Presence of mucilage and low omega-3 fatty acid contents in phytoplankton impair the quality of phytoplankton diet for zooplankton

Phytoplankton are a crucial basal dietary source for consumers in lake ecosystems. The dietary quality of phytoplankton for zooplankton has been investigated in terms of molecules (e.g., fatty acids) and stoichiometry. However, little is known on the effects of algal mucilage on the dietary quality of phytoplankton. Therefore, this study aimed to investigate the effects of mucilage and fatty acid contents in phytoplankton on dietary quality. Zooplankton (Daphnia pulicaria) were reared with a single species of phytoplankton as the diet. Eighteen phytoplankton types were used. Algal dietary quality was evaluated based on the growth, reproduction, and survival rates of D. pulicaria for each dietary algal species during the feeding experiments. The results demonstrated that the dietary quality of phytoplankton with mucilage decreased significantly among the green algae. Green algae without mucilage had high omega-3 fatty acid contents, which increased its dietary quality. The quality of cyanobacteria diet was low for D. pulicaria regardless of the absence or presence of mucilage and fatty acid contents. The results show that the presence of mucilage may have reduced ingestion and digestion by zooplankton and indicate that this algal succession will have a negative impact on lake ecosystems.