Trophic upgrading via the microbial food web may link terrestrial dissolved organic matter to Daphnia

Direct consumption of allochthonous resources generally yields poor growth and reproduction in zooplankton, but it is still unclear how trophic upgrading of terrestrial dissolved organic matter (tDOM) via the microbial food web may support zooplankton. We compared survival, somatic growth and reproduction of Daphnia magna fed with the heterotrophic flagellate Paraphysomonas vestita and three algal diets. Paraphysomonas was fed lake bacteria that used tDOM as a substrate to simulate an allochthonous diet that zooplankton encounter in lakes. The highest survival, growth and reproduction was achieved with a diet of Cryptomonas, while Daphnia performance was the worst when fed Microcystis. Paraphysomonas and Scenedesmus diets lead to intermediate growth and reproduction. Cryptomonas contained high amounts of essential polyunsaturated fatty acids (PUFA) and phytosterols that supported high somatic growth and reproduction, whereas poor performance of Daphnia on the cyanobacterial diet was most likely due to lack of sterols. Paraphysomonas contained some phytosterols, but not in sufficient amounts, and also essential PUFA (eicosapentaenoic and arachidonic acid) that enhance zooplankton growth and reproduction. Our results indicate that the tDOM-based microbial food web supports Daphnia performance even as a sole food source, and may be important in providing zooplankton with essential biochemical components when phytoplankton quantity or quality is low.