Abstract Carbon isotope fingerprinting, or multivariate analysis using δ 13 C values of individual compounds, is a powerful tool in ecological studies, particularly measurements of essential amino acids (EAA δ 13 C). Despite the widespread application of this technique, there has been little methodological validation to determine (a) whether multivariate EAA δ 13 C signatures (fingerprints) of primary producer groups vary across space and time and (b) what biochemical mechanisms drive these patterns. Here, we evaluate the spatiotemporal consistency in EAA δ 13 C fingerprints among nearshore primary producers: Chlorophyta ( Ulva sp.), Ochrophyta (kelps), particulate organic matter (POM) and phytoplankton, and Rhodophyta. We analysed 135 samples from 14 genera collected in Alaska, California and Chile. The collections included historical museum samples (1896–1980 CE) of the giant kelp, Macrocystis pyrifera . We employed canonical analysis of principal coordinates and generalized linear models (GLMs) to, respectively, characterize isotopic fingerprints and evaluate the effect of taxonomy, local upwelling regimes, ecological setting, and time on individual EAA δ 13 C values and associated fingerprints. We also calculated amino acid discrimination values (∆ 13 C) to identify key biochemical pathways responsible for these patterns. We found remarkable consistency in EAA δ 13 C fingerprints of marine algae across space and through time. Kelps and rhodophytes exhibited statistically distinct multivariate isotopic patterns regardless of geographical location, species identity or time (kelps). In contrast, isotopic fingerprints of POM/phytoplankton and Ulva overlapped substantially. GLMs indicated that producer family, presumably due to the presence/absence of carbon concentrating mechanisms, and site locality are important determinants of individual amino acid δ 13 C values. Taxonomy was also a key variable for EAA δ 13 C fingerprints. The calculated discrimination values suggest variation in (a) metabolism of pyruvate and oxaloacetate‐derived amino acids and (b) production of storage and structural carbohydrates are responsible for taxonomic differences in isotopic fingerprints. We conclude EAA δ 13 C fingerprinting is a robust method for tracing the contribution of diverse primary producer taxa to coastal food webs. We show that this technique can be applied to modern and historical samples, as well as consumers collected across continental scales. The high fidelity of EAA δ 13 C multivariate patterns coupled with biochemical mechanisms provides a powerful framework for future studies of carbon flow across broad biogeographical and ecological contexts. Read the free Plain Language Summary for this article on the Journal blog.