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Threshold dietary polyunsaturated fatty acid concentrations for Daphnia pulex growth and reproduction

Abstract Knowing when, where, and why food quality limits herbivore growth and reproduction is the core question in nutritional ecology. We investigated dietary threshold concentrations of polyunsaturated fatty acids (PUFAs) for the herbivorous zooplankter Daphnia pulex when fed the cyanobacterium Microcystis aeruginosa or the green algae Scenedesmus obliquus. Phosphatidylcholine liposomes were used to supplement diets with arachidonic acid (ARA, 20:4ω6), stearidonic acid (SDA, 18:4ω3), or eicosapentaenoic acid (EPA, 20:5ω3) at concentrations (per mg carbon) ranging from 0.04 to 10 μg (mg C)−1. ARA supplementation had no effect on Daphnia somatic growth or reproduction. SDA supplementation did not affect Daphnia growth, however egg production dropped below a 50% saturation threshold when dietary SDA concentrations were <0.34–0.39 μg SDA (mg C)−1. Daphnia egg production dropped below a 50% saturation threshold when EPA concentrations for the Microcystis and Scenedesmus treatments were <0.25 and 0.17 μg EPA (mg C)−1, respectively. Growth declined below the 50% threshold when EPA concentrations for the 2 diets were <0.14 and 0.04 μg EPA (mg C)−1, respectively. These results show the demands for EPA are more intense for reproduction than for somatic growth. We used the EPA results to develop an overall model between Daphnia production and dietary EPA: normalized production = (EPA/C)/(0.15 + EPA/C), R2 = 0.72, n = 40. The bootstrapped 50% saturation threshold for this model was 0.15 ± 0.03 (± SE) μg EPA (mg C)−1. Our results suggest Daphnia pulex dietary EPA limitation may be common in eutrophic lakes