The potential long-term environmental impact of manufactured nanoparticles (NPs) remains poorly understood, in part due to the complexity of NPs themselves and the range of physico-chemical parameters that may influence their biological impacts (such as size, shape and chemistry), as well as their dynamic interactions with their environment, leading to acquisition of an eco-corona and a range of other possible transformations. A key hypothesis in nanosafety assessment is that the NP core chemistry is a primary factor controlling toxicity. This work aims to compare the uptake and impacts of 5 nm zinc oxide (ZnO) NPs which are highly soluble and cerium dioxide (CeO2) NPs which are considered to be poorly soluble, where both particles were produced using an identical protocol and capped with polyvinylpyrrolidone (PVP), on Daphnia magna (D. magna). These well-characterised NPs were developed as part of a systematic library, and were intended to allow controlled variation of one property at a time, e.g. the core composition as evaluated here. Half-maximal effective concentrations (EC50) were determined in pure medium and medium conditioned with biomolecules secreted by D. magna, as the presence of biomolecules in the environment has the ability to alter NP stability and biological effects. NPs were characterised by size and zeta potential measurements under both conditions. NP uptake concentrations and removal over 24 hours post exposure (without feeding) were determined by inductively coupled plasma – optical emission spectroscopy (ICP-OES) of the exposure and receiving media, respectively. Results showed that PVP capped ZnO NPs were more toxic than PVP capped CeO2 NPs. The NP impact and behaviour was due to physical effects with CeO2 NPs which showed signs of agglomeration while a chemical effect was apparent in the case of ZnO NPs which underwent partial dissolution in the gut following uptake.