One of the central paradigms of ecology is that only about 10% of organic carbon production of one trophic level is incorporated into new biomass of organisms of the next trophic level. Many of energy-yielding compounds of carbon are designated as 'essential', because they cannot be synthesized de novo by consumers and must be obtained with food, while they play important structural and regulatory functions. The question arises: are the essential compounds transferred through trophic chains with the same efficiency as bulk carbon? To answer this question, we measured gross primary production of phytoplankton and secondary production of zooplankton and content of organic carbon and essential polyunsaturated fatty acids of ω-3 family with 18-22 carbon atoms (PUFA) in the biomass of phytoplankton and zooplankton in a small eutrophic reservoir during two summers. Transfer efficiency between the two trophic levels, phytoplankton (producers) and zooplankton (consumers), was calculated as ratio of the primary production versus the secondary (zooplankton) production for both carbon and PUFA. We found that the essential PUFA were transferred from the producers to the primary consumers with about twice higher efficiency than bulk carbon. In contrast, polyunsaturated fatty acids with 16 carbon atoms, which are synthesized exclusively by phytoplankton, but are not essential for animals, had significantly lower transfer efficiency than both bulk carbon, and essential PUFA. Thus, the trophic pyramid concept, which implicitly implies that all the energy-yielding compounds of carbon are transferred from one trophic level to the next with the same efficiency of about on average 10%, should be specified for different carbon compounds.

Emerging aquatic insects, including mosquitoes, are known to transfer to terrestrial ecosystems specific essential biochemicals, such as polyunsaturated fatty acids (PUFA). We studied fatty acid (FA) composition and contents of dominant mosquito populations (Diptera: Culicidae), that is, Anopheles messeae, Ochlerotatus caspius, Oc. flavescens, Oc. euedes, Oc. subdiversus, Oc. cataphylla, and Aedes cinereus, inhabited a steppe wetland of a temperate climate zone to fill up the gap in their lipid knowledge. The polar lipid and triacylglycerol fractions of larvae and adults were compared. In most studied mosquito species, we first found and identified a number of short-chain PUFA, for example, prominent 14:2n-6 and 14:3n-3, which were not earlier documented in living organisms. These PUFA, although occurred in low levels in adult mosquitoes, can be potentially used as markers of mosquito biomass in terrestrial food webs. We hypothesize that these acids might be synthesized (or retroconverted) by the mosquitoes. Using FA trophic markers accumulated in triacylglycerols, trophic relations of the mosquitoes were accessed. The larval diet comprised green algae, cryptophytes, and dinoflagellates and provided the mosquitoes with essential n-3 PUFA, linolenic, and eicosapentaenoic acids. As a result, both larvae and adults of the studied mosquitoes had comparatively high content of the essential PUFA. Comparison of FA proportions in polar lipids versus storage lipids shown that during mosquito metamorphosis transfer of essential eicosapentaenoic and arachidonic acids from the reserve in storage lipids of larvae to functional polar lipids in adults occurred.

Summary 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.