Ecological importance of rare phytoplankton species revealed by their high contribution to unsaturated fatty acids in tropical waters

There is increasing evidence that rare species play an important role in trophic interactions, but the function of rare species with low biomass in these processes remains unclear. Phytoplankton is placed at the base of lentic and marine food webs and is characterised by a few dominant species and many rare species. While the dominant species contribute most to the primary production, they are often low-quality food for primary consumers. The rare species may instead provide the essential biochemical nutrients for consumers, especially in eutrophic waters. We hypothesised that the biomass of rare eukaryotic phytoplankton species significantly determines the concentrations of sestonic long-chain highly unsaturated fatty acids, directly linking them to the functioning of aquatic ecosystems. We applied redundancy analysis and Lasso regression models to identify the species whose population dynamics explain the variations of sestonic fatty acids concentrations in tropical reservoirs and lakes. The lasso models predicted that the dominant phytoplankton species determined the concentration of saturated sestonic fatty acids and that rare phytoplankton species were the main determinant for polyunsaturated fatty acids, which are critical for the food quality of consumers such as zooplankton and promote the energy transfer from primary producers to higher trophic levels in natural waters. In particular, the biomass of the rare species Scenedesmus obliquus was a key variable explaining the variations of alpha-linolenic acid, alpha-linoleic acid, gamma-linolenic acid, and eicosapentaenoic acid concentrations. We conclude that the population dynamics of rare phytoplankton species can define the food quality associated with eicosapentaenoic acid for consumers and thus play a critical role in the trophic transfer in the food webs of tropical waterbodies.