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Stable isotopes of amino acids from reef fishes uncover Suess and nitrogen enrichment effects on local ecosystems

A. J. M. Sabadel*, L. M. Durante, S. R. Wing

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ABSTRACT: In 1979, the “Suess effect” was described as decreasing δ13C of the oceans linked to anthropogenic CO2 emissions. After years of over-fertilization of farming soils and runoff, we hypothesized that δ15N in coastal environments would also decline, whereby synthetic fertilizers lead to depletion of the heavy isotope 15N. We used museum preserved and modern samples of three fishes from Otago, New Zealand, to reconstruct the isotopic baselines of C and N, and assess specific trophic positions through time (1955 to present) based on bulk and amino acid stable isotope values. Our sample set included Odax pullus, a strictly herbivorous species, and two commercially important species: Nemadactylus macropterus and Parapercis colias. Muscle tissue of the fishes recorded the change in δ13CBulk through time, which matched estimated Suess effect values for New Zealand. We also resolved the effects on the C isotopic baseline from natural changes in the food web using analysis of δ13CEAA and found that while P. colias kept a steady diet, the food web position of N. macropterus likely changed. Analysis of δ15NPhe of O. pullus indicated a decrease of 0.023‰ per year since 1955, which corroborates our coastal N enrichment hypothesis. Furthermore, we found that isotopic changes for N. macropterus were consistent with overfishing and habitat degradation in the region. These data provide vital information for our resolution and understanding of how past environments have changed in terms of both anthropogenic influences on coastal food web structure and biogeochemical cycles of C and N in marine ecosystems.