Abstract

Stable isotope analysis is a powerful tool that is widely used in modern marine ecology. However, its utility depends on researchers accounting for spatial and temporal variability in baseline values, e.g., for the correct interpretation of food web dynamics or animal movement in marine ecosystems. Here, we present the first observation-based delta C-13 and delta N-15 isoscapes for particulate organic matter (POM) in the southeastern Pacific, spanning similar to 2300 km of the Chilean coastline. POM samples were collected during four research cruises (2017-2018) across three latitudinal zones. Using a Bayesian hierarchical modeling framework, we predicted spatial variation in delta(13)C(POM )and delta(15)N(POM )values based on in situ and satellite-derived environmental data. Because temporal overlap among stations was limited, these predictions represent integrated isotopic baseline estimates for 2017-2018 rather than season-specific surfaces. Results revealed a latitudinal gradient, with delta C-13 values increasing and delta N-15 values decreasing with latitude (i.e., southward). Best-fit models explained 61% (delta C-13) and 44% (delta N-15) of spatial variability. Chlorophyll-a concentration emerged as the most consistent environmental predictor across both isoscapes, emphasizing the importance of primary productivity in shaping isotopic baselines. Salinity was additionally associated with spatial variation in delta N-15, suggesting an influence of water-mass structure and regional nitrogen cycling processes. Spatially structured variation captured by the SPDE component further indicated that broad-scale oceanographic processes contributed to isotopic patterns beyond the measured environmental covariates. These findings demonstrate that isotopic baselines vary systematically along the Chilean coast and reveal contrasting environmental controls on carbon and nitrogen isotope dynamics within the Humboldt Current System. The resulting isoscapes provide an important baseline for studies of trophic ecology, animal movements, ecosystem connectivity, and geographic assignment in the Southeast Pacific.