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  • 1
    In: Limnology and Oceanography, May 2014, Vol.59(3), pp.724-732
    Description: The potential contribution of swimming zooplankton to the vertical mixing of stratified waters has been the topic of an ongoing scientific debate. Current estimates, which are primarily based on scale analyses and numerical simulations, range from negligible effects to significant contributions that are comparable in magnitude to physical driving forces, such as wind and tides. Here, we analyzed laboratory observations of fluid mixing that are caused by vertically migrating zooplankton () in a density‐stratified water column. Mixing rates were quantified at the scale of individual organisms in terms of the dissipation rates of small‐scale spatial variance of tracer concentration measured by laser‐induced fluorescence. At the bulk scale, we analyzed temporal changes in the mean density stratification. Organism and bulk scale observations were used to estimate apparent diffusion coefficients in trails of individuals and organism groups. Mean diffusivities of 0.8–5.1 × 10 m s, which were averaged over trail volumes of 1.5–13 × 10 m, are on the same order of magnitude as the molecular diffusivity of salt. A comparable diffusivity (1.1 × 10 m s) was estimated on the bulk scale, and the initial density stratification, although frequently passed by migrating , was preserved over the 5 d experimental period. The present results agree with scaling arguments and suggest the negligible enhancement of vertical transport in comparison with the turbulent mixing that is typically observed in oceans and lakes.
    Keywords: Oceanography;
    ISSN: 0024-3590
    E-ISSN: 1939-5590
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  • 2
    Language: English
    In: PLoS ONE, 01 January 2014, Vol.9(3), p.e92383
    Description: 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.
    Keywords: Sciences (General)
    E-ISSN: 1932-6203
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  • 3
    Language: English
    In: PLoS ONE, Jan 30, 2015, Vol.10(1)
    Description: 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.sup.-2 d.sup.-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 O.sub.2 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.
    Keywords: Turbulence (Fluid Dynamics) ; Sensors
    ISSN: 1932-6203
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Limnology and Oceanography, 2014, Vol.59(3), p.724(9)
    Description: The potential contribution of swimming zooplankton to the vertical mixing of stratihed waters has been the topic of an ongoing scientific debate. Current estimates, which are primarily based on scale analyses and numerical simulations, range from negligible effects to significant contributions that are comparable in magnitude to physical driving forces, such as wind and tides. Here, we analyzed laboratory observations of fluid mixing that are caused by vertically migrating zooplankton (Daphnia magna) in a density-stratified water column. Mixing rates were quantified at the scale of individual organisms in terms of the dissipation rates of small-scale spatial variance of tracer concentration measured by laser-induced fluorescence. At the bulk scale, we analyzed temporal changes in the mean density stratification. Organism and bulk scale observations were used to estimate apparent diffusion coefficients in trails of individuals and organism groups. Mean diffusivities of 0.8-5.1 X [10.sup.-9] [m.sup.2] [s.sup.-1], which were averaged over trail volumes of 1.5-13 x [10.sup.-5] [m.sup.3], are on the same order of magnitude as the molecular diffusivity of salt. A comparable diffusivity (1.1 x [10.sup.-9] [m.sup.2] [s.sup.-1]) was estimated on the bulk scale, and the initial density stratification, although frequently passed by migrating Daphnia, was preserved over the 5 d experimental period. The present results agree with scaling arguments and suggest the negligible enhancement of vertical transport in comparison with the turbulent mixing that is typically observed in oceans and lakes. doi:10.4319/lo.2014.59.3.0724
    Keywords: Zooplankton – Environmental Aspects
    ISSN: 0024-3590
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Chemosphere, July, 2014, Vol.107, p.13(10)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.chemosphere.2014.02.055 Byline: Christoph Stang, Matthias Valentin Wieczorek, Christian Noss, Andreas Lorke, Frank Scherr, Gerhard Goerlitz, Ralf Schulz Abstract: acents Aquatic macrophytes determine how dispersion and sorption mitigate PPPs in streams. acents Sparse vegetation fosters dispersion. acents Dense vegetation fosters mass retention. acents Compound related and time limited mass retention compensates diminished dispersion. Article History: Received 11 September 2013; Revised 21 January 2014; Accepted 4 February 2014 Article Note: (miscellaneous) Handling Editor: X. Cao
    Keywords: Closed Experimental Ecosystems
    ISSN: 0045-6535
    Source: Cengage Learning, Inc.
