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The effect of methane oxidation in aerobic sediment on oxygen consumption and phosphate flux was investigated in diffusion chambers. The diffusion chambers consisted of two compartments separated by a Teflon membrane. In the upper chamber a thin sediment layer was present and the lower chamber was continuously flushed with gas. The hydrophobic membrane allowed for diffusion of gases from the lower chamber through the sediment layer toward the headspace of the upper chamber. In experiments with a methane oxidation rate of 9.8 mmol m(-2) day(-1), the oxygen consumption rate increased by a factor of two compared with controls without methane oxidation (8.6 vs 17.7 mmol m(-2) day(-1)). Methane oxidation significantly decreased oxygen penetration depth (2.5-4.0 vs 1.0-2.0 mm). However, despite the shrinkage of the oxidized microlayer, no differences were found in phosphate flux across the sediment water interface. Batch experiments with standard additions of methane revealed that the growth of methanotrophic bacteria contributes to the phosphate uptake of aerobic sediment. From the batch experiments a molar ratio of carbon to phosphate of 45 mol:mol was calculated for the growth of methanotrophs. Results suggest that a decrease in chemical phosphate adsorption caused by a decrease in the oxygen penetration depth could be compensated for entirely by the growth of methanotrophic bacteria.

An extraction method to determine the contribution of bacterial processes to phosphate uptake of aerobic freshwater sediment was tested on Fe hydroxyphosphate that was either synthesized or formed under in situ conditions and a pure culture of Acinetobacter 210A. A mild extraction with H2SO4 solubilized the entire Fe hydroxyphosphate fraction but did not extract bacterial phosphate. Phosphate uptake of randomly sampled surface layers of the sediment of Lake Loosdrecht was considerable, ranging from 11 to 138 µmol g−1 on a dry weight basis. The contribution of bacterial processes ranged from 12 to 32%. Addition of an easily degradable substrate, such as acetate, to the sediment stimulated the uptake of phosphate and augmented the biologically bound phosphate fraction. The results indicated that growing bacteria play a considerable role in phosphate uptake by aerobic sediment.

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Lake internal phosphorus-recycling from sediments is a significant process in most eutrophic shallow lakes.The mechanisms for P-liberation in the sediments are coupled microbial processes depending on water transport at the sediment-water interface. Thereby electron acceptors are supplied to sessile microassemblages of bacteria and inhibitory products, such as hydrogen sulphid, are removed by water transport. This relations where shown by timeseries in interstitial water strata and lake-budget calculations for several lakes in the Berlin-region, the Schlei-estuary and Swedish lakes.In shallow eutrophic lakes up to 1 g/m 2 /month phosphorus have been liberated by the process of internal fertilisation from sediments during short periods in summer and autumn. This was equal to the whole phosphorus content of the upper 2 mm of surface sediments and resulted in a large increase of phosphate concentration in water column. The periods of internal P-liberation where coupled with high rates of plankton sedimentation and depleted oxidized nitrogen by denitrification in sediment surface. In these periods desulfurication became the main respiration-process in sediment metabolism.The sediment studies showed a monocausal hierachy of processes. The energyflow partly resulting in water transport and in chemical reactions control the sediment metabolism with respect to space and time distribution of microbial activity.In aquatic sediments the mineralization of organic matter regenerates phosphates, resulting in an accumulation of phosphates in the interstitial water and the formation of a concentration gradient. The subsequent diffusive transport to the overlying water is affected by chemical and microbial processes. Especially the redox state of the sediment is known to be an important factor determining the actual phosphate flux. The presence of an oxidized microlayer at the sediment surface is considered to be an important trap for phosphates. The high adsorption capacity of the oxidized microzone is generally ascribed to the presence of iron(III)-hydroxides. Lately, also the role of microorganisms has come into focus. Microorganisms in the surface layer of the sediment might act as a redox dependent source/sink mechanism of phosphates.This contribution is focused on the role of methane oxidizing bacteria in the sediment surface. In Lake Loosdrecht almost the entire methane flux diffusing upward is oxidized in the sediment surface layer. In periods with high methane production the major fraction of the oxygen consumption of the sediment can be ascribed to methanotrophic bacteria. An experimental setup will be described which enabled us to manipulate the methane flux toward the sediment surface. Microelectrode equipment was used to quantify the oxygen consumption and the oxygen penetration depth. In columns with high activity of methanotrophic bacteria a decrease in the oxygen penetration depth could be demonstrated. However, these columns appeared to have a higher phosphate adsorption capacity than the controls. Batch experime...