Photochemically Mediated Toluene Oxidation through a Copper Complex.

A method is described to photochemically oxidize toluene selectively to benzaldehyde, an essential compound in the chemical industry. Copper(I) complexes with different ligands were applied in combination with [Ru(bipy)3 ](PF6 )2 and dioxygen as the oxidant. As a result, a "dioxygen adduct" copper complex, for example, a peroxido complex, is formed as the active species. The copper(II) complex obtained after oxidation can be photochemically reduced to the starting copper(I) species, and the process can be repeated continuously. The ligand tris(2-methylpyridyl)amine (tmpa) led to the highest conversion rates.

Structural and functional development of twelve newly established floodplain pond mesocosms.

Ecosystems are complex structures with interacting abiotic and biotic processes evolving with ongoing succession. However, limited knowledge exists on the very initial phase of ecosystem development and colonization. Here, we report results of a comprehensive ecosystem development monitoring for twelve floodplain pond mesocosms (FPM; 23.5 m × 7.5 m × 1.5 m each) located in south-western Germany. In total, 20 abiotic and biotic parameters, including structural and functional variables, were monitored for 21 months after establishment of the FPMs. The results showed evolving ecosystem development and primary succession in all FPMs, with fluctuating abiotic conditions over time. Principal component analyses and redundancy analyses revealed season and succession time (i.e., time since ecosystem establishment) to be significant drivers of changes in environmental conditions. Initial colonization of both aquatic (i.e., water bodies) and terrestrial (i.e., riparian land areas) parts of the pond ecosystems occurred within the first month, with subsequent season-specific increases in richness and abundance for aquatic and terrestrial taxa over the entire study period. Abiotic environmental conditions and aquatic and terrestrial communities showed increasing interpond variations over time, that is, increasing heterogeneity among the FPMs due to natural environmental divergence. However, both functional variables assessed (i.e., aquatic and terrestrial litter decomposition) showed opposite patterns as litter decomposition rates slightly decreased over time and interpond differences converged with successional ecosystem developments. Overall, our results provide rare insights into the abiotic and biotic conditions and processes during the initial stages of freshwater ecosystem formation, as well as into structural and functional developments of the aquatic and terrestrial environment of newly established pond ecosystems.

Exploring flow-biofilm-sediment interactions: Assessment of current status and future challenges.

Biofilm activities and their interactions with physical, chemical and biological processes are of great importance for a variety of ecosystem functions, impacting hydrogeomorphology, water quality and aquatic ecosystem health. Effective management of water bodies requires advancing our understanding of how flow influences biofilm-bound sediment and ecosystem processes and vice-versa. However, research on this triangle of flow-biofilm-sediment is still at its infancy. In this Review, we summarize the current state of the art and methodological approaches in the flow-biofilm-sediment research with an emphasis on biostabilization and fine sediment dynamics mainly in the benthic zone of lotic and lentic environments. Example studies of this three-way interaction across a range of spatial scales from cell (nm - µm) to patch scale (mm - dm) are highlighted in view of the urgent need for interdisciplinary approaches. As a contribution to the review, we combine a literature survey with results of a pilot experiment that was conducted in the framework of a joint workshop to explore the feasibility of asking interdisciplinary questions. Further, within this workshop various observation and measuring approaches were tested and the quality of the achieved results was evaluated individually and in combination. Accordingly, the paper concludes by highlighting the following research challenges to be considered within the forthcoming years in the triangle of flow-biofilm-sediment: i) Establish a collaborative work among hydraulic and sedimentation engineers as well as ecologists to study mutual goals with appropriate methods. Perform realistic experimental studies to test hypotheses on flow-biofilm-sediment interactions as well as structural and mechanical characteristics of the bed. ii) Consider spatially varying characteristics of flow at the sediment-water interface. Utilize combinations of microsensors and non-intrusive optical methods, such as particle image velocimetry and laser scanner to elucidate the mechanism behind biofilm growth as well as mass and momentum flux exchanges between biofilm and water. Use molecular approaches (DNA, pigments, staining, microscopy) for sophisticated community analyses. Link varying flow regimes to microbial communities (and processes) and fine sediment properties to explore the role of key microbial players and functions in enhancing sediment stability (biostabilization). iii) Link laboratory-scale observations to larger scales relevant for management of water bodies. Conduct field experiments to better understand the complex effects of variable flow and sediment regimes on biostabilization. Employ scalable and informative observation techniques (e.g., hyperspectral imaging, particle tracking) that can support predictions on the functional aspects, such as metabolic activity, bed stability, nutrient fluxes under variable regimes of flow-biofilm-sediment.

Effects of small-scale turbulence at the air-water interface on microcystis surface scum formation.

