Manipulating the chemosynthetic and photosynthetic support of river food webs

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We are probably all familiar with the basic principle that life on earth is reliant on primary production i.e. photosynthetic plants driven by energy from the sun. 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 reliant upon chemical energy (chemosynthesis). Few, if any, would suspect that such chemosynthetic life may be significant in the classic chalk rivers of southern England. However, a fortuitous finding, as part of a wider NERC LOCAR project into the ecological significance of river water and groundwater exchange, suggests that this is the case. We measured the stable carbon isotope values of common aquatic invertebrates (small crustacea and insects) and their putative food sources in one of our focal model systems (the River Lambourn) because we can use stable isotopes to trace energy sources and fluxes through food webs. Whereas the values for small shrimps and blackfly larvae reflected that of the dominant photosynthetic production, the cased larvae of the common caddisflies were distinctly different. Remarkably, such isotope values characterise an input of methane-derived carbon and our calculations suggest that the caddisflies were receiving a 20-25 % chemosynthetic carbon 'subsidy'. Freshwater may comprise only 3% of the Earth's total water, and rivers a vanishingly small percentage of that, yet it is this tiny percentage with which we think we are most familiar, and upon which we rely in our everyday lives. Our earlier research suggests we do not know as much about the processes in rivers as we first thought; a completely novel source of carbon, in effect, fuelling life in the river. 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. These first findings prompted us to examine the relative proportion of chemosynthetic to photosynthetic production under simple conditions in the laboratory and we showed that chemosynthesis was indeed a significant source of energy; around 6% but with the potential to be higher under natural conditions. What we need to do now is to scale up these simple measurements in the laboratory to realistic field-trials in which we can manipulate both the amount of methane and sunlight. Then we can map the stable isotope 'patterns' we see in the insects directly onto the processes which we hypothesised were the drivers of that pattern and close this knowledge gap. At the River Laboratory of the Freshwater Biological Association, there are a number of stream channels which we can use as the basis for our experimentation, although we will need to modify those to our specific requirements. The channels are fed with water from the R Frome which we have previously found to have the highest summer concentration of methane, ideal for our experiments. In a series of experiments, we will manipulate methane concentration, sunlight and animal numbers, while measuring concurrent photosynthetic and chemosynthetic production. If we can demonstrate that the whole food web, including the plants, are ultimately affected by methane cycling, then our first calculations of the importance of methane subsidy (20-25%) are underestimates, and chemosynthetic production is even more important to the life in these rivers. In summary, we will combine the traditional river ecology expertise of Hildrew and Woodward, with the stable isotope expertise of Grey, and gas and nutrient cycling expertise of Trimmer in a new collaboration to re-appraise how productivity in our rivers is governed.