Abstract: Nanoparticle pollution is known to affect various organisms. However, how nanoparticles influence species interactions, and in turn, species coexistence and biodiversity effects on ecosystem functions, remains poorly understood. Here, we addressed this knowledge gap with ten pairs of freshwater phytoplankton subjected to a gradient of copper oxide nanoparticle concentration. We found that nanoparticles reduced phytoplankton species growth rate and community productivity. More importantly, we consistently found that nanoparticles had a greater negative effect on species with smaller cell sizes, such that nanoparticle pollution weakens the competitive dominancy of the smaller species and promoted coexistence in bi-species communities. More mechanistically, we found that nanoparticles significantly narrowed the growth rate difference and competitive ability difference of competing species, while having little effect on species niche difference. As a result, nanoparticle pollution reduced the selection effect on abundance but increased the selection effect on biomass without changing complementarity effect on neither abundance nor biomass. Our results suggest that nanoparticle pollution may alter species competitive balance and the relationships between biodiversity and ecosystem functions.
