Abstract
Abstract Organisms at the base of aquatic food webs synthesize essential nutrients, such as omega‐3 polyunsaturated fatty acids (n‐3 PUFA), which are transferred to consumers at higher trophic levels. Many consumers, requiring n‐3 long‐chain (LC) PUFA, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have limited ability to biosynthesize them from the essential dietary precursor α‐linolenic acid (ALA) and thus rely on dietary provision of LC‐PUFA. We investigated LC‐PUFA metabolism in freshwater zooplankton using stable hydrogen isotopes (δ2H) of fatty acids as tracers. We conducted feeding experiments with the freshwater keystone grazer Daphnia to quantify changes in the δ2H value of body FA in response to the FA composition of their food and the δ2H value of the ambient water. The isotopic composition of LC‐PUFA changed in Daphnia, depending on the integration of 2H from ambient water during de novo synthesis or bioconversion from dietary precursors, allowing us to distinguish dietary from bioconverted EPA in body tissue. We tested the applicability of these laboratory findings in a field setting by analysing δ2H values of PUFA in primary producers and consumers in eutrophic ponds to track EPA sources of zooplankton. Multilinear regression models that included conversion of ALA to EPA correlated better with zooplankton δ2HEPA than seston δ2HEPA at low dietary EPA supply. This study provides evidence that zooplankton can compensate for low dietary EPA supply by activating LC‐PUFA biosynthesis and shows that herbivorous zooplankton play a crucial role in upgrading FA for higher trophic levels during low dietary EPA supply. A free Plain Language Summary can be found within the Supporting Information of this article.
Highlights
Tracing and quantifying the transfer of dietary energy and nutrients within aquatic food webs is critical for understanding trophic interactions among species
This study provides evidence that zooplankton can compensate for low dietary eicosapentaenoic acid (EPA) supply by activating LC-PUFA biosynthesis and shows that herbivorous zooplankton play a crucial role in upgrading Fatty acids (FA) for higher trophic levels during low dietary EPA supply
Omega-3 polyunsaturated fatty acids (n-3 PUFA), such as α-linolenic acid (ALA; 18:3n-3), eicosapentaenoic acid (EPA; 20:5n-3) or docosahexaenoic acid (DHA; 22:6n-3), are important for trophic studies (Carlson & Neuringer, 1999), since most consumers are unable to synthesize them de novo, that is, from low-molecular-weight precursors
Summary
Tracing and quantifying the transfer of dietary energy and nutrients within aquatic food webs is critical for understanding trophic interactions among species. Fatty acids (FA) are increasingly used as specific dietary source biomarkers in aquatic consumers and for disentangling autochthonous versus allochthonous diet sources (Nielsen et al, 2018). Models from controlled experiments need to take food limitations into account, which might change the FA profiles of consumers (Taipale et al, 2015; Twining et al, 2020). Various animals have been shown to be capable of elongating and desaturating dietary C18 PUFA to C20 PUFA to adapt their n-3 PUFA profile to their specific physiological needs, which is a complex process involving several enzymes and comes at higher energy costs than direct dietary acquisition (Taipale et al, 2011; Twining et al, 2016)
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