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  • 6
    In: PLoS ONE, 2013, Vol.8(11)
    Description: Due to their surface characteristics, nanosized titanium dioxide particles (nTiO 2 ) 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 nTiO 2 (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.
    Keywords: Research Article
    E-ISSN: 1932-6203
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  • 7
    In: Geophysical Research Letters, 16 September 2017, Vol.44(17), pp.8901-8909
    Description: Solute transport across the sediment‐water interface has major implications for water quality and biogeochemical cycling in aquatic ecosystems. Existing measurement techniques, however, are not capable of resolving sediment‐water fluxes of most constituents under in situ flow conditions. We investigated whether relaxed eddy accumulation (REA), a micrometeorological technique with conditional sampling of turbulent updrafts and downdrafts, can be adapted to the aquatic environment. We simulated REA fluxes by reanalyzing eddy covariance measurements from a riverine lake. We found that the empirical coefficient that relates mass fluxes to the concentration difference between both REA samples is invariant with scalar and flow and responds as predicted by a joint Gaussian distribution of linearly correlated variables. Simulated REA fluxes differed on average by around 30% from eddy covariance fluxes (mean absolute error). Assessment of the lower quantification limit suggests that REA can potentially be applied for measuring benthic fluxes of a new range of constituents that cannot be assessed by standard eddy covariance methods. Applicability of relaxed eddy accumulation (REA) to measure solute fluxes across the sediment‐water interface was assessed The empirical coefficient in flux calculations is similar for heat, oxygen, and particles and does not change with flow REA has the potential to provide flux estimates for a broad range of substances in aquatic ecosystems but may be limited by analytical precision
    Keywords: Conditional Sampling ; Benthic Boundary Layer ; Internal Load ; Monitoring ; Solute Transport
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 8
    In: Earth Surface Processes and Landforms, May 2018, Vol.43(6), pp.1241-1251
    Description: A novel hand‐held laser‐based stream bed survey system is presented. The system facilitates the capture of detailed 3D mapping of shallow (〈 0.7 m) riverbed topography in sections approximately 4 m by 2 m. The system includes a stationary reference system, which projects three laser sheets (two at offset angles), within which a hand‐held monitoring pole is moved. The unique configuration of the light sheets intercepts with the pole as it moves within the survey area providing an exact horizontal location. Pole tilt is compensated for by an inertial measurement unit on the pole, and the height above the bed of the pole and submerged scanning laser are monitored relative to the horizontal laser sheet. Verification and application measurements demonstrate high resolution and accuracy in the horizontal (~5 mm) and vertical (~1 mm) direction. The system can be applied at sites where a free view is blocked and other optical through‐water methods fail. It is appropriate for studies on riverbed statistics and dynamics, which necessitate non‐invasive in‐situ surveys. Copyright © 2017 John Wiley & Sons, Ltd. Key findings
    Keywords: Positioning ; Topography ; Riverbed ; Roughness ; Flow Depth ; New Survey Method
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 9
    Language: English
    In: Water Research, 15 December 2017, Vol.127, pp.211-222
    Description: 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.
    Keywords: Bacteria ; Cyanobacteria ; Algae ; AAL-Specific Glycoconjugates ; Near Streambed Turbulence ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 10
    Language: English
    In: Chemosphere, July 2014, Vol.107, pp.13-22
    Description: 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; log 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 ), 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 log 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.
    Keywords: Pesticide ; Plant Protection Product ; Dispersion ; Sorption ; Vegetated Stream Mesoscosms ; Tracer ; Chemistry ; Ecology
    ISSN: 0045-6535
    E-ISSN: 1879-1298
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