Non-technical summary:

We are probably all familiar with the basic principle that life on earth is reliant on plant production driven by the sun's energy. There was a great deal of interest in 1977 when images of bizarre 6ft tubeworms and giant clams came up from the depths of the Pacific to reveal significant production, indeed whole communities, far below the influence of the sun's rays. These organisms were being fuelled, not by light, but by chemical energy from the core of the Earth itself.

Such a mode of life would seem rather strange in the classic chalk rivers of southern England fished by J.R. Hartley! However, using a technique known as stable isotope analysis, we have demonstrated that this may actually be the case.

We measured the stable carbon isotope values of common aquatic crustaceans and insects and their food sources in the River Lambourn. Some, such as Gammarus shrimps and Simulium fly larvae, reflected that of the dominant plant food sources in the river, whereas the cased larvae of the caddis flies Agapetus, Silo and Drusus indicated that their food source was likely to be derived partly from methane as a carbon source (up to 25%).

We have identified such alternative 'fuelling' of food webs before in stagnant lakes but not in 'pristine' chalk rivers. Of course, methane is a powerful greenhouse gas and the more we know about how it is produced and cycled in the environment, the better. What could be even more exciting is the prospect that the original source of methane in groundwater fed rivers is both a subsidy to the system and ancient i.e. potentially formed thousands of years ago. This would revolutionise the way in which we perceive carbon and energy flux in these more commonplace habitats.