Fatty acid stable isotopes add clarity, but also complexity, to tracing energy pathways in aquatic food webs

Impact of Glaciers on Trophic Dynamics and Polyunsaturated Fat Accumulation in Southern Greenland Fjord Ecosystems

The primary production of fjords across the Arctic and Subarctic is undergoing significant transformations due to the climatically driven retreat of glaciers and ice sheets. However, the implications of these changes for upper trophic levels remain largely unknown. In this study, we employ both bulk and compound-specific stable isotope analyses to investigate how shifts at the base of fjord food webs impact the carbon and energy sources of consumers. Focusing on two rapidly changing fjords in Southern Greenland, we used the migratory Arctic char as an indicator species, sampling populations along environmental gradients within the fjords, building upon the assumption that char populations feed primarily close to their natal stream, thereby integrating a dietary gradient. Our analysis of bulk stable isotopes in Arctic char tissue confirmed this premise, revealing a consistent change in resource use from the outer to the inner fjord, which nonetheless served as preferred feeding grounds. Essential amino acid analysis further indicated shifts in carbon and nitrogen sources, consistent with changes in nutrient use near glacier inputs characterized by low turbidity and high iron levels. Notably, these changes in the source of primary production were associated with shifts in trophic positions and the transfer of polyunsaturated fatty acids, with Arctic char in glacier-influenced inner fjords feeding at lower trophic level (size-corrected) and accumulating higher levels of high-quality docosahexaenoic acid (DHA). These findings highlight the usefulness of new analytical tools in revealing that glacial retreat can substantially alter food web dynamics, enhancing both carbon flow and the nutritional quality of fish in fjord ecosystems. The two Southern Greenland fjords studied could represent the future of other fjords, where retreating glaciers become land-terminating and glacial inputs decrease. Our study underscores the critical role of glacier dynamics in affecting high-level consumers, such as salmonids, with implications for fjords globally.

Quantifying energy and nutrient fluxes in coral reef food webs

The movement of energy and nutrients through ecological communities represents the biological 'pulse' underpinning ecosystem functioning and services. However, energy and nutrient fluxes are inherently difficult to observe, particularly in high-diversity systems such as coral reefs. We review advances in the quantification of fluxes in coral reef fishes, focusing on four key frameworks: demographic modelling, bioenergetics, micronutrients, and compound-specific stable isotope analysis (CSIA). Each framework can be integrated with underwater surveys, enabling researchers to scale organismal processes to ecosystem properties. This has revealed how small fish support biomass turnover, pelagic subsidies sustain fisheries, and fisheries benefit human health. Combining frameworks, closing data gaps, and expansion to other aquatic ecosystems can advance understanding of how fishes contribute to ecosystem functions and services.

Chytrids enhance Daphnia fitness by selectively retained chytrid-synthesised stearidonic acid and conversion of short-chain to long-chain polyunsaturated fatty acids

Chytrid fungal parasites convert dietary energy and essential dietary molecules, such as long-chain (LC) polyunsaturated fatty acids (PUFA), from inedible algal/cyanobacteria hosts into edible zoospores. How the improved biochemical PUFA composition of chytrid-infected diet may extend to zooplankton, linking diet quality to consumer fitness, remains unexplored.Here, we assessed the trophic role of chytrids in supporting dietary energy and PUFA requirements of the crustacean zooplankton Daphnia, when feeding on the filamentous cyanobacterium Planktothrix.Only Daphnia feeding on chytrid-infected Planktothrix reproduced successfully and had significantly higher survival and growth rates compared with Daphnia feeding on the sole Planktothrix diet. While the presence of chytrids resulted in a two-fold increase of carbon ingested by Daphnia, carbon assimilation increased by a factor of four, clearly indicating enhanced carbon transfer efficiency with chytrid presence.Bulk carbon (δ 13C) and nitrogen (δ 15N) stable isotopes did not indicate any treatment-specific dietary effects on Daphnia, nor differences in trophic position among diet sources and the consumer. Compound-specific carbon isotopes of fatty acids (δ 13CFA), however, revealed that chytrids bioconverted short-chain to LC-PUFA, making it available for Daphnia. Chytrids synthesised the ω-3 PUFA stearidonic acid de novo, which was selectively retained by Daphnia. Values of δ 13CFA demonstrated that Daphnia also bioconverted short-chain to LC-PUFA.We provide isotopic evidence that chytrids improved the dietary provision of LC-PUFA for Daphnia and enhanced their fitness. We argue for the existence of a positive feedback loop between enhanced Daphnia growth and herbivory in response to chytrid-mediated improved diet quality. Chytrids upgrade carbon from the primary producer and facilitate energy and PUFA transfer to primary consumers, potentially also benefitting upper trophic levels of pelagic food webs.