Abstract
In 1979, the Suess effect was described as decreasing δ13C in 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 3 fishes from Otago, New Zealand, to reconstruct the isotopic baselines of C and N and assess specific trophic positions through time (1955-present) based on bulk and amino acid stable isotope values. Our sample set included Odax pullus, a strictly herbivorous species, and 2 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 the δ13C of essential amino acids and found that while P. colias maintained a steady diet, the food web position of N. macropterus likely changed. Analysis of δ15N of phenylalanine in O. pullus indicated a decrease of 0.023‰ yr-1 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.
Highlights
The impacts of anthropogenic activities have become increasingly evident in both coastal and open ocean environments, with consequences for the structure and function of marine ecosystems (Harley et al 2006, Shackell et al 2010)
TPGlx−Phe indicated that both O. pullus and P. colias had no significant differences in their TPGlx−Phe, from 1.75−1.70 and from 3.67−3.63, respectively
Because Odax pullus is a primary consumer with a narrow diet niche mainly consisting of macroalgae, its isotope chemistry represents an interesting proxy for tracking the isotopic baseline changes in organic matter source through time
Summary
The impacts of anthropogenic activities have become increasingly evident in both coastal and open ocean environments, with consequences for the structure and function of marine ecosystems (Harley et al 2006, Shackell et al 2010). Δ15NPhe, as well as the δ13C of essential AAs (δ13CEAA) of primary producers can record environmental variations, such as changes in temperature, CO2, and/or nutrient concentration (Yamaguchi & McCarthy 2018, Sabadel et al 2019), representing a solid baseline to track changes in the isotopic composition at the base of a food web through time. This ability is valuable in situations where isotopic information on the position of the base of the food web is lacking, such as in archaeological and paleontological studies. The homogeneity of change of all individual AA values still allows for precise measurements of TP for historic samples (Durante et al 2020b)
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