Michael Hupfer

Long term effects of phosphorus precipitations with alum in hypereutrophic Lake Süsser See (Germany)

Precipitation and inactivation of phosphorus (P) by the addition of aluminum (Al) is a widely applied lake restoration measure. Many studies about short-term effects are reported, but little is known about long-term effects. To reduce this lack of knowledge we investigated the German hardwater lake, Süsser See, which was treated almost annually from 1977 to 1992 with aluminum sulfate (alum). We conducted sediment core experiments in laboratory, as well as in situ investigations of sediment and pore water. The treatment is clearly recognizable in sediment depth profiles as increases of total phosphorus, NaOH-extractable phosphorus, and NaOH-extractable aluminum. The molar ratio of added Al to additionally bound P is approximately 2.1:1. Pore water profiles of soluble reactive phosphorus taken in situ and in sediment core experiments, as well as sorption batch experiments, illustrate that the Al(OH)3 layers sorptive capacity is still not exhausted with further P sorption occurring in different P fractions. The P release of the sediment is affected by the magnitude of the downward flux into the P sorbing layer. However, sediment core experiments and a modeling exercise indicate that a buried P sorbing layer has little or no effect on the P release of the uppermost fresh sediment layers.

Retention and early diagenetic transformation of phosphorus in Lake arendsee (Germany) : consequences for management strategies

Repeated sediment core investigations over one decade, mass balance calculations, and vertical flux measurements by traps and dialysis samplers, were used to determine P retention rates, release potential, and early diagenetic transformation processes in the sediment of Lake Arendsee (Germany). Sediment cores were dated by varve counting, by 137 cesium, and by a distinct layer originating from a restoration attempt in 1995, which involved the capping of the sediment with calcium rich material from the lake shore. P retention rates and the internal P cycle have not been altered by the sediment capping. The sharp decline of total P content within the first two centimeters of the sediment shows that diagenetic P mobilization is a rapid process. The temporary P pool in the sediment, calculated from core analysis (mean ± SE: 709 ± 82mg m -2 , n = 7), was small compared to the rates of hypolimnetic SRP increase (10.7 ± 0.45 mg m -2 d -1 , 1992-1997) and total P losses in the epilimnion caused by sedimentation (11.7 ± 0.53 mg m -2 d -1 , 1992-1997), both of the latter calculated by mass balances during summer stratification. Without additional supply of freshly settled material, the temporary P pool in the sediment would be exhausted in less than three months. The fast P release of freshly settled material was also demonstrated during summer by the three times higher vertical P sedimentation rates calculated on the basis of mass balance data, compared to rates determined on the basis of cylindrical traps, in which some P was released during four weeks exposure time. The driving process for the rapid P release in Lake Arendsee is the remineralisation of organic P. This study demonstrates that high hypolimnetic P accumulation rates are not always correlated with a large total or potentially mobile P pool in the sediment. In lakes with small temporary P pools in the sediment, a decrease of P in the water body would immediately decrease the hypolimnetic P accumulation in summer; capping or dredging as in-lake measures are ineffective in such lakes.

Immobilisation of phosphorus by iron-coated roots of submerged macrophytes

To observe effects on the phosphorus retention mechanisms of a lake after re-colonisation by macrophytes, Potamogeton crispus L. and Elodea canadensis Michx. were planted in lab aquariums containing an iron and phosphorus rich sediments from the highly eutrophic Lake Müggelsee (Germany). Microsensors were used to analyse oxygen gradients near roots and diurnal changes of oxygen saturation in the rhizosphere. Under light conditions oxygen is detectable in a 0.5–1 mm thick zone around the roots. The detected maximum oxygen saturation at the root surface was approximately 30%. The sharp redox gradient at the root surface led to an oxidation of ferrous iron and a deposition of Fe(III) crusts around the roots. X-ray micro-analyses have shown that those crusts contain iron as well as phosphorus in high quantities. Chemical extraction of roots with iron crusts showed that more than 90% of the phosphorus they contain is reductive soluble phosphorus. Based on mesocosm experiments a phosphorus retention by iron precipitates around the roots of 0.08 g P per m for E. canadensis and 0.48 g P for P. crispus per m2 sediment surface was determined. These first results have shown, that the root oxygen release of submerged plants can form iron crusts in anaerobic sediment leading to an enhanced sorptive phosphorus fixation.

Redox sensitivity of iron in phosphorus binding does not impede lake restoration.

