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Rivers transport carbon from continents to oceans. Surprisingly, this carbon has often been found to be centuries old, not originating from contemporary plant biomass. This can be explained by anthropogenic disturbance of soils or discharge of radiocarbon–depleted wastewater. However, land enclosure and channel bypassing transformed many rivers from anabranching networks to single–channel systems with overbank sediment accumulation and lowered floodplain groundwater tables. We hypothesized that human development changed the fluvial carbon towards older sources by changing the morphology of watercourses. We studied radiocarbon in the Elbe, a European, anthropogenically–transformed lowland river at discharges between low flow and record peak flow. We found that the inorganic carbon, dissolved organic carbon (DOC) and particulate organic carbon was aged and up to 1850 years old. The ∆14C values remained low and invariant up to median discharges, indicating that the sources of modern carbon (fixed after 1950) were disconnected from the river during half of the time. The total share of modern carbon in DOC export was marginal (0.04%), 72% of exported DOC was older than 400 years. This was in contrast to undisturbed forested subcatchments, 72% of whose exported DOC was modern. Although population density is high, mass balances showed that wastewater did not significantly affect the ∆14C-DOC in the Elbe river. We conclude that wetlands and other sources of contemporary carbon were decoupled from the anthropogenically transformed Elbe stream network with incised stream bed relative to overbank sediments, shifting the sources of fluvial carbon in favor of aged stores.

Long-term monitoring shows evidence of persistent changes in the magnitude and timing of the seasonal pattern and C-Q relations of nitrate concentrations in rivers, with possibly grave effects on aquatic ecosystems. Seasonal patterns of riverine nutrient concentrations are driven by a complex interplay of inputs, transport, and in-stream processing. Over multi-decadal periods, each of these factors may change due to socio-economic factors such as consumption patterns, governance regimes, or technological control measures. Here we test the hypothesis that observed multi-decadal changes in stream nitrate seasonality could be explained by changes in the relative importance of catchment nutrient sources over time. We analyze 66 years of shifting nitrate seasonality in a large, central-European river (Elbe) during a period of significant socio-political changes (1954 to 2019), with correspondingly significant changes in the sources of anthropogenic nitrate emissions. We show that the in-stream nitrate seasonality of the River Elbe changed significantly from a weak seasonal pattern (chemostatic) with peak concentrations during summer in the 1950s to a strong seasonal pattern (chemodynamic) with peak concentrations during winter in the 1990s. We link these shifts to a succession of technical/ political developments which influence the contribution of point and diffuse sources over time. Such shifts in seasonal concentration patterns can significantly impact the macronutrient (carbon, nitrogen, phosphorus) ratios in rivers, which in turn highly affect the health of aquatic ecosystems.

Environmental data are the key to define and address water quality and quantity challenges at catchment scale. Here, we present the first large-sample water quality data set for 1386 German catchments covering a large range of hydroclimatic, topographic, geologic, land use and anthropogenic settings. QUADICA (water QUAlity, DIscharge and Catchment Attributes for large-sample studies in Germany) combines water quality with water quantity data, meteorological and nutrient forcing data, and catchment attributes. The data set comprises time series of riverine macronutrient concentrations (species of nitrogen, phosphorus and organic carbon) and diffuse nitrogen forcing data at catchment scale (nitrogen surplus, atmospheric deposition and fixation). Time series are generally aggregated to an annual basis; however, for 140 stations with long-term water quality and quantity data (more than 20 years), we additionally present monthly median discharge and nutrient concentrations, flow-normalized concentrations and corresponding mean fluxes as outputs from weighted regressions on time, discharge, and season (WRTDS). The catchment attributes include catchment nutrient inputs from point and diffuse sources and characteristics from topography, climate, land cover, lithology and soils. This comprehensive, freely available data collection can facilitate large-sample data-driven water quality assessments at catchment scale as well as mechanistic modeling studies.

Over the last decades, treatment of domestic wastewater promoted by environmental regulations have reduced human health risks and improved water quality. However, ecological risks caused by effluents of wastewater treatment plants (WWTPs) discharged into rivers still persist. Moreover, the evolution of these ecological risks in the future is intimately related to effects of changing climate, especially regarding streamflow in receiving rivers. Here, we present a systematic and transferable framework for assessing the ecological risks posed by individual WWTP-effluents at the catchment-scale. The framework combines the size-class k of WWTPs, which is a proxy for load, with the location of their outflows in river networks, represented by its stream-order ω. We identify ecological risks by using three proxy indicators: the urban discharge fraction and the local-scale concentrations of each total phosphorous and ammonium-nitrogen discharged from WWTPs. About 3,200 WWTPs over three large catchments (Rhine, Elbe, and Weser) in Central Europe were analyzed by incorporating simulated streamflow for the most extreme projected climate change scenario. We found that WWTPs causing ecological risks in future prevail in lower stream-orders, across almost all size-classes. Distinct patterns of ecological risks are identified in the k-ω framework for different indicators and catchments. We show that, as climate changes, intensified risks are especially expected in lower stream-orders receiving effluents of intermediate size WWTPs. We discuss implications of our findings for prioritizing WWTPs advancement and urging updates on environmental regulations. Further applications of the k-ω framework are discussed to help achieving global long-term commitments on freshwater security.

