Do autochthonous resources enhance trophic transfer of allochthonous organic matter to aquatic consumers, or vice versa?

Autochthonous and allochthonous resources are known to differ in nutritional quality and trophic support for aquatic food webs, but it is less clear how these high- and low-quality resources interact to affect trophic transfer and consumer production. We conducted 30-d feeding trials to investigate the resource assimilation, somatic growth, and fatty-acid (FA) composition of the widespread benthic generalist isopod Asellus aquatints, in response to different ratios of low-quality allochthonous (leaf litter) to high-quality autochthonous diets (algae). Wet mass growth of Asellus was lowest when fed 100% leaf litter or algae (0.53 +/- 0.46 and 0.55 +/- 0.57 mg center dot g(-1)center dot d(-1), respectively; mean +/- SE) and highest (4.95 +/- 0.51 mg center dot g(-1)center dot d(-1)) with a diet of 90:10 leaf litter:algae ratio. Asellus tended to grow slower with increasing dietary algal proportions (10-100%), yet stable isotopes and Bayesian mixing models revealed consistently high algal assimilation (>= 94%) by Asellus. Therefore, among the mixed-diet treatments, Asellus biomass production using algal resources was optimized when terrestrial organic matter (OM) dominated over algae. Eicosapentaenoic acid (EPA):total FA, EPA:omega-3 FA, and arachidonic acid:total FA declined, but docosahexaenoic acid (DHA):omega-3 FA increased, with increasing growth of Asellus. Tissue EPA concentrations of Asellus were similar among treatments, so reductions in EPA:omega-3 and EPA:total FA were due to increases in DHA concentration. Overall, our results suggest synergistic effects between autochthonous and allochthonous resources on Asellus growth and that allochthonous OM particularly facilitates the trophic transfer of autochthonous resources. Asellus preferentially retains DHA at low algal availability. This may improve its neural tissue development and so its success in accessing algae. The growth and FA responses of the widespread Asellus can enhance resource and DHA transfer to visual predators that have greater DHA demands, particularly when brownification of boreal freshwaters likely intensifies upon global climate change.