Chin H. Wu

Lake microbial communities are resilient after a whole-ecosystem disturbance

Disturbances act as powerful structuring forces on ecosystems. To ask whether environmental microbial communities have capacity to recover after a large disturbance event, we conducted a whole-ecosystem manipulation, during which we imposed an intense disturbance on freshwater microbial communities by artificially mixing a temperate lake during peak summer thermal stratification. We employed environmental sensors and water chemistry analyses to evaluate the physical and chemical responses of the lake, and bar-coded 16S ribosomal RNA gene pyrosequencing and automated ribosomal intergenic spacer analysis (ARISA) to assess the bacterial community responses. The artificial mixing increased mean lake temperature from 14 to 20 °C for seven weeks after mixing ended, and exposed the microorganisms to very different environmental conditions, including increased hypolimnion oxygen and increased epilimnion carbon dioxide concentrations. Though overall ecosystem conditions remained altered (with hypolimnion temperatures elevated from 6 to 20 °C), bacterial communities returned to their pre-manipulation state as some environmental conditions, such as oxygen concentration, recovered. Recovery to pre-disturbance community composition and diversity was observed within 7 (epilimnion) and 11 (hypolimnion) days after mixing. Our results suggest that some microbial communities have capacity to recover after a major disturbance.

Resistance, resilience and recovery: aquatic bacterial dynamics after water column disturbance

For lake microbes, water column mixing acts as a disturbance because it homogenizes thermal and chemical gradients known to define the distributions of microbial taxa. Our first objective was to isolate hypothesized drivers of lake bacterial response to water column mixing. To accomplish this, we designed an enclosure experiment with three treatments to independently test key biogeochemical changes induced by mixing: oxygen addition to the hypolimnion, nutrient addition to the epilimnion, and full water column mixing. We used molecular fingerprinting to observe bacterial community dynamics in the treatment and control enclosures, and in ambient lake water. We found that oxygen and nutrient amendments simulated the physical-chemical water column environment following mixing and resulted in similar bacterial communities to the mixing treatment, affirming that these were important drivers of community change. These results demonstrate that specific environmental changes can replicate broad disturbance effects on microbial communities. Our second objective was to characterize bacterial community stability by quantifying community resistance, recovery and resilience to an episodic disturbance. The communities in the nutrient and oxygen amendments changed quickly (had low resistance), but generally matched the control composition by the 10th day after treatment, exhibiting resilience. These results imply that aquatic bacterial assemblages are generally stable in the face of disturbance.

A Comparison of Two- and Three-Dimensional Wave Breaking

The influence of directionality on wave-packet evolution and in particular on the onset of breaking was explored through laboratory experiment. Lateral tapering was applied to the input signal to produce a range of crest lengths, with greater directionality created by diffraction for the shorter crests. The wave shape, local and global wave steepness, and surface displacement spectra were used to characterize the wave fields. The observations suggest that directionality can accelerate or suppress the onset of breaking, and additionally can influence both the local wave steepness at breaking as well as the breaking severity. Directionality, however, did not alter the observed up-frequency energy transfer associated with wave focusing. When no breaking occurred this energy shift was completely reversed. With breaking the shifted energy was lost, that is, passed from the wave to the turbulent energy field. The short-crested wave packet lost 16% of its energy as a result of breaking, while a comparable two-dimensional breaker lost 22%.

Development and Application of an Automated River-Estuary Discharge Imaging System

AbstractAn automated river-estuary discharge imaging system (AREDIS) is developed to measure the velocity of a wide tidal estuary. The system contains near- and far-field cameras that capture the entire 370-m-wide channel from an oblique angle with high image resolution. A new rotational scheme is developed to calibrate the cameras. Wakes generated by flow past bridge piers are found to be natural tracers for large-scale particle image velocimetry (LSPIV) to reliably and accurately measure the surface velocity, confirmed by buoy tracking velocimetry (BTV). The success of AREDIS is demonstrated in three field measurements on the Danshui River, the largest estuary in Taiwan. First, AREDIS is used to measure discharge over the entire tidal cycle under normal flow, and a 10% difference is found between discharges measured with AREDIS and a boat-mounted acoustic Doppler profiler. Second, AREDIS is employed to measure discharge under a typhoon event, resulting in discharges 45% greater than those at normal flow...

