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 21 ). 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 x3 : x6 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.

Population structure, life cycle, and trophic niche of the glacial relict amphipod, Gammaracanthus lacustris, in a large boreal lake

“…Our previous research (Hiltunen, Strandberg, Keinänen, Taipale, & Kankaala, 2014;Hiltunen et al, 2015) shows that these fatty acids are generally only found in L. macrurus, and not in other zooplankton taxa or seston. Our previous research (Hiltunen, Strandberg, Keinänen, Taipale, & Kankaala, 2014;Hiltunen et al, 2015) shows that these fatty acids are generally only found in L. macrurus, and not in other zooplankton taxa or seston.…”

“…vertical profiles of oxygen, chlorophyll, and light) are available from authors and in the study byHiltunen et al (2015) More environmental information on the study lake (e.g.…”

Selective transfer of polyunsaturated fatty acids from phytoplankton to planktivorous fish in large boreal lakes

“…Differences in the FA composition of seston (primary producers, heterotrophic bacteria and small heterotrophic flagellates), different zooplankton groups and fish were significant, and the taxonomic grouping explained~72% of the observed variation in the FA composition of the pelagic food web (PERMANOVA Pseudo-F 9,305 = 80.588, P = 0.001). The remaining variation is likely due to seasonal and spatial differences in the FA composition (see Hiltunen et al, 2015;Strandberg et al, 2015 for details). The PUFA with the highest effect on the sample ordination were DHA and ALA.…”

“…Vendace are known to feed extensively on both cladocerans and copepods (Viljanen, 1983;Northcote and Hammar, 2006), presumably also in our study area since the δ 15 N of vendace should be higher if they solely consumed copepods. However, it is well established that cladocerans contain very little DHA, while freshwater copepods in general are rich in DHA (Persson and Vrede, 2006;Smyntek et al, 2008;Ravet et al, 2010;Burns et al, 2011;Hiltunen et al, 2015). Elongation and desaturation of shorter chained analogs (C 18 PUFA or EPA) could account for the high proportion of DHA in vendace.…”

“…For zooplankton and the dorsal muscle tissue of fish, total lipids were extracted with a chloroform:methanol (2:1 v/v) solution, after which the extracted lipids were transmethylated under acidic conditions (1% H 2 SO 4 in methanol) and heated for 16 h at 50°C. FA methyl esters were analyzed gas chromatographically using mass selective detection (details in Hiltunen et al, 2015;Strandberg et al, 2015).…”