Whereas it is acknowledged that the C:N:P stoichiometry of consumers and their resources affects both the structure and the function of food webs, and eventually influences broad‐scale processes such as global carbon cycles, the mechanistic basis for the variation in stoichiometry has not yet been fully explored. Empirical evidence shows that the specific growth rate is positively related to RNA concentration both between and within taxa in both unicellular and multicellular organisms. Since RNA is rich in P and constitutes a substantial part of the total P in organisms, a high growth rate is also connected with a high P content. We argue that the reason for this pattern is that the growth of all biota is closely linked with their protein synthesis rate, and thus with the concentration of ribosomal RNA. Dynamic energy budget theory supports the positive relationship between RNA and specific growth rate in microorganisms, whereas the predictions concerning multicellulars only partially agrees with the observed pattern. In a simple model of consumer growth, we explore the consequences of various allocation patterns of RNA, protein, carbohydrates/lipids, and other biochemical constituents on organism potential growth rate and C:N:P stoichiometry. According to the model the percentage of N and especially percentage of P per dry mass increases with increasing specific growth rate. Furthermore, the model suggests that macromolecule allocation patterns and thus N:P stoichiometry are allowed to differ substantially at low growth rates whereas the stoichiometry at high growth rates is much more constricted at low N:P. The model fits empirical data reasonably well, but it is also acknowledged that complex life cycles and associated physiological constraints may result in other patterns. We also use a similar approach of modeling organism growth from basic biochemical principles to illustrate fundamental connections among biochemical allocation and C:N stoichiometry in autotroph production, which is based on allocation patterns between carbohydrates and rubisco. Similar to the RNA–protein model, macromolecular composition and C:N ratios are more constrained at high than at low growth rates. The models and the empirical data together suggest that organism growth is tightly linked with the organisms' biochemical and elemental composition. The stoichiometry of growth impinges on nutrient cycles and carbon fluxes at the ecosystem level. Thus, focus on the biological basis of organism C:N:P stoichiometry can mechanistically connect growth strategy and biochemical and cellular mechanisms of biota to major ecological consequences.

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial-aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)-the messenger between terrestrial and lake ecosystems-with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.

1. Food quality has major effects on the transfer of energy and matter in food webs, and essential long-chained polyunsaturated fatty acids (PUFAs) can affect the quality of phytoplankton as food. In a study of oligotrophic lakes in north-western Sweden, we investigated the fatty acid composition of four planktonic cladocerans and two calanoid copepods, representing herbivorous and carnivorous species. We also collected seston samples. 2. The proportions of long-chain PUFAs in the organisms increased with their increasing trophic position. Thus, both their quality as food for other organisms, as well as their requirement for fatty acids (FAs), differed among taxa and depended on their trophic position. 3. We found taxon-specific differences in the FA composition of zooplankton that were not related to sestonic FA composition. This implies that the variation in zooplankton FA composition is constrained by phylogenetic origin, life history characteristics, or both. 4. The cladoceran taxa contained 12-23% eicosapentaenoic acid (EPA) but only 0.9-2.1% docosahexaenoic acid (DHA) of the total FA content. In contrast, the calanoid copepods contained 7-11% EPA and 14-21% DHA. Thus, our results show that differences in the PUFA content among zooplankton species could have repercussions for both food web structure and function.

Summary 1. Fatty acids (FAs) have been widely applied as trophic biomarkers in aquatic food web studies. However, current knowledge of inter‐ and intraspecific variation in consumer FA compositions across spatial and temporal scales is constrained to a few pelagic taxa. 2. We analysed the FAs of 22 taxa of benthic macroinvertebrates, zooplankton and fish collected from the littoral, pelagic and profundal habitats of nine boreal oligotrophic lakes over spring, summer and autumn. We quantified and compared the FA variance partitions contributed by species identity (i.e. an integrative effect of phylogenetic origin, life history and functional feeding guild of individual taxa), site and season using partial redundancy analysis both on all consumers and on benthic arthropods alone. 3. Species identity alone contributed 84.4 and 72.8% of explained FA variation of all consumers and benthic arthropods, respectively. Influences of site, season and all joint effects accounted for 0–11.3% only. Fatty acid profiles of primary consumers differentiated below class level, but those of predators were distinguishable only when they became more taxonomically distinct (i.e. among classes or higher). 4. Pelagic and profundal consumers showed stronger reliance on autochthonous resources than did their littoral counterparts as reflected by their higher ω3 to ω6 FA ratios. Polyunsaturated FAs (PUFAs) were increasingly retained with trophic levels, and saturated FAs (e.g. FA 16 : 0) gradually reduced. Ecologically, this trade‐off enhances the trophic transfer efficiency and confirms the importance of PUFA‐rich autotrophs in aquatic food webs. 5. Our findings indicate strong interspecific differences in FA requirements and assimilation among aquatic consumers from a wide range of taxonomic levels, habitats and lakes. Consumers were able to maintain homoeostasis in FA compositions across spatial and temporal changes in resource FAs, but consumer homoeostasis did not limit the effectiveness of FAs as trophic biomarkers.

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