Comparison of spectral features in the FUSE spectra of Mrk 509 from 1999 November (blue) and 2000 September (red). Normalized relative fluxes are plotted as a function of velocity relative to the systemic redshift of z = 0.034397. The individual panels (from top to bottom) show O viλ1032, O viλ1037, Lyβ, and Lyγ. Foreground Galactic ISM features are marked. Error vectors for each spectrum are shown as thin lines in the same color as the data.