Science Spotlight: What Are Stable Isotopes, and How Can They Inform Marine EBM?

The Skimmer on Marine Ecosystems and Management

On 10 May, Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) is hosting a symposium on how knowledge about food webs can be best integrated into marine EBM. Specifically the symposium is focusing on the movement of stable isotopes through food webs (www.csiro.au/org/Stable-Isotope-Symposium.html).

What are "stable isotopes"? For insights MEAM asked Beth Fulton, who leads the ecosystem modeling group in marine and atmospheric research at CSIRO.

Please describe what a stable isotope is.

Beth Fulton: An isotope is a natural variant of a chemical element that has more or fewer neutrons than normal, and so has a slightly different atomic mass. When we say an isotope is stable, this means it does not lose or gain neutrons - its atomic mass remains constant over time. The stable isotope carbon-13, for example, is taken up in different amounts by different primary producers, but its signal persists as it moves through a food web. That is, the signal of where that production ultimately originated is largely conserved even as it moves through many predator-prey interactions. As a result, the isotope can be used as a tracer for pathways of food through food webs.

Another useful isotope is nitrogen-15, which is enriched with each step up the trophic chain. As a result, it can be a good indicator of trophic level.

How does the use of stable isotopes differ from other food web research, such as stomach content analyses?

Fulton: Because stable isotopes are stored in tissue and are not subject to bias about what you can see in the gut (and differential digestion rates of hard vs. soft parts of prey), they can be a source of more representative trophic relationships. In other words, they are better at telling you who really eats what - including if the source is unexpected, like material from land entering the sea.

There are other chemical tracers now (fatty acids and DNA of material from stomachs) that are complementing the information provided by stable isotopes. But the isotopes remain a key potential information source, especially as it is often easier to grab a quick tissue biopsy rather than keep a whole fish's stomach contents for taxonomic sorting.

Are there examples in practice of stable isotopes informing coastal or marine EBM?

Fulton: To my knowledge, no single decision to date has been made solely on the back of stable isotope data. Instead it has a supporting role to help elucidate food web structure, which is then used to understand connections in the system and guide levels of exploitation and protection for key groups. There is an example where a mudflat crab species that had been assumed to be mainly a flesh eater turned out to get a good proportion (>30%) of its diet from mangrove leaf litter and small things that lived on mangroves; crabs macerate their prey so finely that we would never have known this, or at least not easily, without the isotopes.

For more information:

Beth Fulton, CSIRO, Australia. E-mail: Beth.Fulton [at] csiro.au

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