Lake eutrophication and its consequences is still an important water quality problem being an effect of nutrient input to surface waters. In most lakes of the temperate zone, phosphorus is the nutrient responsible for eutrophication. Bottom sediments are the main pool involved in the retention and cycling of this element. Bottom sediments, depending on their chemical composition and aeration of the overlying water, may take up or release dissolved phosphorus i.e. the form easy utilizable by the plants. This study was aimed at comparing the exchange (uptake/release) of dissolved reactive phosphorus (DRP) in experiments that simulated natural conditions in various types of bottom sediments originating from different river-lake habitats typical of Masurian Lakeland (north-east Poland). Several river-lake systems typical for postglacial landscape were selected like river Jorka (15 km long, 5 lakes in cascade) and river Krutynia (~100 km long flowing through 17 lakes). Sediments used in experiments were taken from the littoral and profundal zones of four lakes (meso- meso-eu-, eutrophic and hypertrophic), from a humic lake and from ecotone zones at the land-water border and at the border between lake and river (from through-flow lakes). In total, 154 experiments were performed to assess the intensity of P exchange at a high (> 8 mg O2 L-1) and low (<2 mg O2 L-1) concentration of oxygen in water overlying undisturbed sediment cores. The following P fractions were isolated using the sequential extraction method and their importance was further analysed: easily exchangeable P (NH4Cl-RP – loosely bound, most available P; BD-RP – redox-dependent P associated with metal (Fe, Mn) hydroxides; NaOH-RP – phosphorus adsorbed mainly on metal (Fe, Al) oxides), hardly exchangeable P (BD-NRP – mainly organic P, whose stability depends on redox potential; NaOH-NRP – phosphorus in microorganisms, polyphosphates and part of organic P bound to detritus and humic substances) and non-exchangeable P (Hcl-P – phosphorus bound to carbonates, apatite-P and phosphorus released during total dissolution of metal oxides; P-residual – non-exchangeable P together with part of organic P). The effect of various factors (i.e. Fe, Mn, Mg, Al, Ca, organic matter, total P content and its fractions) on the intensity of DRP uptake/release was analysed with Pearson correlation and multiple regression. In sediments (both littoral and profundal) from lakes of the Jorka River trophic gradient, high oxygen conditions were always accompanied by P uptake (from –0.9 to –2.8 mg P m-2 d-1) while reduced oxygen concentrations were followed by DRP release (from 2.3 to 18.6 mg P m-2 d-1). These values were several dozen times higher than those noted for sediments from humic lake. Profundal sediments released more P than the littoral ones. In profundal sediments of all lakes of the Jorka River, the intensity of DRP release tooverlying water under reduced O2 concentrations was higher than the uptake rate under aerobic conditions. It means that DRP release prevailed over its uptake. Release rate of DRP tended to be higher from sediments of eutrophic and hypertrophic lakes than from those of meso- and mesoeutrophic lakes both in the two studied habitats (littoral, profundal) and seasons (spring and summer). Sediments of humic lake (from both littoral and profundal zones) showed a low dynamics of DRP uptake/release with a small prevalence of the latter (0.02 to 0.08 mg P m-2 d-1). River-lake-river sediments (from the inflows and outflows of the Krutynia River to a lake) were different in comparison with typical lake sediments – they released DRP to aerated overlying water in both meso- and meso-eutrophic lake. Phosphorus was released from in-shore bog sediments at reduced oxygen concentration in overlying water in both seasons (spring and summer) while under aerobic conditions DRP was weakly taken up and/or released. Fe, Mn, Mg, total P content and redox-dependent easily exchangeable BD-RP fraction had a significant effect on the intensity of P uptake at high concentration of oxygen and P release under reduced oxygen concentration (Pearson correlation, P <0.01). Factor analysis showed that at a high O2 concentration the intensity of DRP uptake by sediments was determined by redoxdependent fraction of P bound to Fe and Mn hydroxides (BD-RP) and the P fraction bound to carbonates and apatite (HCl-P) (r2 = 0.48). At reduced O2 concentration in overlying water the intensity of DRP release was affected by redox-dependent fraction of P associated with Fe and Mn hydroxides (BD-RP), P fraction bound to metal oxides (NaOH-RP), organic P in detritus, P in microorganisms and combined in humic substances (NaOH-NRP) and P fraction bound to carbonates and apatites (HCl-P) (r2 = 0.63). Sediments from eutrophic and hypertrophic lakes in the lower course of the Jorka River are most intensively eutrophicated. They showed the highest values of DRP release and the predominance of P release over P uptake was the highest (up to seven fold). Sediments of these lakes contained the highest amounts of redox-dependent elements – Fe and Mn. Hence, these lakes easily accumulate phosphorus at high concentrations of oxygen but equally easily release it when oxygen in water is depleted. More stable are meso- and meso-eutrophic lakes situated higher in the Jorka River system. Sediments of these lakes released smaller amounts of DRP than eutrophic and hypertrophic lakes and the prevalence of DRP release over uptake was threefold. In-shore bog sediments form a stable system when overlying waters are rich in oxygen. Under reduced oxygen concentrations, however, these habitats become an important P source (comparable with profun dal sediments) for lake littoral zone in case of theirclose contact with lake waters. A system able to bind phosphorus stronger and faster will hamper the delivery of available P to lake water and thus will delay lake eutrophication; that able to release P will accelerate eutrophication. In this case, internal loading may have a decisive effect on the lake trophic status. Profundal mid-lake sites, in-shore bogs and to a smaller extent littoral sediments (gyttja type) are the systems accelerating eutrophication. Humic lake sediments (dy type) are more equilibrated among the studied systems – the differences between uptake and release are small there.
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