The effect of hardness on the stability of citrate-stabilized gold nanoparticles and their uptake by Daphnia magna

► Citrate-stabilized AuNPs is readily aggregated in synthetic fresh waters. ► Hardness induces aggregation by compressing double layer of particles. ► In reaction-limited regime, AuNPs form rigid particle-cluster aggregates. ► Micro-sized aggregates of AuNPs are well accumulated by daphnia. ► Daphnia molt to remove AuNPs on their carapace.