Chironomid larvae stimulate biogeochemical and microbial processes in a riverbed covered with fine sediment

We determined if the influence of chironomid larvae on biogeochemical and microbial processes in fine surface sediments of a riverbed depended on the characteristics of the sedimentary habitat. The influence of chironomids was measured on sediment reworking, biogeochemical processes and bacterial characteristics in infiltration sediment columns having three different surface sediments (with different organic matter characteristics). The results showed that chironomids exhibited similar bioturbation activities (construction and irrigation of U-shaped tubes in the upper layers of sediments) and significantly stimulated O2 uptake and percentage of active bacteria in all sediment treatments. In contrast, the release of NH4+ in anoxic sediment layers was strongly increased by chironomids in the presence of organicrich sediments, whereas they had no effect on this process with organic-poor sediments. Our results indicated that the influence of chironomids on biogeochemical processes in river bed sediments are linked to the redox conditions of the system. Moreover, when the effects of chironomid larvae are compared to those measured with tubificid worms in the same experimental conditions, we conclude that the mode of bioturbation (sediment reworking, biogenic structure building, burrowing depth, bioirrigation) can also determine the impact of benthic animals on microbial activities and biogeochemical processes in the sediment.