Iron salts have been regarded as unsuitable precipitants for sustainable sedimentary P retention, because Fe-bound P is released at low redox potential. The longevity of an Fe(3+) application (500 g m(-2)) in 1992 was studied in a dimictic lake. Release of Fe and P as well as their co-precipitation were observed dependent on artificial aeration in 2010 and only natural oxygenation in 2011. Sediment core stratigraphy by μX-ray fluorescence analysis revealed that Fe is relocating towards the surface, representing a dynamic P trap with a molar Fe:P ratio of 7. Even at this favourable ratio, P release cannot be suppressed. Settling fluxes of Fe, Mn and P, determined by a multi trap at two day resolution, during aeration and oxygenation, showed that released P can be efficiently precipitated independent of the nature of the oxygen supply. Thus, P release is not relevant for the P supply to the epilimnion, since at overturn most P is co-precipitated by the concurrently hypolimnetically accumulated Fe. To increase the availability of reactive (dithionite extractable) Fe for P binding, our Fe dosage calculation considers Fe in surplus. Beside external and internal P sources to be precipitated in a stoichiometric Fe:P ratio of 5, additional Fe equivalents of 25% for sedimentary organic carbon and to bind soluble sulfides are required. A long-term effect can be achieved only if the external P loading is sufficiently reduced, and Fe is added to ≥ 200 g m(-2).

Tube‐dwelling invertebrates: tiny ecosystem engineers have large effects in lake ecosystems

There is ample evidence that tube-dwelling invertebrates such as chironomids significantly alter multiple important ecosystem functions, particularly in shallow lakes. Chironomids pump large water volumes, and associated suspended and dissolved substances, through the sediment and thereby compete with pelagic filter feeders for particulate organic matter. This can exert a high grazing pressure on phytoplankton, microorganisms, and perhaps small zooplankton and thus strengthen benthic-pelagic coupling. Furthermore, intermittent pumping by tube-dwelling invertebrates oxygenates sediments and creates a dynamic, three-dimensional mosaic of redox conditions. This shapes microbial community composition and spatial distribution, and alters microbe-mediated biogeochemical functions, which often depend on redox potential. As a result, extended hotspots of element cycling occur at the oxic-anoxic interfaces, controlling the fate of organic matter and nutrients as well as fluxes of nutrients between sediments and water. Surprisingly, the mechanisms and magnitude of interactions mediated by these organisms are still poorly understood. To provide a synthesis of the importance of tube-dwelling invertebrates, we review existing research and integrate previously disregarded functional traits into an ecosystem model. Based on existing research and our models, we conclude that tube-dwelling invertebrates play a central role in controlling water column nutrient pools, and hence water quality and trophic state. Furthermore, these tiny ecosystem engineers can influence the thresholds that determine shifts between alternate clear and turbid states of shallow lakes. The large effects stand in contrast to the conventional limnological paradigm emphasizing predominantly pelagic food webs. Given the vast number of shallow lakes worldwide, benthic invertebrates are likely to be relevant drivers of biogeochemical processes at regional and global scales, thereby mediating feedback mechanisms linked to climate change.

Decision support for the selection of an appropriate in-lake measure to influence the phosphorus retention in sediments

Many in-lake measures which aimed to influence the phosphorus retention in lake sediments have failed to improve the trophic state of the lakes. The present paper introduces a systematic approach to select an appropriate in-lake measure. Before selecting an in-lake measure the goal of the measure should be defined, the problems of the lake must be identified, and the probability of success must be estimated. The proposed decision support consists of two parts. Part A, pre-selection, excludes inappropriate measures. It uses six criteria, which are mainly based on a simple mass-balance model, and the targets of restoration. The criteria describe the magnitude of the external versus the internal phosphorus loading, the dynamics of the internal load, and the lake morphometry. Each measure is weighted differently with respect to importance and suitability by specific quantified limits. Part B, selection, uses qualitative criteria, which are specific to the measure in question. Checking these criteria will help to select a measure with a low risk of failure. The suggested decision support is illustrated in flow charts and exemplified by Lake Arendsee in Germany.

Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality.

114 lakes treated with aluminum (Al) salts to reduce internal phosphorus (P) loading were analyzed to identify factors driving longevity of post-treatment water quality improvements. Lakes varied greatly in morphology, applied Al dose, and other factors that may have affected overall treatment effectiveness. Treatment longevity based on declines in epilimnetic total P (TP) concentration averaged 11 years for all lakes (range of 0-45 years). When longevity estimates were used for lakes with improved conditions through the end of measurements, average longevity increased to 15 years. Significant differences in treatment longevity between deeper, stratified lakes (mean 21 years) and shallow, polymictic lakes (mean 5.7 years) were detected, indicating factors related to lake morphology are important for treatment success. A decision tree developed using a partition model suggested Al dose, Osgood index (OI, a morphological index), and watershed to lake area ratio (related to hydraulic residence time, WA:LA) were the most important variables determining treatment longevity. Multiple linear regression showed that Al dose, WA:LA, and OI explained 47, 32 and 3% respectively of the variation in treatment longevity. Other variables (too data limited to include in the analysis) also appeared to be of importance, including sediment P content to Al dose ratios and the presence of benthic feeding fish in shallow, polymictic lakes.

