Copper ion (Cu2+) and copper oxide (CuO) nanoparticle (NP) ecotoxicity are of increasing concern as they are considered to be a potential risk to marine systems. This study represents the first attempt to evaluate CuO NP impacts on the seaweeds and Cu2+ on the chlorophyte Ulva rigida. Effects on oxidative stress, antioxidant defence markers, photosystem II function, thalli growth, and cell viability in U. rigida exposed for 4 up 72h to1 and 5mg L-1 Cu2+ and CuO NPs were examined. Hydrogen peroxide (H2O2) generation, superoxide dismutase (SOD) activity, malondialdehyde (MDA) content, and growth inhibition seemed to be reliable and early warning markers of toxicity. The most important variables of the principal component analysis (PCA): H2O2 generation, antioxidant stress markers, and growth-based toxicity index, were higher at 1mg L-1 CuO NPs compared to CuSO4 and at 5mg L-1 CuSO4 compared to CuO NPs. Intracellular uptake kinetics fit well to the Michaelis-Menten equation. The higher toxicity at 5mg L-1 CuSO4 compared to 1mg L-1 was due to the higher Cu uptake with increasing concentration, suggesting and higher accumulation ability. On the contrary, 1mg L-1 CuO NPs induced more strongly toxicity effects than 5mg L-1. The relatively stronger effect of CuO NPs at 1mg L-1 than the respective CuSO4 concentration could be attributed to the higher rate of initial uptake (Vc) and the mean rate of Cu uptake [Cmax/(2 × Km)] at CuO NP treatment. The intracellular seaweed experimental threshold of Cu, which coincided with the onset of oxidative stress, was within the Cu concentration range recorded in Mediterranean Ulva spp., indicating that it may pose a substantial risk to marine environments.