Fe(III) hydroxides stabilize organic carbon (OC) and P in soils. Observations of rising stream Fe concentrations are controversially posited to result from a flushing of iron‐rich deeper soil layers or a decrease of competing electron acceptors inhibiting Fe reduction (NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ and SO42− $\mathrm{S}{{\mathrm{O}}_{4}}^{2-}$). Here, we argue that catchment topography constrains the release of Fe, OC, and P to streams. We therefore incubated organic topsoil and mineral subsoil and modified the availability of NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$. We found that Fe leaching was highest in topsoil. Fe, OC, and P released at quantities proportional to their ratios in the source soil. Supply of NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ reduced Fe leaching to 18% and increased pore water OC:Fe and P:Fe ratios. Subsoil, however, was an insignificant Fe source (, Plain Language Summary: Iron is the second most abundant metal in the crust; its cycle is tightly connected to those of carbon, oxygen, and sulfur. The oxidized form (FeIII) is almost insoluble, but Fe can be mobilized by complexation or microbial Fe reduction. Both processes depend on availability of organic C. We found that Fe concentrations in streams were constrained by the topography of catchments and NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ abundance. Shallower catchments are characterized by higher groundwater tables connecting the organic topsoils efficiently to streams. NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ suppresses Fe reduction as a competing electron acceptor to Fe. We conclude that trends in soil wetness or atmospheric N deposition can change the stability of Fe and thus the release of PO43− $\mathrm{P}{{\mathrm{O}}_{4}}^{3-}$ and harmful metals to surface waters., Key Points: Organic topsoils leach substantial amounts of Fe when incubated in the absence of NO3, a competing electron acceptor that inhibits Fe reduction. Shallow catchments with fluvially coupled topsoils and low NO3 availability release 200 fold more Fe than steep ones with high NO3 abundance. Catchment topography and NO3 availability explain 62%–64% of the variability of Fe concentration and OC:Fe and P:Fe ratios across 88 streams., EFRE‐Europe, https://doi.org/10.4211/hs.43601618877945c5a46b715aa98db729

High-frequency water quality monitoring is increasingly used in examining the nutrient fluxes within catchments. Despite this, no studies have assessed the impact of monitoring frequency on the uncertainty of nitrate estimates obtained from distributed or semi-distributed catchment models. This study aims to evaluate the impacts of two different frequencies of nitrate sampling on the performance of a catchment hydrology model, including the uncertainty in both predictions and calibrated parameters. The investigation uses the HYPE model to simulate streamflow and nitrate concentrations (2010–2015) in the Selke catchment, a heterogeneous mesoscale catchment in central Germany. The Bayesian inference scheme of the DREAM code was employed for calibration and uncertainty analysis, and to explore differences between fortnightly and daily nitrate sampling strategies. The results indicate that: (a) the posterior uncertainty intervals of nitrogen-export process parameters were narrower when the model was calibrated to daily nitrate measurements, while similar maximum likelihood parameter values were obtained regardless of the sampling frequency; (b) the model calibrated using daily nitrate data better represented both daily and fortnightly nitrate measurements relative to that obtained using fortnightly sampling; (c) the daily nitrate dataset produced significantly smaller parametric prediction uncertainty, but only modest reduction in total prediction uncertainty, relative to the fortnightly nitrate dataset; (d) model structural error and measurement errors are the primary sources of total prediction uncertainty, and these combine to inhibit the benefits of high-frequency monitoring. We conclude that the adequacy of sampling frequency is dependent on model structure and measurement errors, such that higher-frequency nitrate monitoring may not markedly reduce the uncertainty of nutrient predictions, depending on other levels and sources of uncertainty.

Climate change impacts on natural environments and human-built landscapes have been extensively studied from the meteorological, hydrological, agricultural, and urban point of views. Embracing the inevitability of climate change, there is a need for investigating and establishing adaptation strategies to changing climate conditions in order to protect essential resources for the survival of humans and ecosystems. Especially for surface water resources, water quality in rivers is a sensitive aspect which might be affected by the impact of climate change on hydrological regimes along river networks.In fact, with a grand target of achieving Good-Ecological-Status for all European surface water bodies, the implementation of the EU Water Framework Directive since year 2000 has facilitated remarkable reductions of point-source nutrient loads discharged from municipal wastewater treatment plants (WWTPs) into rivers. Nevertheless, satisfying the environmental regulations at the emission-pipe-end of individual WWTPs has not guaranteed a perfect resolution of river water quality problems (e.g., eutrophication) at the scale of entire river basins. This likely occurred because decisions concerning WWTPs size and location were mainly influenced by the scale and location of residential areas and driven by efficiency purposes. That is, the hydrological, biogeochemical, and ecological characteristics of river water bodies receiving the WWTPs emissions were less likely to be considered. Climate-change-driven shifts of hydrological regimes in rivers could exacerbate the current situation and accelerate the water quality degradation caused by the urban emissions.To tackle this issue, this study aims to decipher the interplays between WWTPs discharges and hydrological regimes of the receiving river water bodies, and to assess water quality risks due to WWTPs emissions under climate-change-induced alteration of hydrologic regimes, by using systematic and general tools at the scale of entire river networks (e.g., combined dimensions of stream-orders and WWTP-sizes). To this end, we synthesize the EU-scale reliable dataset for river networks and WWTPs and the simulation results of the mesoscale hydrologic model under a climate change scenario. We focus on nutrient concentrations (NH4-N, total P) and urban discharge fraction from WWTPs (i.e., the fraction of treated wastewater in river flows), performing the risk assessments for three large European river basins. Our diagnostic results at the river-network-scale could assist river basin managers and stakeholders to select WWTPs to be preferentially managed for minimizing water quality risks in the future under climate change. The presented concept here for the specific components is generally applicable to assess environmental risks and guide strategic management options for other pollutants in urban emissions (e.g., microplastics and pharmaceuticals).

There is significant debate about why less than half of European rivers and streams are in good ecological status, despite decades of intense regulatory efforts. Of the multiple stressors that are recognized as potential contributors to stream degradation, we focus on discharge from 26,500 European wastewater treatment plants (WWTPs). We tested the hypothesis that stream ecological status degradation across Europe is related to the local intensity of wastewater discharge, with an expected stream-order (ω) dependence based on the scaling laws that govern receiving stream networks. We found that ecological status in streams (ω≤3) declined consistently with increasing urban wastewater discharge fraction of stream flow (UDF) across river types and basins. In contrast, ecological status in larger rivers (ω≥4) was not related to UDF. From a continental-scale logistic regression model (accuracy 86%) we identified an ecologically critical threshold UDF = 6.5% ± 0.5. This is exceeded by more than one third of WWTPs in Europe, mostly discharging into smaller streams. Our results suggest that new receiving water-specific strategies for wastewater management are needed to achieve good ecological status in smaller streams.

