An experimental approach to test the effect of temperature increase and nutrient enrichment on Andean aquatic insects

Climate change and nutrient enrichment are two main stressors of rivers in Andean ecosystems. As temperatures increase globally, species are expected to be affected in various ways, including growth and survival. Here, we performed a novel, 15-day microcosm experiment to test the impact of multiple stressors on algae biomass (measured as chlorophyll-a biomass), the main food supply of two aquatic insect grazers, and its effect on their survival and growth rates. We used Andesiops peruvianus (Ephemeroptera) and Anomalocosmoecus illiesi (Trichoptera), two abundant species in Andean rivers in Ecuador. The experiment tested the addition of nitrogen (N) and phosphorus (P) using nutrient-diffusing substrates (NDS) in three Intergovernmental Panel on Climate Change (IPCC) temperature scenarios (ambient, +2.5 °C, and +5 °C). The single effect of increasing temperature produced an increase in algal biomass. Positive synergistic effects on chlorophyll-a biomass resulted in ambient chambers with a mix of N+P addition and a combination of +2.5 °C with +P and N+P. The single and interactive effects of nutrient addition and warming water produced high mortality for Andesiops peruvianus but did not impact the survival of Anomalocosmoecus illiesi larvae. The growth rates of Andesiops peruvianus larvae showed no significant effect in any treatments except for the ambient NP treatment, in which we observed a marginal impact on body mass. In turn, the growth rates of Anomalocosmoecus illiesi larvae increased significantly only at +2.5 °C with +P addition. Based on our results, we posit that phosphorus is an important nutrient in Andean streams that mediates the growth of larval insects. This multiple-stressor experiment suggests that global warming and nutrient enrichment could critically alter the river’s primary productivity and the metabolism of Andean aquatic insects.