Effects of thermal evolution on the stoichiometric responses to nano-ZnO under warming are not general: insights from experimental evolution

A key challenge for ecological risk assessment of contaminants under global warming is to predict effects at higher levels of biological organisation. One approach to reach this goal is to study how contaminants and warming cause changes in body stoichiometry as these may potentially cascade through food webs. Furthermore, though contaminants typically interact with warming, how rapid adaptation to higher temperatures affects these interactions is poorly studied. Here, we examined the effects of an important contaminant (ZnO nanoparticles, nZnO) and mild warming (4 °C) on body stoichiometry (C, N, P and their ratios) of an aquatic keystone species, the water flea Daphnia magna. To evaluate whether thermal evolution impacts the effects of nZnO at higher temperatures, we compared two sets of clones from a thermal selection experiment where Daphnia were kept in outdoor mesocosms at ambient or ambient +4 °C temperatures for 2 years. Exposure to nZnO decreased key body stoichiometric ratios (C:N, C:P and a trend for N:P) while warming increased the body C:N ratio. The stoichiometric changes to nZnO and warming were mostly independent and could be partly explained by changes in the macromolecules sugars and fat. Exposure to nZnO decreased C-rich sugars contributing to a reduced %C. Warming reduced body %C due to decreased C-rich sugars and fat levels, yet warming decreased body N% even more resulting in a higher C:N ratio. The stoichiometric responses to nZnO at the higher temperature did not differ between the two sets of clones, indicating experimental thermal evolution did not change the effects of nZnO under warming. Studying the stoichiometric responses to nZnO and warming of this keystone species may provide novel insights on the toxic effects of contaminants under warming. Moreover, understanding the influence of thermal evolution on the toxicity of contaminants is important for ecological risk assessment especially in a warming world.status: publishe