Lagrangian profiles of riverine autotrophy, organic matter transformation, and micropollutants at extreme drought

Complete exhaustion of dissolved nutrients in a large lowland river

Riverine phytoplankton takes up phosphate, dissolved silicate, and nitrate. We investigated which nutrients are depleted during a Lagrangian sampling in the free-flowing part of the River Elbe in 2023. As part of this study, we tested the hypotheses that nutrient depletion might be caused by (1) above-average phytoplankton biomass or by (2) decreased nutrient load of the river during previous years. Phytoplankton biomass increased up to 350 km in rivers and stopped increasing exactly when soluble reactive phosphorus had been completely consumed, and molar carbon to phosphorus ratios of seston indicated the beginning phosphorus limitation. The concentrations of dissolved silicate and nitrate dropped below the detection limit as well. In contrast to the results from eight previous longitudinal samplings taken in 2018-2022, nitrate exhaustion was detected for the first time in 2023 within the transect. This was caused neither by an above-average phytoplankton biomass nor by a declined overall nutrient load of the river in 2018-2023. Instead, denitrification appears to be the most plausible explanation for the downstream decrease of nitrate and the loss of total nitrogen which was supported by enrichment of nitrate stable isotopes and a decreasing ratio of nitrate 15N/18O.

Post column infusion of an internal standard into LC-FT-ICR MS enables semi-quantitative comparison of dissolved organic matter in original samples

Ultrahigh resolution mass spectrometry hyphenated with liquid chromatography (LC) is an emerging tool to explore the isomeric composition of dissolved organic matter (DOM). However, matrix effects limit the potential for semi-quantitative comparison of DOM molecule abundances across samples. We introduce a post-column infused internal standard (PCI-IS) for reversed-phase LC-FT-ICR MS measurements of DOM and systematically evaluate matrix effects, detector linearity and the precision of mass peak intensities. Matrix effects for model compounds spiked into freshwater DOM samples ranging from a headwater stream to a major river were reduced by 5-10% for PCI-IS corrected mass peak intensities as compared to raw (i.e., untransformed) intensities. A linear regression of PCI-IS corrected DOM mass peak intensities across a typical DOM concentration range (2-15 mg dissolved organic carbon L-1) in original, non-extracted freshwater samples demonstrates excellent linearity of the detector response (r2 > 0.9 for 98% of detected molecular formulas across retention times). Importantly, PCI-IS could compensate for 80% of matrix effects across an environmental gradient of DOM composition from groundwater to surface water. This enabled studying the ionization efficiency of DOM isomers and linking the observed differences to the biogeochemical sources. With PCI-IS original, non-extracted DOM samples can be analysed by LC-FT-ICR MS without carbon load adjustment, and mass peak intensities can be reliably used to semi-quantitatively compare isomer abundances between compositionally similar DOM samples.

Evidence of deviations between experimental and empirical mixing lengths: Multi-discharge field tests in an arid river system

Despite advances in wastewater treatment plant (WWTP) efficiencies, multiple contaminants of concern, such as microplastics, pharmaceuticals, and per- and poly-fluoroalkyl substances (PFAS) remain largely untreated near discharge points and can be highly concentrated before they are fully mixed within the receiving river. Environmental agencies enforce mixing zone permits for the temporary exceedance of water quality parameters beyond targeted control levels under the assumption that contaminants are well-mixed and diluted downstream of mixing lengths, which are typically quantified using empirical equations derived from one-dimensional transport models. Most of these equations were developed in the 1970s and have been assumed to be standard practice since then. However, their development and validation lacked the technological advances required to test them in the field and under changing flow conditions. While new monitoring techniques such as remote sensing and infrared imaging have been employed to visualize mixing lengths and test the validity of empirical equations, those methods cannot be easily repeated due to high costs or flight restrictions. We investigated the application of Lagrangian and Eulerian monitoring approaches to experimentally quantify mixing lengths downstream of a WWTP discharging into the Rio Grande near Albuquerque, New Mexico (USA). Our data spans river to WWTP discharges ranging between 2-22x, thus providing a unique dataset to test long-standing empirical equations in the field. Our results consistently show empirical equations could not describe our experimental mixing lengths. Specifically, while our experimental data revealed "bell-shaped" mixing lengths as a function of increasing river discharges, all empirical equations predicted monotonically increasing mixing lengths. Those mismatches between experimental and empirical mixing lengths are likely due to the existence of threshold processes defining mixing at different flow regimes, i.e., jet diffusion at low flows, the Coanda effect at intermediate flows, and turbulent mixing at higher flows, which are unaccounted for by the one-dimensional empirical formulas. Our results call for a review of the use of empirical mixing lengths in streams and rivers to avoid widespread exposures to emerging contaminants.

