  We use spectra from the ALFALFA, GASS and COLD GASS surveys to quantify
variations in the mean atomic and molecular gas mass fractions throughout the
SFR-M* plane and along the main sequence (MS) of star-forming galaxies.
Although galaxies well below the MS tend to be undetected in the Arecibo and
IRAM observations, reliable mean atomic and molecular gas fractions can be
obtained through a spectral stacking technique. We find that the position of
galaxies in the SFR-M* plane can be explained mostly by their global cold gas
reservoirs as observed in the HI line, with in addition systematic variations
in the molecular-to-atomic ratio and star formation efficiency. When looking at
galaxies within +/-0.4 dex of the MS, we find that as stellar mass increases,
both atomic and molecular gas mass fractions decrease, stellar bulges become
more prominent, and the mean stellar ages increase. Both star formation
efficiency and molecular-to-atomic ratios vary little for massive main sequence
galaxies, indicating that the flattening of the MS is due to the global
decrease of the cold gas reservoirs of galaxies rather than to bottlenecks in
the process of converting cold atomic gas to stars.
