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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.

The role of hydromorphological degradation and temporal variation for food webs in human-modified rivers is still not fully evaluated. We tested the hypothesis that man-made engineering structures alter macroinvertebrate resource use in the Elbe River (Germany) in relation to seasonal variation. Stable isotopes (δ13C, δ15N) and mixing models revealed that dietary contributions of benthic organic matter (BOM) and phytoplankton were driven by engineering structure. Contributions of biofilm were driven by season, while contributions of terrestrial particulate organic matter (t-POM) were driven by both engineering structure and season. Contributions of t-POM were larger than those of phytoplankton in spring and summer, but not in autumn, which adds to the debate about the sources of organic matter fuelling riverine benthic food webs. Resource availability was not systematically related to resource use, indicating that factors other than resource limitation were responsible for the observed results. By demonstrating that human alterations determine consumer resource use independently from resource availability, our study links hydromorphological modifications to fluxes of matter in riverine food webs. Future studies should quantify organic matter fluxes from various autochthonous and allochthonous pathways in human-modified and natural rivers to allow for a robust synthesis of how hydromorphological modifications alter benthic food webs.

Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A follow-up monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overlooked by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management.

The appearance of a Metalimnetic Oxygen Minimum (MOM) has been recorded in many natural lakes and reservoirs. However, its isotopic characterization with oxygen and carbon stable isotopes have not yet been fully constrained. The purpose of this work is to apply stable isotopes of carbon and oxygen to characterize photosynthetic and respiratory activities in the Rappbode Reservoir, Germany. Here we present the results of a 48 hours intensive sampling in July 2020. We provide preliminary data of dissolved oxygen (DO) concentration and saturation profiles, together with dissolved inorganic carbon (DIC). This includes data of oxygen (δ18ODO) and carbon (δ13CDIC) stable isotopes with particular focus on the metalimnion. Our profiles identified minor differences from day to day, such as an additional respiration-like peak above the MOM. Samples from within the MOM were characterized by the lowest oxygen content and saturation (7.05 mg L-1 and 75 %, respectively) and the highest δ18ODO (up to +30.1‰). These samples also showed the lowest δ13CDIC values (down to -12.8‰). Surface layers (between 0 and 9 meters) and the deepest water sample (65 meters) did not follow the expected depth profiles for δ18ODO and δ13CDIC. Surface layers were likely influenced by equilibration with the atmosphere that was enhanced by windy conditions. On the other hand, samples from 65 meters depth could have been perturbed by the presence of organic material from sediments. Moreover, oxygen and carbon stable isotopes patterns were able to show that the MOM can migrate within the water column up to 5 meters within 12 hours. These findings offer an interesting basis to establish mass balances of oxygen and carbon turnover in water bodies.

Dissolved organic matter plays an important role in aquatic ecosystems and poses a major problem for drinking water production. However, our understanding of DOM reactivity in natural systems is hampered by its complex molecular composition. Here, we used Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and data from two independent studies to disentangle DOM reactivity based on photochemical and microbial-induced transformations. Robust correlations of FT-ICR-MS peak intensities with chlorophyll a and solar irradiation were used to define 9 reactivity classes for 1277 common molecular formulas. Germany's largest drinking water reservoir was sampled for 1 year, and DOM processing in stratified surface waters could be attributed to photochemical transformations during summer months. Microbial DOM alterations could be distinguished based on correlation coefficients with chlorophyll a and often shared molecular features (elemental ratios and mass) with photoreactive compounds. In particular, many photoproducts and some microbial products were identified as potential precursors of disinfection byproducts. Molecular DOM features were used to further predict molecular reactivity for the remaining compounds in the data set based on a random forest model. Our method offers an expandable classification approach to integrate the reactivity of DOM from specific environments and link it to molecular properties and chemistry.

Metalimnetic oxygen minima are observed in many lakes and reservoirs, but the mechanisms behind this phenomenon are not well understood. Thus, we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir (Germany) with a well-established two-dimensional water quality model (CE-QUAL-W2) to systematically quantify the chain of events leading to its formation. We used high-resolution measured data to calibrate the model, which accurately reproduced the physical (e.g. water level and water temperature), biogeochemical (e.g. nutrient and oxygen dynamics) and ecological (e.g. algal community dynamics) features of the reservoir, particularly the spatial and temporal extent of the MOM. The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes. The occurrence of the cyanobacterium Planktothrix rubescens in the metalimnion delayed and slightly weakened the MOM through photosynthesis, although its decaying biomass ultimately induced the MOM. Our research also confirmed the decisive role of water temperature in the formation of the MOM since the water temperatures, and thus benthic and pelagic oxygen consumption rates, were higher in the metalimnion than in the hypolimnion. Our model is not only providing novel conclusions about the drivers of MOM development and their quantitative contributions, it is also a new tool for understanding and predicting ecological and biogeochemical water quality dynamics.

