Caffeine and canopy cover interact to alter biofilm nutrient content, benthic invertebrates, and insect emergence

Abstract

The addition of trace contaminants to surface waters is recognized as a rising environmental concern and caffeine is among those most commonly detected. Classic toxicological analyses of single organisms have described some of the impacts of caffeine on freshwater organisms, but few have examined ecological interactions, food webs, or aquatic‐terrestrial linkages, especially in natural aquatic systems. This study used an in situ manipulative experiment to effects of caffeine addition on aquatic biofilms and biomass of aquatic and emergent insects. We used chemical diffusing substrata to introduce small concentrations of caffeine to streams and analyzed effects on downstream biofilm nutrient content, benthic macroinvertebrates, and emergent insects. We found that caffeine addition decreased the biomass of emergent insects by 56% and altered biofilm nutrient content in complex ways that interact with canopy cover, with stronger effects of caffeine addition under low canopy cover (higher light). Thus, the impacts of this common aquatic contaminant may influence the linked riparian food web by decreasing emergent insects, an important food source for riparian insectivores.