Hee Young Yun

and 4 more

Explaining food web dynamics, stability, and functioning depend substantially on understanding of feeding relations within a community. Bulk stable isotope ratios (SIRs) in natural abundance are well-established tools to express direct and indirect feeding relations as continuous variables across time and space. Along with bulk SIRs, the SIRs of individual amino acids (AAs) are now emerging as a promising and complementary method to characterize the flow and transformation of resources across a diversity of organisms, from microbial domains to macroscopic consumers. This significant AA-SIR capacity is based on empirical evidence that a consumer’s SIR, specific to an individual AA, reflects its diet SIR coupled with a certain degree of isotopic differences between the consumer and its diet. However, many empirical ecologists are still unfamiliar with the scope of applicability and the interpretative power of AA-SIR. To fill these knowledge gaps, we here describe a comprehensive approach to both carbon and nitrogen AA-SIR assessment focusing on two key topics: pattern in AA-isotope composition across spatial and temporal scales, and a certain variability of AA-specific isotope differences between the diet and the consumer. On this basis we review the versatile applicability of AA-SIR to improve our understanding of physiological processes as well as food web functioning, allowing us to reconstruct dominant basal dietary sources and trace their trophic transfers at the specimen and community levels. Given the insightful and opportunities of AA-SIR, we suggest future applications for the dual use of carbon and nitrogen AA-SIR to study more realistic food web structures and robust consumer niches, which are often very difficult to explain in nature.

Thomas Larsen

and 2 more

Marine food webs are highly compartmentalized and characterizing the trophic niches among consumers is important for predicting how impact from human activities affect the structuring and functioning of marine food webs. Biomarkers such as bulk stable isotopes have proven to be powerful tools to elucidate trophic niches, but they may lack in resolution, particularly when spatio-temporal variability in a system is high. To close this gap, we investigated whether carbon isotope (δ13C) patterns of essential amino acids (EAAs), also termed δ13AA fingerprints, can characterize niche differentiation in a highly dynamic marine system. We tested the ability of δ13AA fingerprints to differentiate trophic niches among six functional groups and ten individual species in the Baltic Sea. We also tested whether fingerprints of the common zooplanktivorous fishes, herring and sprat, differ among four Baltic Sea regions with different biochemical conditions and phytoplankton assemblages. Additionally, we investigated how these results compared to bulk C and N isotope data for the same sample set. We found significantly different δ13AA fingerprints among all six functional groups. Species differentiation was in comparison less distinct, due to partial convergence of the species’ fingerprints within functional groups. Herring and sprat displayed region specific δ13AA fingerprints indicating that this approach could be used as a migratory marker. Bulk isotope data had a lower power to differentiate between trophic niches, but may provide more easily interpretable information about relative trophic position than the fingerprints. We conclude that δ13AA fingerprinting has a strong potential to advance our understanding of ecological niches, and trophic linkages from producers to higher trophic levels in dynamic marine systems. Given how management practices of marine resources and habitats are reshaping the structure and function of marine food webs, implementing new and powerful tracer methods are urgently needed to improve the knowledge base for policy makers.