1. Pore water chemistry in peaty sediment was monitored for a year at two representative locations of the eutrophic shallow Loosdrecht lakes. The Netherlands. Phosphorus fluxes over the sediment-water interface were calculated using measured concentration gradients in the pore water and compared to flu.xes measured under laboratory conditions. Results were analysed with Redundancy Analysis to detect pattems of variation in pore water chemistry and in measured and calculated fluxes, that could be ascribed to environmental variables.2. It was demonstrated that phosphorus fluxes measured in long-term laboratory incubations were not correlated to any of the pore water characteristics.3. Initial phosphorus fluxes measured in sediment columns, which varied between -7,7 and 1330 jimol m" day ', were correlated significantly to the calculated phosphorus flux over the sediment-water interface, 4. The high correlation between calculated fluxes of ammonia, phosphorus and methane and measured initial flux of phosphorus, conclusively pointed to mineralization of organic matter as the driving force for phosphorus release from the sediment.5. Redundancy Analysis demonstrated that the rates of mineralization and phosphorus release were only weakly related to temperature. They appeared to be especially stimulated by the autumnal decrease in temperature which was probably related to an extra input of organic matter.

In 1984 the external phosphorus load of the shallow eutrophic Loosdrecht lakes was reduced from 3.3 to l.Omgm-*d-'.Theeff ec o p t f h osphorus release from the sediment on lake restoration was investigated. Diffusive release under aerobic conditions (20 ' C) decreased from 1 mg m -* d -i in 1984 to 0.3 mg m -* d -' in 1990. The generation of inorganic phosphorus due to mineralization during summer equals 3 mg m -* d -', which i s much higher than the measured rate of diffusive release. Despite that, the phosphorus release is hardly stimulated by anaerobic conditions, which indicates that only a small amount of phosphorus is adsorbed by ferric iron in the top sediment layer. This apparent discrepancy is probably caused by the uptake of inorganic phosphorus uptake during resuspension and the loss of inorganic phosphorus with downward seepage.The estimated removal of phosphorus due to downward seepage of 0.8 mg me2 d-' agrees well with the average phosphorus retention in the lake. This indicates that sediment burial and diagenesis are unimportant mechanisms for withdrawing phosphorus from the nutrient cycle.Between 1982 and 1991 the total phosphorus content of the upper 2 cm of the sediment decreased from 0.94 to 0.60 g kg-' DW. At present, about 20% of total phosphorus in this layer is potentially bioavailable, but largely incorporated in easily degradable organic matter. This pool is much smaller in deeper layers. Based on the estimated and measured rates and pool sizes, the annual average phosphorus cycle in the lakes was modelled to evaluate the effects of various restoration measures. The main predictions of the model are: 1) further reduction of the external load may cause a gradual decrease of the total phosphorus concentration in the lake water; 2) dredging and iron addition, without reduction of the external load, may give a rapid improvement followed by a slow return to the present situation; and 3) reduction of the external load, combined with a cut off of downward seepage will not improve the water quality.

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.

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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.