Nitrogen isotopes in otoliths reconstruct ancient trophic position

Abstract

The ratio of 15 N/ 14 N( δ 15 N) from consumer and prey tissue is commonly used in ecological studies to determine trophic level, food web structure, and mean trophic level in aquatic ecosystems. There is a predictable positive relationship between the δ 15 N values in tissue and trophic level, caused by the bioaccumulation of 15 N in tissues of consumers with each step up the food chain. Reconstructing trophic structure or food chain length over time may provide resource managers with insights about ecosystem biodiversity and resilience. Yet, in many marine systems the absence of baseline information before anthropogenic disturbances makes comparative stud- ies addressing ecosystem responses extremely diffi- cult. Here we attempt to retrospectively reconstruct trophic position in four species of fish from the upper Gulf of California, Mexico before perturbations such as overfishing or the damming of the Colorado River. We first validated if otolith δ 15 N approximates the δ 15 N observed in fish tissue. We then used the δ 15 N encapsulated in ancient fish otoliths that are between 1,000 and 5,500 years old to define the food web structure. Our results suggested that δ 15 N in otoliths has slightly more positive δ 15 N than soft tissue. The δ 15 N values from ancient otoliths appropriately defined the fishes' relative trophic position. We found significant differences in δ 15 N between functional