We evaluated the dual role of DOM (i.e., as a source of inorganic nutrients and as an absorber of solar radiation) on a phytoplankton community of the western South Atlantic Ocean. Using a combination of microcosms and a cluster approach, we simulated the future conditions of some variables that are highly influenced by global change in the region. We increased nutrients (i.e., anthropogenic input) and dissolved organic matter (DOM), and we decreased the pH, to assess their combined impact on growth rates (mu), species composition/abundance and size structure, and photosynthesis (considering in this later also the effects of light quality i.e., with andwithout ultraviolet radiation). We simulated two Future conditions (Fut) where nutrients and pHwere similarly manipulated, but in one the physical role of DOM (Fut(out)) was assessed whereas in the other (Fut(in)) the physicochemical role was evaluated; these conditions were compared with a control (Present condition, Pres). The mu significantly increased in both Fut conditions as compared to the Pres, probably due to the nutrient addition and acidification in the former. The highest mu were observed in the Fut(out), due to the growth of nanoplanktonic flagellates and diatoms. Cells in the Fut(in) were photosynthetically less efficient as compared to those of the Fut(out) and Pres, but these physiological differences, also between samples with or without solar UVR observed at the beginning of the experiment, decreased with time hinting for an acclimation process. The knowledge of the relative importance of both roles of DOM is especially important for coastal areas that are expected to receive higher inputs and will be more acidified in the future.