Stoichiometric dietary constraints influence the response of copepods to ultraviolet radiation-induced oxidative stress

We carried out field experiments in two clear mountain lakes of both hemispheres (Lake Los Cantaros, Patagonia, Argentina, and Lake La Caldera, Sierra Nevada, Spain) performing a full factorial design (light 3 nutrients: unfiltered sun light (ultraviolet radiation treatment [UVR]) and screened sunlight (> 380 nm; photosynthetically active radiation treatment), with and without nutrient enrichment. We analyzed the direct effect of UVR on enzymatic antioxidant responses (catalase [CAT], glutathione S-transferase [GST], and glutathione reductase [GR]) of two calanoid copepod species-Boeckella gibbosa and Mixodiaptomus laciniatus-and the indirect effects of food quality (carbon : nutrient ratio) potentially affecting body elemental compositions and hence enzymatic activities. Responses for the three enzymes were different: GST increased its activity under UVR exposure in the two copepods, CAT activity was null and showed no response, and GR activity differed between species. Light treatments also affected sestonic elemental ratios; UVR exposure lowered carbon : phosphorus (C : P) ratios, which in turn affected the C: P elemental compositions of the copepods. However, nutrient addition had different effects on the two species; it did not affect final somatic C: P ratio of B. gibbosa but had a substantial effect on body elemental composition of M. laciniatus. Finally, the relationship between grazer's C: P ratio and GST antioxidant enzyme activity was negative. UVR and nutrient inputs affected food quality, grazer somatic stoichiometry, and subsequently enzymatic responses. The ability of calanoid copepods to overcome increased UVR may depend, at least for GST, on the elemental nutrient balance of the food.