Carbon and nitrogen are two essential elements in the biochemistry of life. Both elements naturally occur in two stable isotopic forms differing in their number of neutrons, a light (12C and 14N) and a heavy (13C and 15N) isotope (Fry and Sherr, 1984). The ratios of the heavy and light isotopes of carbon and nitrogen can be measured with precision using a mass spectrometer and are expressed as δ13C and δ15N. The majority of carbon and nitrogen enters marine food webs after initial uptake by primary producers. Subsequently carbon and nitrogen are transferred through the food web by a series of consumers (trophic levels), firstly by plankton grazers, then planktivores, and through the trophic levels to top predators such as seals and orcas.
Organisms preferentially metabolise the lighter isotopic forms of carbon and nitrogen. As a result the isotope ratios of carbon and nitrogen, δ13C and δ15N, systematically increase as one moves up the food chain. This systematic enrichment of heavy isotopes up the food chain can be used to understand food web dynamics. δ13C tends to increase by approximately 1‰ per trophic level, and therefore tends to be more closely representative of the food web source (primary producer). This is particularly useful when tracing the contributions of different primary producers (e.g., phytoplankton, kelp, seagrass) and carbon sources (e.g., marine vs terrestrial) to food webs. δ15N increases by approximately 3-4‰ and can be used to determine an organisms trophic level, and predator-prey relationships.
At inter-annual scales, stable isotope data provides valuable insights into the response of food webs to long term change. δ13C in marine food webs is positively correlated with temperature (and inversely correlated with CO2 concentrations) and δ13C measured from year to year is therefore a good indicator of changing ocean temperature. The addition of δ15N data provides insights into how both individual species and entire food webs respond to changing ocean conditions (e.g., shortening or lengthening of the food web).