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

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.

Overexploitation and decline in kelp forests inflate the bioenergetic costs of fisheries

“…Sabadel et al. (2020) provide an analysis from the Otago area and time‐specific corrections for the Seuss effect based on the analysis of δ 13 C AA and δ 13 C of a strictly herbivorous fish. These corrections corroborate global assessments of Seuss effect values for the southern Pacific and provide the most appropriate Seuss correction possible for our study.…”

“…Stable isotope analysis of amino acids in bone collagen provided an effective method for resolving δ 15 N values of primary producers at the base of the food web, which shifted over the c. 768-year time period. These shifts were probably attributable to changes in the nitrogen cycle, which might be affected by shifting patterns in N 2 fixation, upwelling of deep pools of nitrogen, denitrification and additions of synthetic nitrogen fertilizers in the coastal zone (Sabadel et al, 2020). Analysis of five commonly exploited fishes with contrasting life histories, two large predators (hāpuka and ling), the smaller, reef-associated blue cod and the smaller, schooling red cod, in addition to the large, highly mobile, predatory barracouta, in archaeological sites and continue to be the source of targeted modern-day commercial and recreational fisheries.…”

“…768‐year time period. These shifts were probably attributable to changes in the nitrogen cycle, which might be affected by shifting patterns in N 2 fixation, upwelling of deep pools of nitrogen, denitrification and additions of synthetic nitrogen fertilizers in the coastal zone (Sabadel et al., 2020). Analysis of five commonly exploited fishes with contrasting life histories, two large predators (hāpuka and ling), the smaller, reef‐associated blue cod and the smaller, schooling red cod, in addition to the large, highly mobile, predatory barracouta, provided a means to compare temporal trends in TL for a diverse subset of the exploited fish community.…”

Changes in trophic structure of an exploited fish community at the centennial scale are linked to fisheries and climate forces

“…δ 13 C values of primary producers have also been globally influenced by anthropogenic activities since the industrialization period in the 1950’s 66 , 67 , and to compare with present values those changes need to be taken into account. These processes are known to influence fishes from different trophic levels and have been reported for the Otago region 68 . The Suess effect (decrease of atmospheric δ 13 C of CO 2 ) was corrected for 69 , which predicts a decrease in δ 13 C Bulk of on average 0.011‰ per year (− 0.014 ± 0.001‰ to − 0.006 ± 0.001‰) in the ventilated South Pacific Ocean.…”

Changes in Trophic Structure of An Exploited Fish Community At The Centennial Scale Are Linked To Fisheries and Climate Forces

“…To account for the effects of fixatives and preservatives on the isotope values of muscle tissue, corrections were applied to all samples from museum collections, for both δ 15 N (Equation 2) and δ 13 C (Equation 3) after Durante et al 51 δ 13 C values of primary producers have also been globally influenced by anthropogenic activities since the industrialization period in the 1950's 54,55 , and to compare with present values those changes need to be taken into account. These processes are known to influence fishes from different trophic levels and have been reported for the Otago region 56 . The Suess effect (decrease of atmospheric δ C of CO2) was corrected after 57 , which predicts a decrease in δ 13 CBulk of on average 0.011‰ per year (−0.014±0.001‰ to −0.006±0.001‰) in the ventilated South Pacific…”