Algae, as primary producers in riverine ecosystems, are found in two distinct habitats: benthic and pelagic algae typically prevalent in shallow/small and deep/large streams, respectively. Over an entire river continuum, spatiotemporal patterns of the two algal communities reflect specificity in habitat preference determined by geomorphic structure, hydroclimatic controls, and spatiotemporal heterogeneity in nutrient loads from point- and diffuse-sources. By representing these complex interactions between geomorphic, hydrologic, geochemical, and ecological processes, we present here a new river-network-scale dynamic model (CnANDY) for pelagic (A) and benthic (B) algae competing for energy and one limiting nutrient (phosphorus, P). We used the urbanized Weser River Basin in Germany (7th-order; ~8.4 million population; ~46 K km2) as a case study and analyzed simulations for equilibrium mass and concentrations under steady median river discharge. We also examined P, A, and B spatial patterns in four sub-basins. We found an emerging pattern characterized by scaling of P and A concentrations over stream-order ω, whereas B concentration was described by three distinct phases. Furthermore, an abrupt algal regime shift occurred in intermediate streams from B dominance in ω≤3 to exclusive A presence in ω≥6. Modeled and long-term basin-scale monitored dissolved P concentrations matched well for ω>4, and with overlapping ranges in ω

Due to global climate change, the past decade has been the warmest for Germany since the beginning of climate records. Not only air temperature but also precipitation patterns are changing and therefore influencing the hydrologic cycle. This will certainly influence the chemical status of ground- and surface water bodies as mobilization, dilution and chemical reactions of contaminants are altered. However, it is uncertain if those alterations will impact water quality for better or worse and how they occur spatially. Since water management in Europe is handled at the regional scale, we suggest that an investigation is needed at the same scale to capture and quantify the different responses of the chemical status of water bodies to climate change and extreme weather conditions. In this study, we use open-access data to (1) quantify changes in temperature, precipitation, streamflow and groundwater levels for the past 40 - 60 years and (2) assess their impacts on nutrient concentrations in surface- and groundwater bodies. To disentangle management from climate effects we pay special attention to extreme weather conditions in the past decade. Referring to the Water Framework Directive, we chose the river basin district Elbe as our area of interest. Preliminary results indicate that especially the nitrate concentrations in surface water bodies of the Elbe catchment were positively affected in the last two years, while no significant impact on nitrate levels in shallow groundwater bodies was witnessed. However, many wells showed the first significant increase in water table depth in both years since 1985, raising the question of how fast groundwater-surface water interactions will change in the next years.

Wastewater treatment plants (WWTP) are considered to be a point source of microplastic (particles5 mm) for riverine environments. However, data on microplastic effluent concentrations in WWTPs is collected with a broad range of methods, which impede comparisons across data sets. We provide an estimate of the annual emissions of microplastic particles by WWTPs into the ten major river basins of Germany. We analyze the concentration patterns of microplastics among different stream orders resulting from the spatial organization of WWTPs along the river network. The local in-stream microplastic concentrations are estimated through a network model that accounts for routing of microplastics through the entire fluvial network under the assumption of no losses by sedimentation, entanglement or degradation. Previous studies have observed microplastic concentrations in treated WWTPs effluents ranging several orders of magnitude. In 19 studies reviewed (2016-2020), the concentrations of observed microplastic concentrations (size range between 10 and 5000 μm) in 79 WWTP effluents ranged between 4 ∗ 10

Elevated concentration and high variance of dissolved organic carbon (DOC) in surface waters are a challenge for the production of drinking water. Past studies have indicated a dominant role of wetlands in DOC mobilization, but have mainly been focused on boreal and oceanic catchments. Here we analyze the observational DOC time series from 89 temperate humid catchments which drain into German drinking water reservoirs. We characterize the DOC concentration median and variability and utilize partial least squares regression in order to quantify the relation to catchment characteristics such as land use, climate, and topography. We found that the long-term median DOC concentration in the catchment is well predicted by the 90th percentile of the distribution of the topographic wetness index (0.9P TWI) over the entire catchment area. The 0.9P TWI can be directly connected to the abundance of riparian wetlands in the catchments. DOC concentration variability (represented as the ratio of the interquartile range and the median concentrations) was also found to be well predictable. Concentration variability was highest in cold and wet catchments with a high 0.9P TWI. Here we also found stronger correlations between DOC concentrations and discharge, with positive concentration-discharge patterns. Catchments with elevated DOC-concentration variance also exhibited the most severe long-term increases in concentrations. Our results thus indicate that, in temperate climates, riparian wetlands can be the dominant source zones of DOC and control the hydrological mobilization and potentially also the spatial difference in long-term concentration trends observed in surface waters. We conclude that the dominance of topography and climatic conditions in controlling spatio-temporal patterns in DOC concentrations leads to very limited management options.

n the framework of the BMBF funded project on Integrated Water Resources Management in Central Asia (Model region Mongolia, MOMO project, www.iwrm-momo.de) the objectives focused on supplementing, validating and extending the existing surveillance monitoring to the entire river basin for the time series 2006-2017. The MOMO monitoring programme was set up in order to observe seasonal variation in various water quality parameters along the main river course and its tributaries. A detailed sampling survey was carried out along the Kharaa River in the spring, summer and autumn of 2006 to 2017, extending from the headwaters in the Khentii Mountains to the outlet of the river basin. An additional continuous monthly monitoring programme for surface water quality was carried out upstream (Deed Guur) and downstream of Darkhan city (Buren Tolgoi) including the outlet of WWTP Darkhan in the time between 2007 and 2017. This strategy provides information for the efficient and effective design of future monitoring programmes with a focus on operational or investigative issues. The types of water sampling programmes included initial surveys as well as investigative and operational monitoring, point-source characterization, intensive surveys, fixed-station-network monitoring, groundwater monitoring, and special surveys involving chemical and biological monitoring. The water analyses have a focus on nutrients, heavy metals and metalloids, chloride, boron and the main physical water parameters. The dataset comprises also fluvial sediment analyses on heavy metals. In addition in 2017 a special hygienic monitoring (total coliforms, E. coli and fecal coliforms) has been carried out and was included in this database.

