the long-term variations in the broad line region in a
luminous blazar, where Comptonisation of broad-line emission within a
relativistic jet is the standard scenario for production of gamma-ray emission
that dominates the spectral energy distribution. We analysed ten years of
optical spectroscopic data from the Steward Observatory for the blazar 3C
454.3, as well as gamma-ray data from the Fermi Large Area Telescope (LAT). The
optical spectra are dominated by a highly variable non-thermal synchrotron
continuum with a prominent Mg II broad emission line. The line flux was
obtained by spectral decomposition including significant contribution from the
Fe II pseudo-continuum. Three methods were used to characterise variations in
the line flux: (1) stacking of the continuum-subtracted spectra, (2)
subtracting the running mean light curves calculated for different timescales,
and (3) evaluating potential time delays via the discrete correlation function
(DCF). Despite very large variations in the gamma-ray and optical continua, the
line flux changes only moderately (< 0.1 dex). The data suggest that the line
flux responds to a dramatic change in the blazar activity from a very high
state in 2010 to a deep low state in 2012. Two interpretations are possible:
either the line flux is anti-correlated with the continuum or the increase in
the line luminosity is delayed by ~600 days. If this time delay results from
the reverberation of poorly constrained accretion disc emission in both the
broad-line region (BLR) and the synchrotron emitting blazar zone within a
relativistic jet, we would obtain natural estimates for the BLR radius
[R_{BLR,MgII} >~ 0.28 pc] and for the supermassive black hole mass [M_SMBH ~
8.5x10^8 M_sun]. We did not identify additional examples of short-term 'flares'
of the line flux, in addition to the previously reported case observed in 2010.
