Nitrogen Isotopes Ratios in Marine/Estuarine Skeletal Remains Reconstruct Ancient Food Chain

The ratio of 15N/14N (d15N) 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 relationship between the d15N values of predators and their prey; a 3.4‰ increase (+/-1‰SD) in d15N with each step up the food chain (Post 2002). Tracking food chain length or mean trophic level of the predators in a system over time provides resource managers with insights about ecosystem biodiversity and resilience. Yet, in many marine systems the absence of baseline information before anthropogenic disturbances makes comparative studies addressing ecosystem responses extremely difficult. We test if we can resolve these gaps in baseline information using organic matter encapsulated in ancient fish otoliths and clam shells to define the food web structure in the Northern Gulf of California before perturbations such as overfishing or the damming of the Colorado River. This approach uses the remains from thousands of years ago to provide the first baseline assessment of the food chain length from before human alterations to the system. While only one other study has reported d15N values for otoliths, our preliminary results from a laboratory calibration agrees with their findings that on average otoliths are 1.1 ‰ lower compared to muscle tissue (Vandermyde and Whitledge 2008). Our results from otoliths of three fish species and clam shells demonstrate that nitrogen in aragonite remains can track food web structure. A past/present comparison of nitrogen isotope ratios in otoliths of Totoaba, a top predator fish in the upper Gulf, suggest that one full trophic level has been removed from the food chain since human alteration to the Colorado River Estuary.