Mapping chemical footprints of organic micropollutants in European streams

There is increasing awareness that chemical pollution of freshwater systems with complex mixtures of chemicals from domestic sources, agriculture and industry may cause a substantial chemical footprint on water organisms, pushing aquatic ecosystems outside the safe operating space. The present study defines chemical footprints as the risk that chemicals or chemical mixtures will have adverse effects on a specific group of organisms. The aim is to characterise these chemical footprints in European streams based on a unique and uniform screening of more than 600 chemicals in 445 surface water samples, and to derive site- and compound-specific information for management prioritisation purposes. In total, 504 pesticides, biocides, pharmaceuticals and other compounds have been detected, including frequently occurring and site-specific compounds with concentrations up to 74 µg/L. Key finding is that three-quarter of the investigated sites in 22 European river basins exceed established thresholds for chemical footprints in freshwater, leading to expected acute or chronic impacts on aquatic organisms. The largest footprints were recorded on invertebrates, followed by algae and fish. More than 70 chemicals exceed thresholds of chronic impacts on invertebrates. For all organism groups, pesticides and biocides were the main drivers of chemical footprints, while mixture impacts were particularly relevant for invertebrates. No clear significant correlation was found between chemical footprints and the urban discharge fractions, suggesting that effluent-specific quality rather than the total load of treated wastewater in the aquatic environment and the contribution of diffuse sources, e.g. from agriculture, determine chemical footprints.

Low discharge intensifies nitrogen retention in rivers - A case study in the Elbe River

Eutrophication due to excessive nutrient inputs is a major threat to coastal ecosystems worldwide, causing harmful algae blooms, seagrass loss and hypoxia. Decisions to combat eutrophication in the North Sea were made in the 1980s. Despite significant improvements during recent decades, high nitrogen loads and resulting eutrophication problems remain. In this study, long-term changes in nitrogen inputs to the Elbe Estuary (Germany) were characterized based on nitrogen data provided by the Elbe River Basin Community from 1985 to 2019. Additionally, surface water samples were taken at the weir separating the river from the estuary from 2011 to 2021 to characterize dissolved inorganic nitrogen concentrations and nitrate stable isotope composition. The findings suggest a close coupling of river discharge with the riverine nitrogen cycle. Nitrogen loads decreased disproportionately with decreasing discharge. This decrease is due to intensified nitrogen retention in the Elbe catchment, which can double nitrogen retention compared to average discharge conditions. Phytoplankton growth was enhanced by long residence times and high light availability at low water levels. This suggests that the recent decreases in nitrogen loads in the Elbe River were not only a result of management measures in the catchment but were also amplified by a recent long-lasting drought in the catchment. Based on projections from the Intergovernmental Panel on Climate Change, more frequent and extensive droughts are anticipated, which may lead to future seasonal shifts to nitrate limitation in the lower Elbe River.

Molecular composition of dissolved organic matter across diverse ecosystems: Preliminary implications for biogeochemical cycling

Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely applied to characterize the molecular composition of dissolved organic matter (DOM) in different ecosystems. Most previous studies have explored the molecular composition of DOM focused on one or a few ecosystems, which prevents us from tracing the molecular composition of DOM from different sources and further exploring its biogeochemical cycling across ecosystems. In this study, a total of 67 DOM samples, including soil, lake, river, ocean, and groundwater, were analyzed by negative-ion electrospray ionization FT-ICR MS. Results show that molecular composition of DOM varies dramatically among diverse ecosystems. Specifically, the forest soil DOM exhibited the strongest terrestrial signature of molecules, while the seawater DOM showed the most abundant of biologically recalcitrant components, for example, the carboxyl-rich alicyclic molecules were abundant in the deep-sea waters. Terrigenous organic matter is gradually degraded during its transport along the river-estuary-ocean continuum. The saline lake DOM showed similar DOM characteristics with marine DOM, and sequestrated abundant recalcitrant DOM. By comparing these DOM extracts, we found that human activities likely lead to an increase in the content of S and N-containing heteroatoms in DOM, this phenomenon was commonly found in the paddy soil, polluted river, eutrophic lake, and acid mine drainage DOM samples. Overall, this study compared molecular composition of DOM extracted from various ecosystems, providing a preliminary comparison on the DOM fingerprint and an angle of view into biogeochemical cycling across different ecosystems. We thus advocate for the development of a comprehensive molecular fingerprint database of DOM using FT-ICR MS across a wider range of ecosystems. This will enable us to better understand the generalizability of the distinct features among ecosystems.