Investigating long-term trophic stability in North Atlantic cod (Gadus morhua) through nitrogen stable isotope analysis of amino acids

Human-induced environmental change and fishing pressure have deleterious effects on marine ecosystems, but beyond that, the longer-term impacts are much harder to assess. Here, we applied bulk nitrogen isotopes (δ15NBulk) and compound-specific isotope analysis of amino acids (δ15NAA) to well-dated cod remains from northeast Scotland to provide insights into the trophic structure in the North Sea over the last 1500 years. Ontogenetic changes were observable in trophic δ15NAA and δ15Ntrophic-source proxies but not in δ15NBulk, questioning the latter's utility for inferring trophic level changes. We deployed a Bayesian GAM model, incorporating size-related uncertainties, to show that the trophic level of cod remained relatively stable from 500 CE to 1800 CE despite major climate and economic transitions. However, in the last 200 years, the δ15Ntrophic-source proxy increased against the expectations of the effects of overfishing. While an increase in the trophic level of cod may be attributable to a restructuring of the North Sea food web due to overfishing, other variables such as stress and diet quality might have affected nitrogen isotope fractionation, leading to similar outcomes. Our results show that multiple factors could drive δ15N through time; thus, physiological and biochemical factors must be considered when evaluating long-term trophic dynamics.