To counteract the severe consequences of eutrophication on water quality and ecosystem health, nutrient inputs have been reduced in many lakes and reservoirs during the last decades. Contrary to expectations, in some lakes phytoplankton biomass did not decrease in response to oligotrophication (nutrient reduction). The underlying mechanisms preventing a decrease in biomass in these lakes are the subject of ongoing discussion. We used a hitherto unpublished long‐term data set ranging from 1961 until 2016 from a German drinking water reservoir (Rappbode Reservoir) to investigate the underlying mechanisms preventing a decrease in biomass. Total phosphorus (TP) concentrations in the Rappbode Reservoir dropped abruptly in 1990 from 0.163 to 0.027 mg/L within three consecutive years, as a result of banning phosphate‐containing detergents. Despite substantial reductions in TP, total annual phytoplankton biomass did not decline in the long‐run, and therefore, the yield of total phytoplankton biomass per unit phosphorus largely increased. Regression analysis revealed a positive association between the yield and potentially phagotrophic mixotrophs (R² = .465, p

The invasive clam Corbicula fluminea was first recorded in Lake Constance (Germany) in 2003 and subsequently established high population densities at various locations in the lake. In 2010, four standardized field experiments were conducted in Lake Constance, one in each season, to investigate seasonal changes in somatic growth and survival of C. fluminea at different water depths. Somatic growth and survival of the clams were related to seasonal variations in water temperature and seston characteristics, that is, phytoplankton species composition and essential nutrient concentrations, and to the elemental and biochemical composition of clam tissues using principal component analyses and regression models. The field experiments revealed that somatic growth and survival of C. fluminea in Lake Constance were primarily determined by water temperature. In winter, low temperatures (

This study investigates over 30 years of dissolved oxygen dynamics in the deep interior of Lake Constance (max. depth: 250 m). This lake supplies approximately four million people with drinking water and has undergone strong re-oligotrophication over the past decades. We calculated depth-specific annual oxygen depletion rates (ODRs) during the period of stratification and found that 50% of the observed variability in ODR was already explained by a simple separation into a sediment- and volume-related oxygen consumption. Adding a linear factor for water depth further improved the model indicating that oxygen depletion increased substantially along the depth. Two other factors turned out to significantly influence ODR: total phosphorus as a proxy for the lake's trophic state and mean oxygen concentration in the respective depth layer. Our analysis points to the importance of nutrient reductions as effective management measures to improve and protect the oxygen status of such large and deep lakes.

Optical sensors for fluorescence of chlorophyll a (f-Chl a) and phycocyanin (f-PC) are increasingly used as a proxy for biomass of algae and cyanobacteria, respectively. They provide measurements at high-frequency and modest cost. These sensors require site-specific calibration due to a range of interferences. Light intensity affects the fluorescence yield of cyanobacteria and algae through light harvesting regulation mechanisms, but is often neglected as a potential source of error for in-situ f-Chl a and f-PC measurements. We hypothesised that diel light variations would induce significant f-Chl a and f-PC suppression when compared to dark periods. We tested this hypothesis in a controlled experiment using three commercial fluorescence probes which continuously measured f-Chl a and f-PC from a culture of the cyanobacterium Dolichospermum variabilis as well as f-Chl a from a culture of the green alga Ankistrodesmus gracilis in a simulated natural light regime. Under light, all devices showed a significant (p

Although the treatment of eutrophied lakes with aluminium (Al) compounds has been established for more than 40 years, publications reporting on long-term consequences for phytoplankton are rare. Here we present observations from Lake Barleber for the period 1985–2016. The lake was treated in autumn 1986 because of high phosphorus (P) concentrations and cyanobacteria blooms, which limited the lake's recreational use. Within six weeks 480 t of Al sulphate solution (37 t of Al) were applied to the lake. This was equivalent to a dose of 36 g Al3+ m−2 or 5.7 mg Al3+ L−1. Already after having applied half of the Al sulphate, the concentration of soluble reactive phosphorus (SRP) reached its analytical limit of quantification (3 μg L−1). Removal rates calculated after completion of the treatment were 98% for SRP and 90% for total phosphorus (TP). In the following 13 years from 1987 to 1999, cyanobacteria were almost absent. In the years 2000 to 2003 as well as in 2005 and 2014 they appeared in low abundances. In the period 1987–2014, almost complete absence of cyanobacteria and high transparency provided good conditions for recreational use of Lake Barleber. Compared to pre-treatment conditions, phytoplankton biomass increased temporarily from 1987 to 2016. This increase in biomass did not interfere with the use for bathing and swimming, because phytoplankton community composition changed towards a dominance of chlorophytes and dinophytes. In 2016, however, P concentration and cyanobacterial biomass rose again to the level of the last pre-treatment years (TP 134 μg L−1, cyanobacterial biomass 1 mg L−1; averages for the period May–October). We conclude that Al treatment is effective and can last for decades. For recreational lake use, the effects of the alum treatment on phytoplankton community composition showed to be more important than its effects on total phytoplankton biomass.