We employed the well-established Horton-Strahler, hierarchical, stream-order (ω) scheme to investigate scaling of nutrient loads (P and N) from ~845 wastewater treatment plants (WWTPs) distributed along the river network in urbanized Weser River, the largest national basin in Germany (~46K km2; ~8.4 million population). We estimated hydrologic and water quality impacts at the reach- and basin-scales, at two steady river discharge conditions (median flow, QR50; low-flow, QR90). Of the five WWTPs class-sizes (1 ≤ k ≤ 5), ~68% discharge to small low-order streams (ω

Hucho taimen, the world's largest salmonid, is a potadromous species that is listed as endangered in Mongolia. While mature individuals are known to have extended longitudinal movements of over 90 km along main river channels, details of the seasonal movements and residency of individuals within and between tributary habitats have been largely undocumented. Therefore, the current research aimed to detect and quantify the seasonal distances moved by adult taimen (65–96 cm; n = 10) within and between the Onon River (Amur River basin) and a major tributary (Balj) over 12 months using passive acoustic telemetry. The median distance moved by taimen in spring was 17.4 km (n = 6), in summer 9.1 km (n = 4), autumn 4.7 km (n = 9) and winter 0.4 km (n = 8). However, there were no statically significant differences amongst these median seasonal home range sizes. Two taimen traversed between the Onon River and the Balj tributary during the study period, recording overall home ranges of 44.5 km and 126.1 km. One of these individuals moved twice, exiting the tributary in autumn and returning again in spring where it remained for 36 days, while the second taimen moved only once into the tributary in spring and re-entered the main channel in late summer after 85 days. Another two taimen entered surrounding smaller tributaries and recorded home ranges of 26.1 km and 29.2 km, while all remaining individuals were detected moving only within the waterway where they were originally captured and released (0.8–20.1 km). Taimen movements within tributaries such as the Balj can be extensive (

Past and present gold mining operations scattered throughout the Kharaa River basin, Mongolia, have been identified as a major source of heavy metal and metalloid contamination. However, the potential accumulation of toxic contaminates including Cr, Zn, As, Cd, Hg, Cu, Ni and Pb in the resident fish fauna and the subsequent human health risks associated with their consumption have previously not been quantified. In the current study, contaminates in water, sediment and five consumed fish species (Leuciscus baicalensis, Thymallus baicalensis, Brachymystax lenok, Lota lota and Silurus asotus) were examined. The results indicated that concentrations of As and Hg exceeded the national permissible limits for drinking water in the Gatsuurt tributary of 10 μg L-1 and 0.05 μg L-1 respectively, while Hg contents detected in the sediment of the Boroo tributary were highly elevated (0.78 μg g-1). Heavy metal and arsenic accumulation was evident in all five fish species sampled across the basin, with maximum muscle contents of Cr, As, Hg and Pb detected in several species caught in the middle and lower river reaches, while Zn was highly elevated in B. lenok collected in the upper tributaries. Elevated median contents of Cr, Cu, Hg and Pb increased with trophic level, with Hg accumulation posing the greatest threat to humans as 10.7 % of all fish sampled in the study exceeded the internationally recommended threshold for Hg in consumable fish tissue. Although recreational fishing is rapidly growing throughout Mongolia, the overall level of fish capture and consumption remains relatively low. However, increasing pollution and accumulation in resident fish species could lead to chronic heavy metal intoxication in people who consume them regularly from the most polluted regions of the basin, while additionally being exposed to other sources of contamination.

High-frequency sensor measurements enable calculation of continuous autotrophic nitrate uptake rate based on its intrinsic relationship with gross primary production (GPP). The spatiotemporally available data offer prospects to advance process understandings across scales. We used continuous 15-min data (2011–2015) from a forest upstream reach and an agricultural downstream reach of the Selke River, Germany. Based on the high-frequency data, we developed a parsimonious approach for regionalizing the autotrophic uptake rate, considering effects of global radiation and riparian shading. For networked modeling, we integrated this approach into the fully distributed mesoscale hydrological nitrate model (mHM-Nitrate). Daily GPP-based uptake rate calculations showed distinct seasonal patterns and ranges in the agricultural and forest streams (mean values were 80.9 and 15.5 m g N m − 2 d − 1 , respectively). Validation in the two streams showed acceptable performance (R2 = 0.47 and 0.45, respectively) and spatial transferability of the regionalization approach, given its parsimony. Networked modeling results showed high spatiotemporal variability in nitrate transport and uptake throughout the river network. The magnitude of gross uptake increased, whereas uptake efficiency decreased significantly along stream order. Longitudinal analysis in the main stem of the Selke River revealed that riparian shading and inter-annual hydrochemical variations strongly influenced daily dynamics of the uptake efficiency. This study provides a parsimonious and transferable procedure for regionalizing in-stream autotrophic nitrate uptake based on high-frequency data at reach scale. Integrating this approach in the mHM-Nitrate model allows detailed nitrate transport and in-stream uptake processes to be investigated throughout river networks.

