  Using an analytically tractable kinetic model of a two dimensional Fermi
liquid of electrons, we characterize the crossovers between zero sound, first
sound and plasmons. For experimentally realized Fermi liquids in a hydrodynamic
limit, both zero and first sound waves are essentially replaced by plasmons.
The plasmon dispersion relation is robust against hydrodynamic effects, up to
acquiring the viscous-limited decay rate of a first sound wave in the
hydrodynamic limit. We discuss implications for experiments in clean two
dimensional electron gases.
