The present study explores the effect of light with different spectral composition on the stability of CuO-nanoparticle (CuO-NP) dispersions and their effects to green alga Chlamydomonas reinhardtii. The results showed that simulated natural light (SNL) and light with enhanced UVB radiation (UVR*) do not affect the dissolution of CuO-NPs as compared to light irradiation conditions typically used in laboratory incubator (INC). Comparable values of ζ-potential and hydrodynamic size during 24 h were found under all studied conditions. Concentrations of CuO-NPs below 1 mg L⁻¹ do not attenuate the light penetration in the algal suspensions in comparison with NP-free system. Exposure to a combination of 8 μg L⁻¹ or 0.8 mg L⁻¹ CuO-NPs and INC or SNL has no significant effect on the algal growth inhibition, algal fluorescence and membrane integrity under short-term exposure. However, an enhancement of the percentage of cells experiencing oxidative stress was observed upon exposure to 0.8 mg L⁻¹ CuO-NPs and SNL for 4 and 8 h. Combination of UVR* and 0.8 mg L⁻¹ CuO-NPs resulted in synergistic effects for all biological endpoints. Despite the photocatalytic properties of CuO-NPs no significant increase in abiotic reactive oxygen species (ROS) production under simulated solar radiation was observed suggesting that the synergistic effect observed might be correlated to other factors than CuO-NP-mediated ROS photoproduction. Tests performed with CuSO₄ confirmed the important role of dissolution as toxicity driving force for lower CuO-NP concentration. However, they failed to clarify the contribution of dissolved Cu on the combined effects at 0.8 mg L⁻¹ CuO-NPs. The results point out the necessity of taking into account the possible interactions between ENPs and changing light conditions when evaluating the potential effects of ENPs to phytoplankton in natural waters.