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

• Published in: Journal of Plankton Research
• Main Authors: Taipale, Sami J., Kainz, Martin J., Brett, Michael T.
• Format: Text
• Language: English
• Published: Oxford University Press 2015
• Subjects: Article; Betula nana
• Online Access: http://plankt.oxfordjournals.org/cgi/content/short/fbv015v1; https://doi.org/10.1093/plankt/fbv015

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 2 > 0.80) fatty acid profiles to their diets, whereas Daphnia that consumed t-POM diets had weak correlations ( r 2 = 0.002–0.56) with the corresponding diet sources. Unusual 16 carbon chain polyunsaturated fatty acids (16:2ω6, 16:3ω3 and 16:4ω3), linoleic acid (18:2ω6) and α-linolenic acid (18:3ω3) were diagnostic fatty acids for Scenedesmus and Daphnia that consumed this alga. Stearidonic acid (18:4ω3) and eicosapentaenoic acid (20:5ω3) 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 ω-3:ω-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 ( r2 = 0.88). These results suggest that Daphnia , and perhaps zooplankton in general, preferentially retain algae-derived ω3 fatty acids, and low ω-3:ω-6 ratios in Daphnia indicate a mainly terrestrial diet or poor nutritional condition.