Although the ecotoxicological effects of various metal oxide nanoparticles on aquatic organisms are being actively studied, the contributions of particles and dissolved ions towards toxicity are still not well understood. The current study aims to assess the contribution of ZnO NP(particle) and ZnO NP(ion) to the overall toxicity and accumulation of ZnO NP(total) in Ceriodaphnia dubia. The aggregation and dissolution kinetics were studied for three different sizes (50 nm, 100 nm and bulk) of ZnO particles at 0.05, 0.12, 0.25 and 0.5 mg/L concentrations in the sterile lake water medium at 6, 12, 24, and 48 h intervals. The 48 h LC50 of ZnO NP(total) was found to be 0.431, 0.605 and 0.701 mg/L for 50, 100 nm and bulk particles exposure. However, LC50 of Zn(ion) was found to be 1.048, 1.343 and 2.046 mg/L for dissolved ions from different sizes (50 nm, 100 nm, and bulk) of ZnO particles. At LC50 concentration, the accumulation of 90–95% was noted for the NP(particles) across the sizes employed, while only about 4–5% contribution was from the NP(ion) to the overall accumulation NP(total). The relative contribution of ZnO NP(ion) to overall toxicity and accumulation was found to be lesser than that of ZnO NP(particles) across the sizes used in the study. © 2016 Elsevier B.V.