Summary For capturing spatial variations of runoff and nutrient fluxes attributed to catchment heterogeneity, multi-site hydrological water quality monitoring strategies are increasingly put into practice. This study aimed to investigate the impacts of spatially distributed streamflow and streamwater Inorganic Nitrogen (IN) concentration observations on the identification of a continuous time, spatially semi-distributed and process-based hydrological water quality model HYPE (HYdrological Predictions for the Environment). A Bayesian inference based approach DREAM(ZS) (DiffeRential Evolution Adaptive Metrololis algorithm) was combined with HYPE to implement model optimisation and uncertainty analysis on streamflow and streamwater IN concentration simulations at a nested meso scale catchment in central Germany. To this end, a 10-year period (1994–1999 for calibration and 1999–2004 for validation) was utilised. We compared the parameters’ posterior distributions, modelling performance using the best estimated parameter set and 95% prediction confidence intervals at catchment outlet for the calibration period that were derived from single-site calibration (SSC) and multi-site calibration (MSC) modes. For SSC, streamflow and streamwater IN concentration observations at only the catchment outlet were used. While, for MSC, streamflow and streamwater IN concentration observations from both catchment outlet and two internal sites were considered. Results showed that the uncertainty intervals of hydrological water quality parameters’ posterior distributions estimated from MSC, were narrower than those obtained from SSC. In addition, it was found that the MSC outperformed SSC on streamwater IN concentration simulations at internal sites for both calibration and validation periods, while the influence on streamflow modelling performance was small. This can be explained by the “nested” nature of the catchment and high correlation between discharge observations from different sites. Results revealed, also, that 95% prediction confidence intervals of streamflow and streamwater IN concentration estimated from MSC were more credible compared with those estimated from SSC, which are reflected by narrower confidence intervals and higher percentage of observations bracketed in the estimated unit confidence intervals. The outcomes of this study pointed out the importance of spatially distributed hydrological water quality observations for improving model parameter identification and provided guidelines for choice of adequate calibration strategy and design of hydrological water quality monitoring campaign.

Increasing concentrations of dissolved iron and DOC are likely linked to decreasing nitrogen depositon.

Two morphologically similar pre-dams (Hassel and Rappbode pre-dams, Harz Mountains, Germany) of a large reservoir with different land use in their catchments were compared with respect to their physical, chemical, and biological properties in order to test if differences can be attributed to the different land use. In addition, local geology, soil types, and topography were evaluated in detail to distinguish its influence from that of present-day land use. Despite a remarkable similarity in physical variables and stratification, the annual development of hydrochemistry and biology differed between Rappbode and Hassel pre-dam. While the Hassel pre-dam received much higher nitrogen and phosphorus inputs from the catchment, the Rappbode pre-dam received more dissolved organic carbon. The quality of dissolved organic carbon also differed between the two catchments, indicating different sources. The higher residence time of Hassel pre-dam amplified the effects of these inputs on the trophic state of both pre-dams. The phytoplankton communities in summer were dominated by diatoms in the Rappbode pre-dam and by cyanobacteria in the Hassel pre-dam. In conclusion, land use appeared to be an important driver for the observed lake characteristics. However, it was itself strongly related to soil types and topography, which on the other hand influences the residence time of water within the catchments.

Summary The estimation of stream metabolism represents a useful technique for characterising the ecological status of natural lotic systems. The existing tools for investigating metabolism in situ demand expert knowledge, provide only a few options, ignore possible groundwater effects and often rely on modelled input data, such as gas exchange, that could be subject to error. We have developed MeCa, a user-friendly MATLAB toolbox that contains different options – including the groundwater correction – for the calculation of metabolism. A detailed explanation of all the calculation steps and the various calculation options is given in the Appendix S1. The functions of MeCa can be tested using the test data set provided. We conducted sensitivity tests to assess the effect of certain parameters (reaeration, travel time, groundwater oxygen concentration, mean depth and width) and other measurements (oxygen, temperature, pressure) on the metabolism estimation. Uncertainties in the parameters have a greater effect than those that apply to the measurements. The parameters reaeration coefficient and mean depth are the most sensitive input data. Results from sensitivity analyses show the complexity of metabolism studies in streams and the importance of direct calculations. MeCa includes all relevant calculation options, and thus makes a contribution towards the standardisation of metabolism studies.

Heat is increasingly used as a natural tracer to quantify water fluxes at the groundwater-surface waterinterface. We present a systematic approach to monitor and evaluate stream and streambed temperatures to derive daily-updated temperature-based water exchange fluxes between the stream and the streambed. Specifically designed multi-level temperature sensors coupled with a data logger and GSM modem are used to monitor temperature in the stream and streambed and transfer this data daily to a database. A suite of MATLAB scripts with structured query language (SQL) commands is applied to extract the data for processing using an inverse numerical model to estimate water flow based on the measured temperatures. Compared to common analytical approaches, which typically require sinusoidal diurnal temperature pattern, our numerical model can utilize temperature records without daily variations. Temperature-based calculations to quantify vertical water fluxes at the stream-groundwater interface can provide a supplement to, or even a replacement of, calculations based on vertical hydraulic gradients and Darcy’ law.

Validation of hydrodynamic two-dimensional (2D) sediment transport models is often difficult, because of their high demands for spatially distributed sediment deposition data. Therefore, the objective of this study is to introduce a new method for the validation of distributed hydrodynamic sediment transport models, which relies on the linear relationship between sedimentation and heavy metal concentration in the topsoil of riverine floodplains. The tracer method was tested in the heavily contaminated 45 km 2 large floodplains of River Mulde near Bitterfeld (Germany), using 129 topsoil concentrations of cadmium (Cd), zinc (Zn), and arsenic (As). Sediment deposition during flood events was simulated using the hydrodynamic and sediment transport model Telemac2D. A monotonic increase in median heavy metal concentrations with increasing sediment deposition classes was found by comparing simulated sediment deposition classes to associated heavy metal concentrations of the topsoil. These findings were confirmed for Cd, Zn, and As for more than three sediment deposition classes. Results of correlation analyses indicate highly significant linear positive relationships between heavy metal concentrations and sediment deposition. The proposed method is easy to apply, and time-consuming event-based measurements are not required, because of the use of time-integrated measurements of topsoil heavy metal concentrations. The new method enables the user to evaluate the validity of 2D sediment transport models at least in a semi-quantitative manner, by comparing given heavy metal concentrations of the topsoil of active floodplains to simulated sediment deposition.