Temperate lakes/reservoirs are warming, this can influence the benthic release of nutrients. They undergo seasonal changes resulting in an array of temperature and oxygen conditions, oxic-low, oxic-high, anoxic-low, and anoxic-high temperature. We sought to understand the interaction of temperature and oxygen conditions on benthic solutes exchange through a two-factorial sediment core incubation experiment by varying either temperature or oxygen conditions of sediment cores from an oligotrophic and eutrophic reservoir. Temperature and oxygen conditions are both important for nutrient release, however, they influence solutes differently, differences in the fluxes of the treatments were explained more by temperature for P, DOC and N, while for Fe, Mn and SO42&minus, differences were explained more by oxygen conditions. The combination of strongly reducing conditions (due to anoxia) and high temperature (20 &deg, C) led to a significant increase in nutrients concentrations in the overlying water. Under these conditions, SRP flux was 0.04 and 0.5 mmol m&minus, 2 d&minus, 1, ammonium was 0.9 and 5.6 mmol m&minus, 1 for the oligotrophic and eutrophic reservoir, respectively. We observed a synergistic interaction between temperature and oxygen conditions which resulted in release of solutes from sediments. An increase in nutrients release under increasing temperatures is more likely and so are algal blooms.

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.

We applied a coupled hydrodynamic-ecological lake model to Lake Constance and investigated the consequences of luxury uptake of mineral resources (phosphate) by phytoplankton on the distribution of this resource in the environment. We compared two different resource limitation models, a static P model with a fixed cell stoichiometry disabling luxury uptake (the Monod approach) and a dynamic P model with a flexible cell stoichiometry enabling luxury uptake (the Droop approach). Our research was based on the finding that a model simulation using the static P model showed good results for phytoplankton dynamics but was unable to simulate the vertical distribution of the algal resource properly. In this model, the resource was only depleted in the euphotic zone (ca. 0–20 m) while in Lake Constance observed phosphate depletion was about twice as deep (down to ca. 40–50 m). A simulation using a dynamic P model reproduced the vertical extension of resource depletion. The driving process behind the deeper resource depletion was luxury uptake of phosphate by algae sedimenting out of the productive zone. Even though the spatial extent of phosphate depletion strongly differed, the difference in phytoplankton dynamics between the two resource limitation models was minor. It is shown that a process acting at the cellular level has wide implications at the ecosystem level. Thereby, the inclusion of a flexible cell stoichiometry of phytoplankton into a complex lake model is important for predicting spatial nutrient gradients within the lake.

New scientific understanding is catalyzed by novel technologies that enhance measurement precision, resolution or type, and that provide new tools to test and develop theory. Over the last 50, years, technology has transformed the hydrologic sciences by enabling direct measurements of watershed fluxes (evapotranspiration, stream flow) at time scales and spatial extents aligned with variation in physical drivers. High frequency water quality measurements, increasingly obtained by in situ water quality sensors, are extending that transformation. Widely available sensors for some physical (temperature) and chemical (conductivity, dissolved oxygen) attributes have become integral to aquatic science, and emerging sensors for nutrients, dissolved CO, turbidity, algal pigments, and dissolved organic matter are now enabling observations of watersheds and streams at time scales, commensurate with their fundamental hydrological, energetic, elemental, and biological drivers. Here we synthesize insights from emerging technologies across a suite of applications, and envision future advances, enabled by sensors, in our ability to understand, predict, and restore watershed and stream systems.

Periphyton growth limitation experiments were conducted in five glacier streams during the main ice melt period in late summer using nutrient diffusing substrata (NDS) that contained nitrate and/or phosphate. Periphyton net growth was determined as chlorophyll a accrual after an exposure time of 4 weeks. In addition, primary water chemistry and physical parameters of the study streams were measured. These chemical and physical parameters characterised the sites as kryal (glacial) systems. Neither nutrient limitation nor a significant correlation between water chemistry and physical data and chlorophyll a values were apparent. A comparison between current velocity and ln-transformed chlorophyll a values revealed a typical optimum curve with highest periphyton accrual at 0.5 m s–1 on NDS. During the summer ablation period, the net growth of periphyton in these glacial streams appeared to be controlled primarily by current velocity.