Spatial and Temporal Patterns of <i>Gammarus</i> sp. in Lowland Streams Analyzed Through Amino Acid Isotope Analysis

ABSTRACT Understanding the trophic ecology and nutrient dynamics of freshwater invertebrates is crucial for evaluating the function and resilience of ecosystems. Although bulk stable isotope analysis is widely used to investigate food web structures, it lacks the resolution required to differentiate between carbon and nitrogen sources at a molecular level. Compound‐specific isotope analysis (CSIA) of amino acids (AAs) offers a more detailed approach; however, its application in freshwater ecosystems, particularly across spatial and temporal gradients, remains limited. This study addresses this issue by applying AA‐specific δ 15 N and δ 13 C analysis to the key detritivore Gammarus spp. across the North Rhine‐Westphalia catchment in Germany over 4 years (2016, 2018, 2020, and 2022). Significant spatial variation in δ 13 C values was observed, while temporal trends were less pronounced. Nonessential AAs exhibited higher δ 13 C values than essential AAs, with glycine being the most enriched in 13 C and lysine and phenylalanine the most depleted. Principal component analysis (PCA) of δ 13 C values revealed variability in carbon sourcing, reflecting shifts between primary producers, microbial inputs, and detrital material. Regarding δ 15 N, trophic AAs such as leucine and glutamic acid were enriched, whereas source AAs such as phenylalanine were depleted, confirming their roles in nitrogen metabolism. PCA of δ 15 N values revealed that some sites maintained stable nitrogen inputs from autotrophic sources, while others showed increasing contributions from microbial or detrital nitrogen over time. Trophic position (TP) estimates for Gammarus spp. were consistent across sites and years, ranging from 1.1 to 1.5 using the TP Glu‐Phe method and from 1.4 to 1.8 using the TP 5AA method. This confirms their role as primary consumers. These findings emphasize the ecological importance of Gammarus spp. in nutrient cycling and demonstrate the effectiveness of CSIA‐AA in unraveling complex trophic interactions and food web dynamics.