Cyanobacterial surface scum (here defined as visible Cyanobacteria colonies accumulating at the lake surface) is a harmful phenomenon that negatively affects water quality, human and animal health. Colony-forming Microcystis is one of the most important and ubiquitous genera that can suddenly accumulate at water surfaces. Turbulent water motion, e.g., generated by wind, can vertically disperse this scum layer, which later can re-establish by upward migration of Microcystis colonies. However, the role of wind-generated turbulence in scum formation and development is still poorly understood. Here we present results from a laboratory mesocosm study where we analysed the processes of scum formation and its response to wind-generated turbulence at low wind speed (≤3.6 m s-1). Microcystis colony size and flow velocity at the water surface and in the bulk water were measured using a microscope camera and particle tracking velocimetry. The surface scum formed by aggregation of colonies at the water surface, where they formed loose clusters of increasing size. The presence of large colony aggregations or of a surface film determined the stability of the scum layer. For the largest applied wind speed, most of the aggregations were broken down to sizes <2 mm, which were dispersed to the bulk water. The surface scum recovered quickly from such disturbances after the wind speed decreased. We further observed reduced momentum transfer from wind to water with the growing scum layer. The presence of the scum increased the threshold wind speed for the onset of flow and reduced the flow velocities that were generated above that threshold. This effect was likely caused by the presence of a film of surface-active material at the water surface (surface microlayer), which is related to the presence of Microcystis. Both the small-scale turbulence and surface microlayer might play an important, yet largely unexplored role in Microcystis surface scum development in aquatic ecosystems. Improved understanding of the interplay of both processes will be instrumental for improving current mechanistic models for predicting surface bloom dynamics.

The role of hydrodynamics in shaping the composition and architecture of epilithic biofilms in fluvial ecosystems.

Previous laboratory and on-site experiments have highlighted the importance of hydrodynamics in shaping biofilm composition and architecture. In how far responses to hydrodynamics can be found in natural flows under the complex interplay of environmental factors is still unknown. In this study we investigated the effect of near streambed turbulence in terms of turbulent kinetic energy (TKE) on the composition and architecture of biofilms matured in two mountainous streams differing in dissolved nutrient concentrations. Over both streams, TKE significantly explained 7% and 8% of the variability in biofilm composition and architecture, respectively. However, effects were more pronounced in the nutrient richer stream, where TKE significantly explained 12% and 3% of the variability in biofilm composition and architecture, respectively. While at lower nutrient concentrations seasonally varying factors such as stoichiometry of dissolved nutrients (N/P ratio) and light were more important and explained 41% and 6% of the variability in biofilm composition and architecture, respectively. Specific biofilm features such as elongated ripples and streamers, which were observed in response to the uniform and unidirectional flow in experimental settings, were not observed. Microbial biovolume and surface area covered by the biofilm canopy increased with TKE, while biofilm thickness and porosity where not affected or decreased. These findings indicate that under natural flows where near bed flow velocities and turbulence intensities fluctuate with time and space, biofilms became more compact. They spread uniformly on the mineral surface as a film of densely packed coccoid cells appearing like cobblestone pavement. The compact growth of biofilms seemed to be advantageous for resisting hydrodynamic shear forces in order to avoid displacement. Thus, near streambed turbulence can be considered as important factor shaping the composition and architecture of biofilms grown under natural flows.

Aquatic eddy correlation: quantifying the artificial flux caused by stirring-sensitive O2 sensors.

In the last decade, the aquatic eddy correlation (EC) technique has proven to be a powerful approach for non-invasive measurements of oxygen fluxes across the sediment water interface. Fundamental to the EC approach is the correlation of turbulent velocity and oxygen concentration fluctuations measured with high frequencies in the same sampling volume. Oxygen concentrations are commonly measured with fast responding electrochemical microsensors. However, due to their own oxygen consumption, electrochemical microsensors are sensitive to changes of the diffusive boundary layer surrounding the probe and thus to changes in the ambient flow velocity. The so-called stirring sensitivity of microsensors constitutes an inherent correlation of flow velocity and oxygen sensing and thus an artificial flux which can confound the benthic flux determination. To assess the artificial flux we measured the correlation between the turbulent flow velocity and the signal of oxygen microsensors in a sealed annular flume without any oxygen sinks and sources. Experiments revealed significant correlations, even for sensors designed to have low stirring sensitivities of ~0.7%. The artificial fluxes depended on ambient flow conditions and, counter intuitively, increased at higher velocities because of the nonlinear contribution of turbulent velocity fluctuations. The measured artificial fluxes ranged from 2-70 mmol m(-2) d(-1) for weak and very strong turbulent flow, respectively. Further, the stirring sensitivity depended on the sensor orientation towards the flow. For a sensor orientation typically used in field studies, the artificial flux could be predicted using a simplified mathematical model. Optical microsensors (optodes) that should not exhibit a stirring sensitivity were tested in parallel and did not show any significant correlation between O2 signals and turbulent flow. In conclusion, EC data obtained with electrochemical sensors can be affected by artificial flux and we recommend using optical microsensors in future EC-studies.