Riverine dissolved organic carbon (DOC) supports the production of estuaries and coastal ecosystems, constituting one of the most actively recycled pools of the global carbon cycle. A substantial proportion of DOC entering oceans is highly aged, but its origins remain unclear. Significant fluxes of old DOC have never been observed in temperate headwaters where terrestrial imports take place. Here, we studied the radiocarbon age of DOC in three streams draining forested headwater catchments of the river Mulde (Ore Mountains, Germany). In a 4 week summer precipitation event DOC aged at between 160 and 270 years was delivered into the watershed. In one stream, the DOC was modern but depleted in radiocarbon compared to other hydrological conditions. The yield was substantial and corresponded to 20–52 % of the annual DOC yields in wet and dry years, respectively. The analysis of long-term data suggested that the DOC export in extreme precipitation events added to the annual yield and was not compensated for by lower exports in remaining periods. We conclude that climate change, along with additional processes associated with human activities, channels old soil carbon into more rapidly cycled carbon pools of the hydrosphere.

Reservoirs can be viewed as sentinels of their catchments and a detailed monitoring of reservoir systems informs about biogeochemical and hydrological processes at the catchment scale. We developed a comprehensive online monitoring system at Rappbode reservoir, the largest drinking water reservoir in Germany, and its inflows. The Rappbode Reservoir Observatory comprises of a set of online-sensors for the measurement of physical, chemical, and biological variables and is complemented by a biweekly limnological sampling schedule. Measurement stations are deployed at the four major inflows into the system, at the outlets of all pre-reservoirs, as well as in the main reservoir. The newly installed monitoring system serves both scientific monitoring and process studies, as well as reservoir management. Particular emphasis is paid to the monitoring of short-term dynamics and many variables are measured at high temporal resolution. As an example, we quantitatively documented a flood event which mobilised high loads of dissolved organic carbon and changed the characteristics of the receiving reservoir from eutrophic to dystrophic within a few days. This event could have been completely missed by conventional biweekly sampling programs, but is relevant for biogeochemical fluxes at the catchment scale. We also show that the high frequency data provide a deeper insight into ecosystem dynamics and lake metabolism. The Rappbode Reservoir Observatory; moreover, offers a unique study site to apply, validate, and develop state of the art lake models to improve their predictive capabilities.

Reductions in emissions have successfully led to a regional decline in atmospheric nitrogen depositions over the past 20 years. By analyzing long-term data from 110 mountainous streams draining into German drinking water reservoirs, nitrate concentrations indeed declined in the majority of catchments. Furthermore, our meta-analysis indicates that the declining nitrate levels are linked to the release of dissolved iron to streams likely due to a reductive dissolution of iron(III) minerals in riparian wetland soils. This dissolution process mobilized adsorbed compounds, such as phosphate, dissolved organic carbon and arsenic, resulting in concentration increases in the streams and higher inputs to receiving drinking water reservoirs. Reductive mobilization was most significant in catchments with stream nitrate concentrations < 6 mg L-1. Here, nitrate, as a competing electron acceptor, was too low in concentration to inhibit microbial iron(III) reduction. Consequently, observed trends were strongest in forested catchments, where nitrate concentrations were unaffected by agricultural and urban sources and which were therefore sensitive to reductions of atmospheric nitrogen depositions. We conclude that there is strong evidence that the decline in nitrogen deposition toward pre-industrial conditions lowers the redox buffer in riparian soils, destabilizing formerly fixed problematic compounds, and results in serious implications for water quality.

Aquatic bacteria are considered to exhibit a paradoxical behaviour. They luxuri-ously consume phosphorus, the element often restricting the abundance of algae,which provide the organic substrates maintaining bacterial growth. Here, we testthe hypothesis that bacteria can limit their uptake of phosphorus and increase theavailability of phosphorus to algae. The physiological costs for bacteria must becompensated for by a surplus of photosynthetic exudates facilitating higherbiomass production. To test the potential of such an economic behaviour, we useda new differential equation model that was parameterized by independent experi-ments. Model results indicate that this potential does exist. As a consequence, weconducted continuous growth chemostat experiments. Bacteria did not leave morephosphorus to, “high exudation” algae compared with algae with low release.Therefore, the hypothesis was not supported by the experiments. However, bac-teria significantly increased production 1.4–1.8-fold in cultures with “high exud-ation” algae. This was explained by an increase in conversion of organic carbonfrom growth medium into bacteria biomass. Algal exudates were quantitativelynegligible but could act as growth factors. The results show that biomass of algaeand bacteria cannot be predicted solely by mineral nutrients and carbon asassumed by the classical theory.KEYWORDS: bacteria; phytoplankton; exudation; phosphorus; differentialequation model

An assessment of water quality measurements during a long-lasting low water period in the Elbe River is presented. Weekly samples were taken from May to December 2003 at a sampling site in the middle part of the Elbe River. For multivariate data analysis, 34 parameters of 46 samplings were considered. As a result of this analysis, 78% of the variance of the data set is explained by five factors. They can be assigned to the following latent variables: season (37.5%) > tributaries (12.7%) > re-suspension (10.4%) > discharge (9.4%) > complexation (8.5%). For the investigated sampling site, two key processes were identified as dominating factors on the water quality during low water conditions. First, seasonal phytoplankton development caused changes in redox conditions with consequences for re-solution of pollutants from sediments. Second, tributaries have a higher impact on the main stream, due to changes in mixing processes. Therefore, in addition to flood investigations, monitoring strategies, and management plans should be developed in order to survey changes in water quality during low water conditions.

