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

Abstract

There is increasing evidence that rare species play an important role in trophic interactions, but how rare species with low biomass disproportionately function 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 biomasses of rare phytoplankton species signi cantly determine the concentrations of particulate PUFAs (polyunsaturated 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 particulate fatty acids concentrations in tropical reservoirs and lakes. The model predicted that the dominant phytoplankton species determined the concentration of saturated particulate fatty acids and that rare phytoplankton species were the main determinant for PUFAs. In particular, the biomass of the rare species Scenedesmus obliquus was a key variable explaining the variations of ALA (α-linolenic acid), LIN (α-linoleic acid), GLA (γ-linolenic acid) and EPA (eicosapentaenoic acid) concentrations in the studied waterbodies. We conclude that the population dynamics of rare phytoplankton species can de ne the food quality associated with PUFAs for consumers and thus play a critical role in the trophic transfer in the food webs of tropical waterbodies.