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Abstract. Compound-specific stable isotope analysis (CSIA) of individual amino acids (AAs) has become a powerful analytical tool in trophic ecology. Heavily fractionating "trophic" AAs (e.g., glutamic acid: Glu) provide a robust indicator of trophic transfer, while minimally fractionating "source" AAs (e.g., phenylalanine: Phe) closely reflect the δ 15 N value at the base of the food web (δ 15 N baseline ). Together, the CSIA-AA approach provides an unprecedented ability to disentangle the influences of δ 15 N baseline values and trophic fractionation on consumer nitrogen isotope values. Perhaps the most important assumption underlying CSIA-AA applications to trophic ecology is that trophic fractionation of Glu and Phe, and thus the trophic discrimination factor TDF Glu-Phe (Δ 15 N Glu − Δ 15 N Phe ), is effectively constant across diverse consumer-resource relationships. To test this assumption, we conducted a comprehensive meta-analysis of controlled feeding experiments that examined individual AA trophic fractionation (Δ 15 N C-D ) and resulting TDF Glu-Phe values. We found tremendous variability in TDF Glu-Phe values from 0‰ to >10‰ across 70 species (317 individuals) and 88 distinct consumer-diet combinations. However, this variability appears to follow predictable patterns driven by two dominant variables: diet quality and mode of nitrogen excretion. Consumers feeding on high-quality diets (small diet-consumer AA imbalances) tend to have significantly lower TDF Glu-Phe values than consumers feeding on low-quality diets. Similarly, urea/uric acid-producing consumers also exhibit significantly lower TDF Glu-Phe values than their ammonia-producing counterparts. While these patterns are certainly not universal, together these factors likely explain many of the observed patterns of TDF Glu-Phe variability. We provide an overview of the biochemical and physiological mechanisms underpinning AA Δ 15 N C-D to explain these patterns. There are several seemingly unique systems, including the remarkably consistent TDF Glu-Phe values across insect food webs and the isotopically "invisible" trophic transfers in microbial food webs, that may provide additional insight into the influence of diet quality and nitrogen cycling on AA fractionation. In this review, we argue that to realize the full potential of CSIA-AA approaches in trophic ecology, we must embrace the variability in TDF Glu-Phe values. This likely requires developing new models of trophic transfer dynamics for some applications, including multi-TDF Glu-Phe equations that directly incorporate variability in TDF Glu-Phe value.

Ecogeochemistry-the application of geochemical techniques to fundamental questions in population and community ecology-has been used in animal migration studies in terrestrial environments for several decades; however, the approach has received far less attention in marine systems. This review includes comprehensive meta-analyses of organic zooplankton d 13 C and d 15 N values at the base of the food web, dissolved inorganic carbon d 13 C values, and seawater d 18 O values to create, for the first time, robust isoscapes for the Atlantic Ocean. These isoscapes present far greater geographic variability in multiple geochemical tracers than was previously thought, thus forming the foundation for reconstructions of habitat use and migration patterns of marine organisms. We review several additional tracers, including trace-element-to-calcium ratios and heavy element stable isotopes, to examine anadromous migrations. We highlight the value of the ecogeochemistry approach by examining case studies on three components of connectivity: dispersal and natal homing, functional connectivity, and migratory connectivity. We also discuss recent advances in compound-specific stable carbon and nitrogen isotope analyses for tracking animal movement. A better understanding of isotopic routing and fractionation factors, particularly of individual compound classes, is necessary to realize the full potential of ecogeochemistry.

Summary1. Analysis of stable carbon isotopes is a valuable tool for studies of diet, habitat use and migration. However, significant variability in the degree of trophic fractionation (D 13 C C-D ) between consumer (C) and diet (D) has highlighted our lack of understanding of the biochemical and physiological underpinnings of stable isotope ratios in tissues.2. An opportunity now exists to increase the specificity of dietary studies by analyzing the d C C-D to variability in protein content and AA composition of the diet as well as differential utilization of dietary constituents contributing to the bulk carbon pool. This variability illustrates the complicated nature of metabolism and suggests caution must be taken with the assumptions used to interpret bulk stable isotope data in dietary studies. 6. Our study is the first to investigate the expression of AA D 13 C C-D values for a marine vertebrate and should provide for significant refinements in studies of diet, habitat use and migration using stable isotopes.