Relative influences of DOC flux and subterranean fauna on microbial abundance and activity in aquifer sediments: new insights from 13C-tracer experiments

P>1. Aquifers are considered to be controlled bottom-up because of their dependence on organic matter supply from surface ecosystems. Microorganisms are generally assumed to form the base of the food web and to respond strongly to organic matter supply. Although the bottom-up control of microorganisms by carbon sources has been well documented, the potential top-down control of obligate groundwater invertebrates on microorganisms has never been addressed in alluvial aquifers. 2. The main aims of the present study were (i) to quantify the relative influences of the activity of a subterranean amphipod (Niphargus rhenorhodanensis) and the flux of dissolved organic carbon (DOC) on organic matter processing and microbial activity, biomass and abundance in slow filtration columns mimicking an alluvial aquifer, and (ii) to determine the feeding rate of N. rhenorhodanensis on sedimentary microbes by tracing the flux of a 13C-labelled source of DOC in batches (closed systems). 3. Slow filtration column experiments showed that microbial abundance, biomass and activity were primarily controlled by DOC flux, whereas the activity of N. rhenorhodanensis had only a slight effect on the microbial compartment. Modelling of carbon fluxes in the 13C-tracer experiments indicated that the feeding activity of the amphipod was too low to significantly modify microbial growth and activity. 4. Our experiments supported the hypothesis that groundwater ecosystems are controlled bottom-up. The small influence of N. rhenorhodanensis on the microbial compartment was probably linked to its slow metabolism. Our results highlight the need for further experiments to examine the relationship between metabolic rates of subterranean organisms and their role in ecosystem functioning.