A transecting microprofiler documented a pronounced small-scale variation in the benthic O2 concentration at
1450-m water depth (Sagami Bay, Japan). Data obtained during a single deployment revealed that within a
sediment area of 190 cm2 the O2 penetration depth varied from 2.6 mm to 17.8 mm (average; 6.6 6 2.5 mm) and
the diffusive O2 uptake, calculated from the vertical concentration gradient within the diffusive boundary layer,
ranged from 0.6 mmol m22 d21 to 3.9 mmol m22 d21 (average; 1.8 6 0.7 mmol m22 d21, n 5 129). However,
correction for microtopography and horizontal diffusion increased the average diffusive O2 uptake by a factor of
1.26 6 0.06. Detailed 2D calculations on the volume-specific O2 consumption exhibited high variability. The oxic
zone was characterized by a mosaic of sediment parcels with markedly different activity levels. Millimeter- to
centimeter-sized ‘‘hot spots’’ with O2 consumption rates up to 10 pmol cm23 s21 were separated by parcels of low
or insignificant O2 consumption. The variation in aerobic activity must reflect an inhomogeneous distribution of
electron donors and suggests that the turnover of material within the oxic zone to a large extent was confined to
hot spots. The in situ benthic O2 uptakes, measured during four cruises, reflected a seasonal signal overlying the
observed small-scale variability. The annual benthic mineralization balanced ,50% of the estimated pelagic
production. However, the central bay is characterized by a significant downslope sediment transport, and mass
balance estimates indicate 90% retention of the total organic material reaching the bottom of the central bay.