Nitrogen Isotopic Variation in the Global Ocean: A Potential Anthropogenic Impact Indicator

Abstract

Changes to marine food webs have large global impacts on the economic stability and nutritional well-being of people around the world. It is a priority for conservation to develop qualitative and quantitative frameworks to measure the collective ecological impact of many anthropogenic drivers on various marine environments. This paper proposes the use of a trophodynamic indicator, nitrogen isotope versus length relationships, as a broadly applicable measure of ecosystem health. This study compiled a database of nitrogen isotopes in fish to investigate the relationship between fish size and muscle tissue nitrogen isotopic composition (δ15N) across diverse environments in the global ocean. In an analysis of individual species within each ecosystem, there was a declining relationship between δ15N and size as environmental health declined. In lesser disturbed food webs, fish were more likely to exhibit ontogenetic increases in δ15N, suggesting that a more fully realized trophic structure is indicative of a less-impacted ecosystem. In contrast, highly altered ecosystems observed almost no increases in δ15N. Environmental impact based on trends between δ15N and length significantly matched the results of an ecosystem health model developed by Halpern et al. (2008) which is based on a full range of anthropogenic drivers. Lastly, a case study investigating temporal trends in the relationship between δ15N and length within a single fish population confirmed that δ15N and length relationships strengthened following implementation of a marine protected area. δ15N vs. size relationships may therefore provide a quantitative measure of anthropogenic impact that could support conservation and management decisions.