The concentration of dissolved organic matter (DOM) in aquatic ecosystems has manifold direct and indirect consequences for primary and secondary production. Theoretical approaches suggest a negative effect of DOM on phytoplankton and a positive effect on bacteria, both of which alter the development of metazooplankton, the main consumer of bacteria and phytoplankton. However, experimental observations give heterogeneous results on the effect sizes of plankton in relation to DOM addition. Here, the synthesis of 411 effect sizes of plankton in response to DOM is presented from 59 studies. The referenced studies display effect sizes in relation to various DOM sources and different experimental designs. Thus, the hypothesis that neither DOM quality nor the type of experiment bias the effect size response of plankton was tested; the hypothesis was tested separately for bacterioplankton, phytoplankton and metazooplankton with the use of a meta-analytical approach. DOM quality was represented by the five different sources of DOM that are used most often in these experiments, i.e., natural water, bog leachate, glucose/sucrose, terrestrial plant leachate, and soil leachate. The experiment types comprised in situ experiments, i.e., experiments taking place under manipulated conditions in a natural ecosystem, and ex situ experiments, i.e., those taking place outside of a natural ecosystem. The meta-analyses showed that, although total DOM had a positive influence on the effect size of bacterioplankton and an insignificant influence on metazooplankton, the effect sizes differed significantly in relation to DOM source and the type of experiment. The effect size of phytoplankton, although not significantly related with total DOM, differed in response to DOM source but not experiment type. The results suggest that experimental conditions may produce artifacts in the strength of responses of aquatic biota to DOM concentrations.