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.