Light intensity regulates stoichiometry of benthic grazers through changes in the quality of stream periphyton

In mountain streams, light is a key factor for primary production and primary producer nutrient stoichiometry, or the relative composition of carbon (C) compared to phosphorous (P) and nitrogen (N) in primary producers. Light conditions, therefore, determine the stoichiometric quality of food available to grazers because periphyton primary producers are one of the most important food sources for grazers in stream communities. Here, one of our main goals was to analyze the stoichiometric response of macroinvertebrate grazers to different periphyton nutrient ratios that result from different light conditions. We hypothesized that the elemental imbalance between a consumer and its food source can change depending on food quality and the nutrient content of the consumer. Considering that under high light conditions photoinhibition causes low C∶nutrient ratio in periphyton, we further expected that this inhibition would result in high food quality. Thus, the gap between the food quality needs of the consumer and the quality of food would be smaller. In addition, we expected better food quality to stimulate grazer growth and, thus, increase grazer body size. To test these hypotheses, we conducted a field study and a 3-mo field experiment in which we manipulated light (unshaded and shaded treatments) in a glacier-fed, naturally canopy-free stream in the North-Patagonian Andes. In our field study and during the experiment, grazers were dominated by the stonefly Notoperla magnaspina and the mayfly Meridialaris chiloeensis. C∶P and C∶N ratios of Notoperla magnaspina did not vary significantly in response to different light intensities. In contrast, M. chiloeensis showed higher variability in elemental composition and had significantly higher C∶P and C∶N ratios under reduced light conditions tracking the differences in C content of the periphyton. We interpret the light manipulation experiments to show that the elemental imbalance between periphyton and Notoperla magnaspina increased under low-light conditions as a result of an increase in the periphyton C∶nutrient ratio. Our findings highlight the importance of light as a modulator of C fluxes in the benthic ecosystem. We conclude that light availability can play a key role in determining elemental imbalances within mountain stream food webs.