Taxonomic identity and phytoplankton diet affect fatty acid composition of zooplankton in large lakes with differing dissolved organic carbon concentration

Abstract

An increase in loading of dissolved organic carbon (DOC) from terrestrial to freshwater ecosystems has recently been documented in several boreal regions. We studied how transfer of important biomolecules, fatty acids, may be connected to DOC in pelagic food webs of large boreal lakes. We collected nine zooplankton taxa during three seasons from six lakes along a gradient of DOC (5–10 mg C L−1). Taxonomic identity explained 62% of the variation in zooplankton fatty acids, while the effects of DOC and season were less pronounced. This suggests that the community composition of zooplankton is important in determining the quality of food available for higher trophic level consumers. Seasonal differences in fatty acid composition were most evident in the taxa that overwinter in an active stage, and especially C18 polyunsaturated fatty acids (PUFA) accumulated toward fall. Small part of the within–taxa variation in zooplankton fatty acids was related to lake DOC concentration. However, the proportion of fatty acids indicative of bacteria or terrestrial vegetation were not linked to DOC. Instead, DOC was connected to PUFA, with zooplankton in high DOC lakes having higher ω3 : ω6 ratios. We propose that this is due to decrease of chlorophytes in phytoplankton community with increasing DOC concentration. Our results with significant among–taxa differences in zooplankton fatty acids imply that terrestrial organic matter may affect the transfer of fatty acids more profoundly if it modifies the zooplankton community composition. However, shifts in the phytoplankton community structure could also cause smaller–scale changes in zooplankton fatty acid composition.