Use of Stable Isotopes Within the Project

Stable isotope analysis is a contemporary tool used by ecologists to trace elements through an ecosystem. Isotopes are different types of atoms of the same element, each having a different number of neutrons.

We are interested in the stable isotopes (non-radioactive) which occur naturally in the environment and will use them to track carbon and nitrogen through a food chain (Fig. 2). 

Fig1
Fig 2 – Stable isotope ‘map’ of a simplified food chain that we might find in Windermere
 

The actual stable isotope values that are reported on the x and y axes are simply relative to an international standard so we can, for instance, compare pike caught from Windermere with pike caught in Finland. Obviously, a food web contains many food chains and is far more complex than Fig 2, but we can use mathematical models to estimate the relative contributions of different dietary sources to a consumer such as the pike. Below are some general ‘rules of thumb’.

The ratio of heavy (13C) to light (12C) carbon isotopes does not change much as carbon is transferred up the food chain i.e. it is relatively conservative and thus, is a good indicator of energy source.

The ratio of heavy (15N) to light (14N) nitrogen isotopes changes markedly from diet to consumer and can be equated to ‘trophic level’ as long as some reference point near the base of the chain is known.

Visit the Grey Lab for further information and similar applications.