Fatty acid composition of Cladocera and Copepoda from lakes of contrasting temperature

We studied the fatty acid (FA) composition of six species of Cladocera and six species of Copepoda from five cold-water lakes, situated in the tundra and/or in the mountains, and eight species of Cladocera and four species of Copepoda from eight warm-water lakes (including one reservoir) in temperate regions. We asked whether the contrasting temperature would result primarily simply in changes in the percentages (i.e. percentage of total FAs) and absolute contents (quantities) of the long-chain polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA), or whether there are other FAs with various number of double bonds and/or chain lengths which could be responsible for a putative homeoviscous adaptation. We also aimed to reveal any consistent phylogenetic differences in FA percentages and contents between Cladocera and Copepoda, separable from any temperature effects. Both taxa in warm waters had greater percentages of 18:0, and lower percentages of 14:0 and 18:4n-3, than in cold waters, but there were no differences in percentages of DHA. In addition, Cladocera, besides the lower percentage of EPA, had higher percentages of 20:0 and 22:0 in warm waters. These patterns in the percentages of 14:0, 18:0, 18:4n-3, 20:0 and 22:0 are in a good agreement with the hypothesis of homeoviscous adaptation. Thus, the role of EPA, and particularly DHA, as unique regulators of the homeoviscous adaptation of the zooplankton may have been overestimated. Overall, we confirmed the known differences between Cladocera and Copepoda, namely higher percentages of EPA in Cladocera and higher percentages of DHA in Copepoda. However, there was c.50% overlap in the ranges of the percentage of EPA in Cladocera and Copepoda, while the ranges in the content of EPA per unit organic carbon in Cladocera and Copepoda overlapped completely. Differences in the percentages and content of DHA between Cladocera and Copepoda were statistically significant and invariant with temperature, and therefore are probably due to phylogenetic factors, rather than any temperature adaptation. Contrasting temperature was not associated with significant differences in the contents of EPA and DHA per unit of organic carbon within the taxa studied. If this remained the case in a warming climate, such warming would be unlikely to reduce the accumulation of these important PUFAs in the zooplankton, at least if species composition was unchanged. However, if there were shifts in the proportions of Cladocera and Copepoda in the zooplankton, for example fewer copepods as temperature rises, a decrease of the flux of PUFA in the ecosystem is plausible, taking into account the phylogenetic (and temperature invariant) differences in DHA between the two groups.