We experimentally determine effective interparticle potentials in a two-dimensional (2D) colloidal system of charge-stabilized polystyrene particles at different particle densities ρ. Density variation is achieved by means of a scanned optical laser tweezer which serves to create a boundary box for the system. By changing the size of this boundary box, ρ can be systematically varied without having to prepare a new system. From the measured radial distribution functions we can then obtain the effective pair potentials of the particles. While for low particle densities perfect agreement with Yukawa-like potentials is observed, considerable deviations from this form are found at higher densities. We interpret this result as a many-body effect produced by macroion screening which is expected to become more pronounced as the density is increased.