Chemical Characteristics of Water and Sediment in Acid Mining Lakes of the Lusatian Lignite District

Hard and brown coal mining has a long tradition in central and eastern Europe and covers large areas of mining in Germany, Poland and the Czech Republic (Geller et al., this Vol.). In Germany there are three main districts of lignite surface mining: the Rheinish district near Cologne, the mid-German district around Leipzig and the Lusatian district around Cottbus in the most eastern part of Germany. Surface mining of lignite (brown coal) results in several environmental problems, for example important disturbances of the natural water balance, mass transfer of billions of tons of soil and devastation of nature. Among these, water acidification is an already well-known effect. Sulphide minerals, such as pyrite and marcasite, are commonly associated with coal and most metal ores. Weathering and oxidation of these minerals take place in the host rocks and substrates of the lignite horizons when they are exposed to air. The release of the oxidation products, mainly acidity, iron and sulphate, is known as acid mine drainage (AMD) and has been the subject of intense research for decades (e. g. Singer and Stumm 1970; Lowsen 1982; Nordstrom 1982; van Berk 1987; Morrison et al. 1990; Blowes et al. 1991, Hedin et al. 1994; Wisotzky 1994).

Phosphorus Retention Mechanisms in the Sediment of an Eutrophic Mining Lake

A small, highly eutrophic mining lake (Golpa IV) in eastern Germany with a continuous input of nutrients and metals was used to study the mechanisms of phosphorus (P) fixation in the sediment. The sediment (0-15 cm) is characterised by high contents of iron (96 mg g-1 DW), aluminium (37.3 mg g-1 DW) and sulphur (54.3 mg g-1 DW) and an extreme accumulation of some trace metals. Despite oxygen free conditions in the hypolimnion and intensive sulphate reduction in the sediment, high P retention rates could be calculated from dated sediment cores (1986-1995: 11 g P m-2 a-1). The lake has shown a rapid response to reduction of P loading.In some sediment layers unusually high total sediment P concentrations with more than 24 mg P g-1 DW were observed. More than 80% of total sediment P was bound in the BD-SRP and NaOH-SRP fractions (extraction scheme according to Psenner et al., 1984) which indicates that a substantial portion of deposited P is immobilised in an Fe or Al bound form. This corresponds well with the presence of oxidised Fe species at all sediment depths. Furthermore thermodynamic calculations indicate that vivianite precipitation is favourable in deeper anoxic sediment layers. The inventory or input of Fe or Al seems to be more important for the permanent P immobilisation in the sediment of the investigated mining lake than redox forced mobilisation processes (e.g. iron or sulphate reduction).

Quantification of phosphorus entrainment in a lowland river by in situ and laboratory resuspension experiments

Abstract.Hydrodynamics at the sediment water interface initiate and control the exchange of particles and associated phosphorus (P) between river sediment and the water column. Currently, no general analytical theory for cohesive sediment resuspension is available, so these properties must be inferred from direct measurements. In a moderately slow-flowing (annual mean velocity 0.1–0.3 m s-1) stretch of the lowland River Spree, Germany, an in situ experiment (INS) and a laboratory experiment (LAB) with surface sediment (0–3 cm) were conducted concurrently in May 2005 using an erosion chamber. In both approaches, the entrainment of particles and particulate P (PP) increased significantly as shear velocities were incrementally increased from 0.57 to 1.67 cm s-1. In repetitive INS runs, particle and PP entrainment rates at the lowest shear velocity applied differed by a factor of 5 and 11 (0.25 – 1.18 g m-2 h-1 and 4.1 – 45.9 mg m-2 h-1), respectively, due to in situ river bed heterogeneities, specifically the presence or absence of a high-porosity fluff layer. These rates were on average 48 and 3 times higher, respectively, than those of LAB runs (0.01– 0.02 g m-2 h-1 and 3.1– 14 mg m-2 h-1, respectively), suggesting the temporary availability of local, in situ, fluffy surface layers on the river bed that are not preserved during sediment preparation for LAB runs. Such a transient storage feature has not been described previously in the literature for river sediments. The entrainment of this easily resuspendable and P rich material at a low flow-generated shear velocity can lead to batch-wise P burdens in the water column and consequently to a displacement of eutrophication potential. Those events would be underestimated in LAB experiments devoid of this fluffy layer.