Hydrodynamic trails produced by Daphnia: size and energetics.

This study focuses on quantifying hydrodynamic trails produced by freely swimming zooplankton. We combined volumetric tracking of swimming trajectories with planar observations of the flow field induced by Daphnia of different size and swimming in different patterns. Spatial extension of the planar flow field along the trajectories was used to interrogate the dimensions (length and volume) and energetics (dissipation rate of kinetic energy and total dissipated power) of the trails. Our findings demonstrate that neither swimming pattern nor size of the organisms affect the trail width or the dissipation rate. However, we found that the trail volume increases with increasing organism size and swimming velocity, more precisely the trail volume is proportional to the third power of Reynolds number. This increase furthermore results in significantly enhanced total dissipated power at higher Reynolds number. The biggest trail volume observed corresponds to about 500 times the body volume of the largest daphnids. Trail-averaged viscous dissipation rate of the swimming daphnids vary in the range of 1.8 x 10(-6) W/kg to 3.4 x 10(-6) W/kg and the observed magnitudes of total dissipated power between 1.3 x 10(-9) W and 1 x 10(-8) W, respectively. Among other zooplankton species, daphnids display the highest total dissipated power in their trails. These findings are discussed in the context of fluid mixing and transport by organisms swimming at intermediate Reynolds numbers.

Role of submerged vegetation in the retention processes of three plant protection products in flow-through stream mesocosms.

Quantitative information on the processes leading to the retention of plant protection products (PPPs) in surface waters is not available, particularly for flow-through systems. The influence of aquatic vegetation on the hydraulic- and sorption-mediated mitigation processes of three PPPs (triflumuron, pencycuron, and penflufen; logKOW 3.3-4.9) in 45-m slow-flowing stream mesocosms was investigated. Peak reductions were 35-38% in an unvegetated stream mesocosm, 60-62% in a sparsely vegetated stream mesocosm (13% coverage with Elodea nuttallii), and in a similar range of 57-69% in a densely vegetated stream mesocosm (100% coverage). Between 89% and 93% of the measured total peak reductions in the sparsely vegetated stream can be explained by an increase of vegetation-induced dispersion (estimated with the one-dimensional solute transport model OTIS), while 7-11% of the peak reduction can be attributed to sorption processes. However, dispersion contributed only 59-71% of the peak reductions in the densely vegetated stream mesocosm, where 29% to 41% of the total peak reductions can be attributed to sorption processes. In the densely vegetated stream, 8-27% of the applied PPPs, depending on the logKOW values of the compounds, were temporarily retained by macrophytes. Increasing PPP recoveries in the aqueous phase were accompanied by a decrease of PPP concentrations in macrophytes indicating kinetic desorption over time. This is the first study to provide quantitative data on how the interaction of dispersion and sorption, driven by aquatic macrophytes, influences the mitigation of PPP concentrations in flowing vegetated stream systems.

Three-dimensional analysis of the swimming behavior of Daphnia magna exposed to nanosized titanium dioxide.

Due to their surface characteristics, nanosized titanium dioxide particles (nTiO2) tend to adhere to biological surfaces and we thus hypothesize that they may alter the swimming performance and behavior of motile aquatic organisms. However, no suitable approaches to address these impairments in swimming behavior as a result of nanoparticle exposure are available. Water fleas Daphnia magna exposed to 5 and 20 mg/L nTiO2 (61 nm; polydispersity index: 0.157 in 17.46 mg/L stock suspension) for 96 h showed a significantly (p<0.05) reduced growth rate compared to a 1-mg/L treatment and the control. Using three-dimensional video observations of swimming trajectories, we observed a treatment-dependent swarming of D. magna in the center of the test vessels during the initial phase of the exposure period. Ensemble mean swimming velocities increased with increasing body length of D. magna, but were significantly reduced in comparison to the control in all treatments after 96 h of exposure. Spectral analysis of swimming velocities revealed that high-frequency variance, which we consider as a measure of swimming activity, was significantly reduced in the 5- and 20-mg/L treatments. The results highlight the potential of detailed swimming analysis of D. magna for the evaluation of sub-lethal mechanical stress mechanisms resulting from biological surface coating and thus for evaluating the effects of nanoparticles in the aquatic environment.