| Summary: | Variation in quality and quantity of food resources can affect consumer productivity responses throughout the food chain, particularly the efficiency at which basal resources are converted to consumer biomass. I performed a manipulative feeding experiment to investigate the somatic growth and fatty acid incorporation in the benthic generalist isopod Asellus aquaticus, in response to differing ratios of autochthonous (high quality algae) to allochthonous (low quality leaf litter) foods. I used stable isotopes to quantify the assimilated diet proportions across a range of diet treatments to determine the relative resources that contributed to growth. There were significant differences in growth between treatments, being lowest in treatments A (100% leaf litter) and G (100% algae), with highest growth experienced in treatment B (90% leaf litter/ 10% algae). Stable isotope data revealed that there was very little variation in algal assimilation among combined diet treatments. Fatty acids (FA) indicators eicosapentaenoic acid (EPA):total FA and EPA:omega 3 (ω3) FA and arachidonic acid (ARA):total FA declined with increasing growth and docosahexaenoic acid (DHA):ω3 showed a positive relationship with growth. These findings provide support for previous feeding trials conducted with Asellus, though there are some contrasts with zooplankton. The results suggest a balance between allochthonous and autochthonous dietary sources combine to enhance primary consumer fitness, and the relative availability of each may interact to determine growth and accumulation of important FA compounds. In terms of FA and trophic transfer, temporal and spatial variation in consumer physiological demands might determine the retention and use of FA.
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