Extreme fluvial floods may cause severe contamination with fuel oil and diesel, originating from gasoline pipes and tanks in private households and industrial areas, respectively. Geo-referenced oil spills in the region of Bitterfeld (Germany) after extreme flood events, such as in August 2002, were simulated using the two-dimensional (2D) Finite Element model system Telemac2D, which is subdivided into a hydrodynamic (Telemac-2D) and a transport module (Subief2D). Fuel oil settled via adhesion showed a thickness of less than 1.0 mm. Fuel oil concentrations on the flood wave amounted up to 80 g m -3 in the vicinity of the point sources. At a distance of several hundred meters downstream of the point sources, the fuel oil concentrations were calculated to be zero. Settled areas were only partially contaminated with fuel oil. While one village experienced severe oil contamination, the town of Bitterfeld was almost unaffected by oil spills. It was demonstrated that the 2D transport model applied is capable of simulating fuel oil spills during extreme high waters in the terrestrial environment. Such simulations of fuel oil spills will feed into a GIS-based decision support system of flood protection.

Beside damages of infrastructure in industrial regions, extreme floods can cause contamination with particle-bound pollutants, e.g., due to erosion of soils and sediments. In order to predict contamination with inorganic pollutants, the transport and fate of arsenic, lead, and mercury during a fictive flood event of River Vereinigte Mulde in the region of Bitterfeld (Germany) with 200 years recurrence time was modeled. The finite element model system Telemac2D, which is subdivided into a hydrodynamic (Telemac-2D), a transport (Subief-2D), and a water quality module (wq2subief) was applied. The transport and water quality model models were calibrated using results of sediment trap exposures in the floodplain of River Vereinigte Mulde. Model results exhibited that the spatial patterns of particle-associated arsenic and lead concentrations significantly changed. Extended, mostly agriculturally used areas showed arsenic and lead concentrations between 150 and 200 mg kg -1 and 250 and 300 mg kg -1 , respectively, while urban areas were to a great extent spared from high contamination with arsenic and lead. Concentrations of particle-associated mercury showed a pattern distinct from those of arsenic and lead. Outside of small patches with concentrations up to 63 mg kg -1 , concentrations of particle-associated mercury remained close to zero. Differences in the spatial patterns of the three pollutants regarded mainly arise from significantly different initial and boundary conditions. Sensitivity analyses of initial and boundary conditions revealed a high sensitivity of particle-bound pollutant concentrations, whereas the sensitivities of concentrations of suspended sediments and soluble pollutants were mediocre to negligible.

A strategy to neutralize acidic pit lakes was tested in an upscaling process using field mesocosms of 26 to ca. 4500 m3 volume in the acidic pit Mining Lake 111 in Germany. After addition of the substrates Carbokalk and straw a neutral sediment layer formed, in which microbial sulfate and iron reduction as well as sulfide precipitation occurred. The net rate of neutralization was limited by the precipitation of iron sulfides rather than by microbial reactions. Oxidation of H2S by ferric iron in the anoxic sediment lowered the net sulfate reduction rate. Seasonal fluctuations of iron sulfides in the sediment showed that the reaction products were not necessarily stable. The long-term success of the approach depends on the net partition of the precipitated iron-(mono-/di-) sulfide that is permanently buried in the anoxic sediment. It could be shown by field experiments that the long-term success of the neutralization depends on the spatial scale and duration of the experiments. Volumes from 26 to 4500 m3, exposition times from 4 months to 5 years, and increasingly thick coverings of the sediments with straw, from zero to 40 cm, were used. Net neutralization rates decreased from 41 meq m− 2 d− 1 in laboratory microcosms to a mean rate of 2.3 meq m− 2 d− 1 in the 4500 m3 field experiment. The results show that the success of the microbial treatment of acid pit lakes lastly depends on the limnological conditions in the lake that cannot be simulated by upscaling of simple laboratory experiments.

Phosphorus import by nordic geese (Anser fabalis and Anser albifrons) was investigated in Lake Arendsee, located in the Saxony-Anhalt region, Altmark, Germany during the period 1996 to 1997. Phosphorus contained in geese excrement on the ice was measured in the winters 1996 and 1997. In February 1996 (after 9 days of frozen lake surface) two excrement fields amounted to 80 ha and 30 ha in area and in January 1997, 10 days after ice closure, the excrement field was 106 ha large. The weight of excrement was estimated to be 148 to 266 g m -2 fresh weight (mean 201 g m -2 ) in 1996 and 83 g m -2 to 408 g m -2 (mean of 243 g m -2 ) in 1997. The average of phosphorus content was 8.5 mg g -1 dry weight in 1997 and 9.2 mg g -1 in 1996. Based on these values the phosphorus input attributed to nordic geese was calculated. Our results demonstrated a phosphorus import in 1996 after 9 days of frozen lake surface of 251 kg and in 1997 after 10 days of freezing of 173 kg. During 100 days of wintering, the nordic geese on Lake Arendsee produced a phosphorus load of 2.8 t in 1996 and 1.7 t in 1997. Compared with the annual phosphorus import from different sources, the contribution by nordic geese was 88 % in 1996 and 92 % in 1997. Its yearly phosphorus load during the winter months appears as a significant eutrophication factor for the trophic level of Lake Arendsee. However, the annual external load is approximately 10 % of the phosphorus poolsize in the lake water, and even less when considering the amount lodged in the bottom sediments.

