Introduction
To assess the wider ecological significance of methane-derived carbon, it is important to determine whether it is transferred further up the food web into higher consumers, or indeed out of the ecosystem via emerging imagos. We have so far been using Agapetus spp. (cased caddis) as indicator taxa of methane-derived carbon use and we know they are often extremely numerically abundant in chalk streams. Thus, the potential for transfer is high, if their predators can remove them from their cases!
Cased caddis
From our initial research (Trimmer et al 2009, 2010), it was apparent that it was members of the cased caddis families Glossosomatidae such as Agapetus fuscipes and the Goeridae such as Silo nigricornis that were exhibiting low δ13C values. Maybe it is something to do with grazing / gardening of biofilms on their cases, as Nicola Ings has previously demonstrated for gallery building species (Ings et al 2010). Aurora Sampson will sample all the cased caddis taxa available from her wider survey of rivers to see if this is a common means of acquiring carbon.
Identifying routes of trophic transfer
To trace trophic transfer of methane-derived carbon, we will use both natural abundance stable isotope ratios and diagnostic phospholipid fatty acid biomarkers. At several key chalk river sites, Sarah Tuffin and Jodie Simmonds are collecting basal resources, and both primary and secondary consumers. Key predators may be those fish such as bullhead that can engulf the cases and larvae together, or planarian flatworms that can enter the cases.
Sarah and Jodie will be sampling on a quarterly basis to determine whether trophic transfer is more significant at different times of year (eg when photosynthetic production is low), or to specific ontogenetic stages of the predators.
Methane-derived carbon fixed as secondary production
To measure the contribution of methane-derived carbon to consumer biomass production, Aurora Sampson will measure secondary production of the focal cased caddis taxa using the size-frequency method at nine sites selected from her wider survey. All larvae collected in monthly Surber samples will be counted and measured to give total standing biomass of each population per date and secondary production over the full annual cycle (dry mass, g m-2 y-1). This will then be expressed in terms of carbon production and the fluxes of carbon isotopes from different basal resources (also sampled monthly) and their assimilation into consumer tissue will be measured to quantify the magnitude of the methane subsidy.
Agapetus fuscipes Delta 13C and Geological Type
Figure 1 - The range of δ13carbon values obtained from Agapetus fuscipes from streams from three geological types, sandstone, limestone and chalk.
