On the bed of the Bere Stream, a bullhead (<i>Cottus gobio</i>) amongst stones covered with <i>Agapetus fuscipes</i> and <i>Ancylus fluviatilis</i>
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Overall synthesis:

If we can show that the methane in a river is predominantly groundwater derived (using δ13C and relative concentration) in the winter and we detect an old (14C) and 13C-deplete signal for carbon (derived from methane) in an invertebrate, then we would be confident that groundwater methane represents a spatial and temporal carbon subsidy to the invertebrates living in the chalk river gravels.

Specific objectives and approach:

1. To 14C date and δ13C characterise key invertebrates and their putative food sources

We will collect representative samples of caddis flies, grazing snails, and shredding macroinvertebrates (e.g. Gammarus spp.), Ranunculus spp. (autochthonous basal resource) and terrestrial leaf litter (allochthonous basal resource) on two occasions.

The two occasions reflect the strong seasonal differential in CH4 concentrations and the time of year when we know the δ13C values suggest maximum dependence on methane-derived carbon (Trimmer et al 2009).

Accelerator mass spectrometer (AMS) 14C measurements will be made on pooled invertebrate and on macrophyte samples pretreated to remove any carbonate. The 14C/12C ratio will be measured by AMS at the 14CHRONO Centre, Queen’s University, Belfast by Dr Paula Reimer.

The 13C/12C ratio will be determined by combined elemental analysis and continuous flow isotope ratio mass spectrometry (EA/CF/IRMS) in house at Queen Mary (Trimmer et al 2009).

Hence, we will end up with dual (14C/12C & 13C/12C) isotope ratios for the solid samples.
Adult stage of <i>Silo nigricornis</i>, a common caddis fly in UK chalk streams (Photo courtesy of Melvin Grey)

2. To δ13C characterise the CH4 end-members

We are not seeking to 14C-date the CH4 gas directly but we can characterise its δ13C value in the two end-members, in house at QMUL. This will enable us to apportion the CH4 in a river water sample to either a groundwater or sediment source in winter and summer using a mixing model.

Sediment samples from each river will be incubated (in the laboratory) and the production of CH4 measured using gas chromatography and flame ionisation detector (GC/FID, Sanders et al 2007).

Once the sediments have produced sufficient CH4, we will collect the headspace in gas-tight bulbs using an existing equilibration manifold (Nicholls, Davies & Trimmer 2007).

We will also sample the groundwater for CH4 using boreholes (access via the Environment Agency and Project Partner, Daren Gooddy, BGS) and adjacent river water. δ13C of CH4 will be measured after hot (940°C) nickel oxidation of CH4 to CO2 with cryo-focusing and CF/IRMS.
Dr Mark Trimmer sampling borehole water pumped from 90m into the chalk aquifer