Zusammenfassung Osteochondrale Lasionen des Talus betreffen haufig junge, sportlich ambitionierte Patienten. Verschiedene Pathomechanismen haben einen Einfluss auf die Entstehung einer osteochondralen Lasion: Trauma, ligamentare Instabilitat, Hypovaskularitat, Malalignment. Nach dem Verstandnis der Autoren sollen diese in der Diagnostik abgeklart und in die Therapiewahl einbezogen werden. Die Auswahl an Therapiemoglichkeiten ist sehr breit. Es gibt nur sehr wenige qualitativ gute Studien, nach denen man seine Wahl richten kann. Ein groser Teil der Therapie beruht auf Erfahrung und Wissen des Operateurs. Zur Auswahl stehen chondrale, ossare und osteochondrale Therapiemoglichkeiten. Das vorliegende Review soll einen Uberblick uber den aktuellen Wissensstand und die Therapieoptionen geben.

Artificial-lawn mats were used as sediment traps in floodplains to measure sediment input and composition during flood events. To estimate the natural variability, 10 traps were installed during two flood waves at three different morphological units in a meander loop of the River Elbe. The geochemical composition of deposited and suspended matter was compared. The sediment input showed weak correlations with concentration and composition of river water. It also correlated poorly with flood duration and level as well as distance of trap position from the main river. This is due to the high variability of the inundation, different morphological conditions and the variability of sources. The composition of the deposits and the suspended matter in the river water was comparable. Hence, for the investigated river reach, the expected pollution of the floodplain sediments can be derived from the pollution of the suspended matter in the river during the flood wave.

As benthic biofilms mediate essential functions in stream ecosystems (e.g., carbon flux, storage of nutrients and other substances), the element-specific regulation of the biofilm composition is of great interest. We tested whether (1) the elemental composition of biofilms is related to that of the water column and (2) there are different accumulation patterns from the dissolved phase (adsorption) and the particulate phase (incorporation of suspended matter). We analysed biomass parameters, nutrients and metals in biofilms and surface waters at 28 sites within a stream network (Bode catchment, Germany). Algal biomass in biofilms was dominated by diatoms. The P/C ratio in biofilms was positively related to total phosphorus of surface water (and to the proportion of agricultural area in the catchment) indicating phosphorus limitation of biofilms, whereas the N/C ratio was not related to nitrate levels of surface water, and neither the P/C nor the N/C ratio to the concentration of dissolved organic carbon (DOC) of surface water. Biofilms were enriched in metals compared to their concentrations in water. The metals in biofilms were positively related to the concentration of dissolved metals in surface water for iron and strontium (but not for manganese, copper, zinc, arsenic or lead) and to the concentrations of particle-associated metals of surface waters for strontium and lead. Manganese and arsenic were the metals with a negative effect on the biomasses of biofilm diatoms and cyanobacteria. Overall, we observed element-specific accumulation patterns in biofilms with selected elements being related to the water column while others were probably subject to biofilm-internal processes.

The objective of this case study was to calibrate and verify a detailed sediment transport model in a 4-kilometre stretch of the middle Elbe floodplains in Germany. The hydraulic RMA-2 model and the SED2d-WES sediment transport model were used. The sediment transport model was verified with a flood event by detailed measurements of surface water levels, flow velocities at six profiles, suspended sediment concentration and sediment deposition (by means of 10 sediment traps). Discharge was modelled for three steady-state conditions. Surface water levels could be calculated to an accuracy of less than 5 cm compared to measurements. The differences between calculated and measured velocities were with one exception smaller than 0.2 m/s (measured range 0.1...1.0 m/s). Simulated average sediment input amount to 35 g/(m 2 d) for the given flood event. The highest calculated sedimentation rates of 700 g/(m 2 d) (dry density 90 kg/m 2 ) took place in backwaters and abandoned channels. Twenty-five percent of the deposited sediment settled in the backwaters (which only account for 13 percent of the area). The most sensitive parameters of the sediment transport model were the settling velocity and critical shear stress. The modelling approach allowed a realistic spatial distributed calculation of sediment inputs into the floodplain of the Elbe River.

Dissolved organic carbon (DOC) transported by rivers represents an important link between carbon pools of terrestrial and oceanic ecosystems. However, it is unclear how frequent DOC must be sampled to obtain reasonable load estimates. Here, we used continuous records of the specific UV absorption coefficient (SAC) and discharge from a headwater stream at the Ore Mountains (Germany) to calculate load errors depending on DOC sampling frequency. SAC was used as a proxy for DOC. The results show that the load was underestimated by 13-19% with monthly, 10-13% with bi-weekly and 7-9% with weekly DOC samplings, respectively. We conclude that collecting additional data from high discharge events decrease the error significantly.

Zusammenfassung Ein akzessorischer M. soleus ist ein seltener Befund, welcher sich klinisch als eine Weichteilraumforderung dorsomedial des oberen Sprunggelenkes darstellt. Die uberwiegende Anzahl der Patienten ist asymptomatisch. Beschwerden konnen auftreten und sind haufig mit sportlicher Belastung assoziiert. Das MRT zeigt das charakteristischerweise normale Muskelgewebe in abnormaler Lokalisation und dient zur differentialdiagnostischen Abgrenzung vom pathologischen Befund.

An exact morphometric description of Mining Lake 111, Brandenburg, Germany, was obtained after a new survey in 1996 by the staff of the water research department. Volume and surface area of ML111 were calculated based on values of this survey. An actual bathymetric map was created from the datapool which was the basis for the selection of sampling sites where we obtained sediment cores to describe the geochemistry of the lake sediments. Principal Component Analysis (PCA) was carried out to examine patterns and similarities between concentrations of different heavy metals. The patterns are different for the three basins within the lake. The spatial distributions of 12 elements in the sediment surface were estimated with a geostatistical procedure (Ordinary Kriging) as well as with a conventional interpolation method. The structure found with PCA was confirmed by the plots from spatial interpolation. Due to the complexity of the lake morphometry and bathymetry 66 sediment cores taken at 47 sampling sites were not sufficient to reduce the spatial variance distributions to acceptable values.