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The sediments and exchange processes of the shallow (mean depth 4.9 m), polymictic, eutrophic Miiggelsee have been studied over more than 30 years. Here, previous results are summarised and completed by new data. Sediment analyses were used to identify changes in loadings and migration processes within the sediments of the 12000 year old lake. The surficial oligohumic sediments are rich in Fe (8.8 f 1.9% DW) and in P (2.7 ? 0.5 mg P g-' DW). Mass balances of P and Fe as well as sediment trap measurements and sinking velocity analyses showed that the retention of matter strongly varies within seasons and years. The lake amplifies the seasonal fluctuations of P in the River Spree system by a strong redox-controlled, benthic P-release in summer. Since 1988, P-release has exceeded retention so that the lake changed from a P-sink to a source. The retention rate dropped in summer to -7.3 mg P m-* d-' and in winter to 1.4 mg P m-2 d-I. Water Sampling and AnalysisThe water of the lake and of River Spree is regularly studied (weekly from March to October and biweekly in winter from 1979 to 1994) at the Spree inflow station (Muggelhort), at the deepest site in the lake (M7) and at the Spree outflow station (Spreetunnel) presented in Fig. I . Since 1987, samples from 21 single points (5 sites (M 3, 5 , 7, 8, and 10) at different depths) have been pooled to decrease the variance caused by spatial inhomogeneities (DRIESCHER et ul. 1993).After filtration, phosphorus (P) was measured as soluble reactive P (SRP) according to MURPHY and RILEY (1962). For the determination of total P (TP) and total dissolved P (TDP) unfiltered and filtered samples were digested (H,SO,, 2 h at 170 "C) as described in GELBRECHT et al. (1991) prior to measuring SRP (see list of abbreviations). Particulate P (PP) is calculated as the difference between T P and TDP (KOZERSKI et al., 1993). Total iron (TFe) was analyzed according to LEGLER et af. (1986). Sediment Sampling and AnalysisTo study the horizontal and vertical distribution of different sediment parameters, 23 sampling stations were investigated. Intact sediment cores up to a length of 0.5 m were taken with a UWITEC-corer (flutter valve, acrylic glass tubes i.d.

Summary 1. Although macrophytes play a key role in the structure and functioning of lowland rivers, most of the basic plant, hydrodynamic and sediment‐water interactions have only been described qualitatively. We therefore studied quantitatively, the seasonal dynamics of matter deposition and mobilisation inside and outside (free path) a representative patch of arrowhead, Sagittaria sagittifolia, in the lowland River Spree, NE Germany, in August 2006. Our in situ study combined resuspension experiments, a hydrodynamically calibrated erosion chamber and concurrent measurements of the prevailing flow characteristics and bed load. 2. Increasing entrainment rates (E) of particles (ESPM) and total P (ETP), with increments of shear velocity (U*) from 0.53 to 2.42 cm s−1, were significantly higher inside the plant patch than outside. Indeed, ESPM and ETP at the lowest U* were 8‐ and 12‐fold higher inside than outside the patch, reflecting the resuspension potential of the upper nutrient‐enriched layer and the extent of pulsed P inputs even at small increases in U*. 3. Vertical distribution of velocity (u) revealed a flow pattern of a mixing layer inside the S. sagittifolia patch, and that of a boundary layer in the free path. The highest gradient of u in the mixing layer was located in the water column at about 0.5 m depth, whereas the highest gradient of u for the boundary layer was found near the riverbed. The maximum of U* (1.65 cm s−1) was only 4 mm above the sediment. 4. A plant mosaic provides a low‐energetic environment promoting extensive particle trapping and the accumulation of a fine‐grained, nutrient‐enriched sediment, and forming a large resuspension potential. Consequently, after plant decay and the concomitant increase of U* this material is preferentially entrained at higher rates. Hence, the key role of submerged macrophytes in lowland rivers is more directly related to modifying the dynamic equilibria between vegetation trapping and resuspension, than to the retention of nutrients, particularly P, and the reduction of P loads downstream to other waters.

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An increasing number of studies constrain the importance of iron for the long-term retention of phosphorus (P) under anoxic conditions, i.e. the formation of reduced iron phosphate minerals such as vivianite (Fe3(PO4)2⋅8H2O). Much remains unknown about vivianite formation, the factors controlling its occurrence, and its relevance for P burial during early sediment diagenesis. To study the occurrence of vivianite and to assess its relevance for P binding, surface sediments of two hydrologically contrasting waters were analysed by heavy-liquid separation and subsequent powder X-ray diffraction. In Lake Arendsee, vivianite was present in deeper sediment horizons and not in the uppermost layers with a sharp transition between vivianite and non-vivianite bearing layers. In contrast, in lowland river Lower Havel vivianite was present in the upper sediment layers and not in deeper horizons with a gradual transition between non-vivianite and vivianite bearing layers. In both waters, vivianite occurrence was accompanied by the presence of pyrite (FeS2). Vivianite formation was favoured by an elevated iron availability through a lower degree of sulphidisation and was present at a molar ratio of total sulphur to reactive iron smaller than 1.1, only. A longer lasting burden of sediments by organic matter, i.e. due to eutrophication, favours the release of sulphides, and the formation of insoluble iron sulphides leading to a lack of available iron and to less or no vivianite formation. This weakening in sedimentary P retention, representing a negative feedback mechanism (P release) in terms of water quality, could be partly compensated by harmless Fe amendments.