Bottom-up processes drive isotopic variation in the South American sea lion Otaria flavescens across a 2300 km latitudinal gradient

Spatial differences in the isotope values of widely distributed marine apex consumers may reflect geographical differences in the isotopic composition of basal resources (e.g., phytoplankton) fueling food webs (bottom-up effects) or spatial differences in the trophic ecology of the taxon of interest (top-down effects). We examined spatial variation in δ¹³C and δ¹⁵N values from 264 South American sea lions (SASL, Otaria flavescens) of different age classes (adults, subadults and juveniles), their putative prey consisting of pelagic and benthic coastal fishes, and particulate organic matter (POM) measured from locations situated across >2300 km of the Chilean coast (between 18°42′ and 39°17′ S). We used generalized least squares (GLS) models to compare the form of the relationship between δ¹³C and δ¹⁵N and latitude between the three functional groups. Our results show that SASL from northern, central, and southern areas were isotopically distinct, with individuals from the north having lower δ¹³C and higher δ¹⁵N values in comparison to individuals from the south. When the relationship for each functional group was modelled individually using GLS, results indicated that for each degree of increasing latitude δ¹⁵N decreased on average by 0.12‰ (POM), 0.15‰ (prey), and 0.14‰ (SASL), while δ¹³C increased by 0.06‰ (POM) and 0.05‰ in both prey and SASL. We suggest that the latitudinal differences observed in SASL δ¹³C and δ¹⁵N values reflect baseline isotopic variation rather than marked differences in trophic ecology of these widely distributed consumers.