Oxygen consumption in the profundal gyttja and littoral sands of Lake Vechten was determined with pore-water analyses and core incubations. Methane (58-64%), ammonium (lo-13%), iron (2-6%), and sulfide (< 12%) oxidation accounted for -75% of the oxygen consumed in the profundal sediments. Almost the complete diffusive flux of CH, out of the anoxic layer into the oxic surface layer (7.9-9.4 mmol mm2 d-l) was oxidized, while only 27-36% of the NH,+ flux (5.1-5.2 mmol m-I d-l) was oxidized in the oxic surface layer and 64-73% diffused into the overlying water. In contrast to the profundal sediments, oxidation of reduced end products of anoxic respiration was of minor importance (< 15%) in littoral sandy sediments. Also, sediment oxygen consumption rate was higher in the profundal gyttja (29 mmol m-* d-l at 7°C) during overturn than in the littoral sandy sediments in both winter (8.7 mmol m-2 d-' at 7°C) and summer (18.6 mmol me2 d..' at 22°C). Poisoning the sediment cores with formaldehyde stopped bacterial activity, but oxygen profile measurements showed that this treatment does not reveal the bacterial or chemical nature of the oxidative processes.

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1. The overlying water of intact sediment cores was constantly stirred with an impeller at a rate sufficient to mix turbulently the water column and maintain the diffusive boundary layer at a determined thickness. The system allowed standardization of water circulation in laboratory sediment core experiments.2. Both oxygen concentration and oxygen penetration depth in the sediments decreased, the former by 10% and the latter from 4.2 mm to 2.0 mm, when the overlying water was not stirred for 24 h, as measured with oxygen microelectrodes in a lake sediment core.3. Oxygen profiles measured in sediment cores in the laboratory were similar to those measured in situ when the overlying water was stirred with an impeller at such a rate that a similar thickness of the diffusive boundary layer at the sediment-water interface developed in the laboratory as that in siiii.4. Sediment oxygen consumption was calculated from: (I) measured oxygen profiles in the diffusive boundary layer and the molecular diffusion coefficient for oxygen in water; (2) the measured oxygen decrease in the top of the sediments and the estimated diffusion coefficient in the sediment; and (3) by oxygen differences in the overlying water after incubation of sediment cores.

Whole‐sediment molecular difusion coefficients (Ds) for tritiated water in pore waters of various lakes were determined experimentally by adding 3H2O to the overlying water of asphyxiated (without bioirrigation) and unasphyxiated cores and measuring the resulting pore‐water profiles after a period of time. Our objectives were to determine the relationship between Ds and Do (the diffusion coefficient in pure water) in sediments with a wide range of porosities and organic contents and to examine the influence of bioirrigation on solute transport and on the predictability of Ds. We found that Do/Ds did not change as much as expected with increasing porosity, i.e. in low‐porosity sediments the average Do/Ds was 1.8±0.1 and in high‐porosity sediments it was 1.5±0.2. We also found that the effect of faunal activity on the predictability of Ds was only significant in sediments with high (14,000 ind. m−2) invertebrate populations. This result means that in most freshwater sediments, the sediment diffusion coefficient can be predicted reliably from the molecular diffusion coefficient at in situ temperature.