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

MARTYNIUK, NICOLÁS; MODENUTTI, BEATRIZ; BALSEIRO, ESTEBAN G.

Freshwater Science

Chicago UNiversity press

Lugar: Chicago; Año: 2019 vol. 38 p. 391 - 405

2161-9549

In mountain streams,light is a key factor for primary production and primary producer nutrientstoichiometry, or the relative composition of carbon (C) compared tophosphorous (P) and nitrogen (N) in primary producers. Light conditions thereforedetermine the stoichiometric quality of food available to grazers because periphytonprimary producers are one of the most important food sources for grazers instream communities. Here, one of our main goals was to analyze thestoichiometric response of macroinvertebrate grazers to different periphyton nutrientratios that result from different light conditions. We hypothesized that theelemental imbalance between a consumer and its food source can change dependingon food quality and the nutrient content of the consumer. Considering that underhigh-light conditions photoinhibition causes low C:nutrient ratio in periphyton, we further expected that this inhibition wouldresult in high food quality. Thus, the gap between the food quality needs ofthe consumer and the quality of food would be smaller. In addition, we expectedbetter food quality to stimulate grazer growth and, thus, increase grazer bodysize. To test these hypotheses, we conducted a field study and a 3-mo fieldexperiment in which we manipulated light (unshaded and shaded treatments) in aglacier-fed, naturally[A1] canopy-free stream in the North-PatagonianAndes. In our field study and during the experiment, grazers were dominated by thestonefly Notoperla magnaspina and themayfly Meridialaris chiloeensis. C:Pand C:N ratios of Notoperla magnaspinadid not vary significantly in response to different light intensities. Incontrast, M. chiloeensis showedhigher variability in elemental composition and had significantly higher C:Pand C:N ratios under reduced light conditions tracking the differences in C content ofthe periphyton. We interpret the light manipulationexperiments to show that the stoichiometric imbalance between periphyton and Notoperla magnaspina increased under lowlight conditions as a result of an increase in the periphyton C:nutrient ratiocaused by reduced photoinhibition. Our findings highlight the importance of light as a modulatorof C fluxes in the benthic ecosystem. We conclude that light availability can playa key role in determining elemental imbalances within mountain stream food webs.