The effect of sediment-dwelling macrofauna on nitrifying bacteria was investigated by direct probing in their overlapping habitat, i.e., the upper few millimeters of freshwater sediments. Microsensors (O-2, NH4+, NO3-, and diffusivity) were used at the sediment surface and inside animal burrows to record steady-state and dynamic distributions of reactants, respectively. Short-term changes of metabolic activity (actual and potential nitrification rates) and long-term changes of abundance (fluorescence in situ hybridization) of nitrifying bacteria were determined. The presence of insect larvae (Chironomus riparius) increased the availability of O-2 and NO3- in the sediment pore water and inside animal burrows, suggesting promotion of nitrification and dissimilatory NO3- reduction, particularly in the burrowing layer of C riparius. At the sediment surface (i.e., in the feeding layer of C riparius), however, nitrification was inhibited by low NH4+ availability and high macrofaunal grazing pressure. Consequently, both actual and potential nitrification rates decreased in the feeding layer. Inside burrows, no net nitrification was detected, despite high NH4+ availability and frequent O-2 injections by larval ventilation activity. nitrifying bacteria increased measurably in the burrowing layer after prolonged incubation, but only in sediments in which the larvae were able to construct and ventilate stable burrows.
