A low ω-3:ω-6 ratio in Daphnia indicates terrestrial resource utilization and poor nutritional condition

Abstract

It has been hypothesized that terrestrial particulate organic matter (t-POM) makes important contributions to Daphnia production in some lakes. We conducted a series of feeding experiments to explore the fatty acid responses in Daphnia to diets comprised of different terrestrial resources (i.e. Alnus rubra, Phragmites australis, Betula nana and Betula pendula) and mixed diets with terrestrial and phytoplankton (Scenedesmus or Cryptomonas) resources. When fed 100% phytoplankton, Daphnia had very similar (r . 0.80) fatty acid profiles to their diets, whereas Daphnia that consumed t-POM diets had weak correlations (r 1⁄4 0.002–0.56) with the corresponding diet sources. Unusual 16 carbon chain polyunsaturated fatty acids (16:2v6, 16:3v3 and 16:4v3), linoleic acid (18:2v6) and a-linolenic acid (18:3v3) were diagnostic fatty acids for Scenedesmus and Daphnia that consumed this alga. Stearidonic acid (18:4v3) and eicosapentaenoic acid (20:5v3) were diagnostic for Cryptomonas and Daphnia that consumed this diet. All of the t-POM resources were characterized by a high content of saturated fatty acids (SAFA; 79+ 12%), especially the diagnostic long-chain SAFA (20:0, 22:0, 24:0, 26:0, 28:0). Daphnia that consumed t-POM assimilated very little of these terrestrial biomarkers, but the shorter chain SAFA 16:0 and 18:0 were very prevalent in juvenile and adult Daphnia that consumed terrestrial plant matter. The v-3:v-6 ratios were distinctive between terrestrial (0.3–1.6) and phytoplankton resources ( 3–15), and this ratio in Daphnia was strongly associated with their diets (r 1⁄4 0.88). These results suggest that Daphnia, and perhaps zooplankton in general, preferentially retain algae-derived v3 fatty acids, and low v-3:v-6 ratios in Daphnia indicate a mainly terrestrial diet or poor nutritional condition.