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Comparison of bottom-water chemistry in the marine-limnic habitat gradient shows greater phosphorus availability in marine waters, primarily because of enhanced iron sequestration by sulfide. In the oxidative hydrolysis of iron and the concomitant precipitation of phosphate, a minimum of two iron atoms are needed to precipitate one phosphate molecule (Fe : P ϭ 2). However, dissolved Fe : P Ͻ 2 predominates in anoxic marine waters, therefore leaving some phosphate in solution after oxygenation because of a shortage of dissolved iron for phosphate coprecipitation by iron oxyhydroxide. In contrast, anoxic bottom waters in most freshwater lakes show Fe : P Ͼ 2, allowing almost complete phosphate removal on oxygenation. This difference is a consequence of the high sulfate content of sea salt, and a main reason why nitrogen normally limits net primary production in temperate coastal waters, in contrast to the predominant phosphorus limitation of near-neutral lakes.

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In 1989 sedimentation in seven basins of oligotrophic Lake Lucerne was measured by means of cylindrical traps below the epilimnion (30-50 m) and in the hypolimnion 5 m above lake bottom. The aim of this study was to evaluate horizontal P sedimentation differences and the possible causes thereof. By means of measuring autochthonous primary production, mean sestonic PP concentration, major allochthonous P inputs and the P output, and increments in bottom sediments, the fate of P in the different lake basins might be elucidated, despite the complicated morphological structure of Lake Lucerne.The annual PP flux in the seven basins ranged between 0.6 and 2.0 g PP. m -2 * yr-. PP fluxes varied not only horizontally, but also seasonally and vertically between 0.4 and 11.7 mg PP' m-2 . d -, with one extreme sedimentation rate of 32 mg PP m-2 d-in a sediment trap 5 m above lake bottom, measured after a rainfall event in one basin and indicating a turbidity current. The ratio between maximum and minimum flux value of 6.6 for seasonal variances, 3.1 for vertical differences, and 21.3 for horizontal differences showed that differences from basin to basin were far the largest. This is basically caused by differences in allochthonous PP input rather than autochthonous PP production by phytoplankton, since the horizontal differences in primary production were negligible. Because horizontal seston differences were much smaller than those in settling flux, a fast settling velocity of inorganic particles with high density is assumed. (As usual, calculations dividing flux by mean concentration, yielding 0.18-1.35 m d-in the different basins, underestimated true settling velocity considerably.) Typically, the PP flux was lower in winter than in summer, reflecting again the high summer discharge of glacier fed rivers. The vertical differences in PP fluxes indicated usually a greater hypo flux by about 68% in all basins indicating bottom sediment resuspension or P uptake during settling. Evidence of resuspension events is limited due to the great depth of the basins; however, density currents in basins with large river input cannot be excluded. There was no evidence, however, that such currents stratified in any depth of the upper hypolimnion, since additional traps exposed at various hypolimnetic depths in two basins did never show increased settling flux. On the other hand, P uptake experiments with settling particles revealed that bacteria attached at settling particles can incorporate significant amounts of phosphorus during sedimentation.Comparative studies on the exchange processes of phosphorus over the sediment-water interface in marine and freshwater environments are reported. The study is based on laboratory experiments with