The benthic amphipod Diporeia represents a crucial trophic link that conveys vital nutrients and energy to predators at higher trophic levels. The current decline of Diporeia populations, mostly in the North American Great Lakes, may, in part, be related to concurrent declines in food quantity and/or quality. We hypothesized that somatic growth and survival of Diporeia would be positively related to dietary supply and subsequent retention of polyunsaturated fatty acids (PUFA); a class of chemicals known to affect diet quality. We examined how different algal PUFA concentrations in; a) Ankistrodesmus falcatus (Chlorophyta), b) a naturally occurring diatom assemblage from Lake Ontario, c) a non-toxic strain of Microcystis aeruginosa (Cyanophyta), and, d) fasting for 30 d, affected PUFA concentrations, somatic growth, and survival of Diporeia. Total PUFA concentrations were significantly higher in A. falcatus than in diatoms and Microcystis, but only diatoms contained considerable amounts of eicosapentaenoic (EPA) and docosahexaenoic acid (DHA). EPA, DHA, and arachidonic acid (ARA) were highly retained in Diporeia even in the absence of dietary supply with ARA being the most efficiently bioaccumulated PUFA. Survivorship of Diporeia ranged from 60% (diatom-fed), 68% (A. falcatus-fed), to 70% (fasting treatment), but was 0% in the M. aeruginosa diet treatment. Nucleic acid ratios (RNA:DNA), commonly used as proxies for somatic growth potential, were highest in Diporeia feeding on diatoms and lowest in fasting animals. We conclude that overall condition of Diporeia improved with dietary access to EPA and DHA, but survival was not related to this food quantity and/or quality.
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Vol. 36 • No. 2