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.
We studied biogeochemical characteristics, including organic carbon and nitrogen contents, fatty acid (FA) composition, stable isotope ratios, and primary production in conjunction with species composition of bacterioplankton, using next generation sequencing, in the Yenisei River along a distance ∼1800 km. Basing on FA composition of particulate organic matter (POM) and on other indicators of sources of POM, the river was subdivided into four sections. The upper section 1, situated in mountain region, was the net source of high‐quality autochthonous organic matter, produced primarily by diatoms and partly consumed by specialized bacteria species. Section 2 in plain taiga was net sink of high quality allochthonous and autochthonous organic matter, produced by cyanobacteria and green algae. Section 3 was net sink of organic matter, primarily allochthonous, consumed by the specialized species of bacteria. The lowest section 4, situated in tundra, was primarily the conduit of recalcitrant terrestrial organic matter, but also the net source of autochthonous organic matter, produced by diatoms. Biogeochemical traits of sections of the Yenisei River evidently shaped dominant species composition of bacterioplankton of these sections. Regarding the biogeochemical and microbiological data, we concluded that the Yenisei River ecosystem complexly combines features of river mosaic, river continuum, and “neutral pipe.”
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