Incipient breaking of unsteady waves on sheared currents

Incipient breaking of unsteady waves on sheared currents is experimentally investigated. A new wave-generation technique, based on the iterative frequency-focusing concept with the consideration of effects of Doppler shift and current shear, is developed. The surface displacement, the wavelength, and the phase speed of waves at the breaking onset on shear currents are measured. It is found that the steepness of unsteady, incipient breaking waves is altered by the sign and magnitude of current shear (or vorticity). A current with a positive shear, as would be the case in a wind-driven current, reduces the steepness of an unsteady incipient breaking wave. A negatively sheared current, such as the jet-like ebb current at a tide inlet, leads to steeper incipient breaking waves. The magnitude of reduction/increase in wave steepness is proportional to the strength of a current shear. In particular, a negative shear can alter the wave steepness more significantly in comparison to a positive shear of the same mag...

Energy Dissipation of Unsteady Wave Breaking on Currents

Energy dissipation for unsteady deep-water breaking in wave groups on following and opposing currents, including partial wave-blocking conditions, was investigated by detailed laboratory measurements. A range of focusing wave conditions, including current strengths, wave spectrum slopes, and breaking intensities, were examined. Observations show that weak following and opposing currents do not alter the limiting wave steepness. The kinematics of unsteady breaking can be characterized as the one without currents simply by the Doppler shift. In contrast, strong opposing currents can cause partial wave blockings that narrow the spectral frequency bandwidth and increase the mean spectral slope. Dependence of the significant spectral peak steepness on the spectral bandwidth parameter was identified, confirming threshold behavior of breaking inception of nonlinear wave group dynamics. Loss of excessive energy fluxes due to breaking was found to depend strongly on the mean spectral slope. Wave groups of a steeper spectral slope yield fewer energy losses. In addition, the spectral distribution of energy dissipation due to breaking has the following two main characteristics: (a) significant energy dissipation occurred at frequency components that were higher than the spectral peak frequency, and little energy change at the peak frequency was found; (b) below the spectral peak frequency a small energy gain was observed. The energy-gain-to-loss ratio varies with the spectral bandwidth parameter. Higher gain‐ loss ratios (up to 40%) were observed for breakers on strong opposing currents under the partial wave-blocking condition. Comparison and assessment of proposed and existing parameterizations for breaking-wave energy dissipation were made using the measured data. The new proposed form provides the features for addressing these two main spectral energy distribution characteristics due to breaking with and without currents.

Reconstruction of a meteotsunami in Lake Erie on 27 May 2012: Roles of atmospheric conditions on hydrodynamic response in enclosed basins

On 27 May 2012, atmospheric conditions gave rise to two convective systems that generated a series of waves in the meteotsunami band on Lake Erie. The resulting waves swept three swimmers a 0.5 mi offshore, inundated a marina, and may have led to a capsized boat along the southern shoreline. Analysis of radial velocities from a nearby radar tower in combination with coastal meteorological observation indicates that the convective systems produced a series of outflow bands that were the likely atmospheric cause of the meteotsunami. In order to explain the processes that led to meteotsunami generation, we model the hydrodynamic response to three meteorological forcing scenarios: (i) the reconstructed atmospheric disturbance from radar analysis, (ii) simulated conditions from a high-resolution weather model, and (iii) interpolated meteorological conditions from the NOAA Great Lakes Coastal Forecasting System. The results reveal that the convective systems generated a series of waves incident to the southern shore of the lake that reflected toward the northern shoreline and reflected again to the southern shore, resulting in spatial wave focusing and edge wave formation that combined to impact recreational users near Cleveland, OH. This study illustrates the effects of meteotsunami development in an enclosed basin, including wave reflection, focusing, and edge wave formation as well as temporal lags between the causative atmospheric conditions and arrival of dangerous wave conditions. As a result, the ability to detect these extreme storms and predict the hydrodynamic response is crucial to reducing risk and building resilient coastal communities.

Observations of surface waves interacting with ice using stereo imaging

A powerful Automated Trinocular Stereo Imaging System (ATSIS) is used to remotely measure waves interacting with three distinct ice types: brash, frazil, and pancake. ATSIS is improved with a phase-only correlation matching algorithm and parallel computation to provide high spatial and temporal resolution 3-D profiles of the water/ice surface, from which the wavelength, frequency, and energy flux are calculated. Alongshore spatial frequency distributions show that pancake and frazil ices differentially attenuate at a greater rate for higher-frequency waves, causing a decrease in mean frequency. In contrast, wave propagation through brash ice causes a rapid increase in the dominant wave frequency, which may be caused by nonlinear energy transfer to higher frequencies due to collisions between the brash ice particles. Consistent to the results in frequency, the wavelengths in pancake and frazil ices increase but decrease in brash ice. The total wave energy fluxes decrease exponentially in both pancake and frazil ice, whereas the overall energy flux remain constant in the brash ice due to thin layer thickness. The spatial energy flux distributions also reveal that wave reflection occurs at the boundary of each ice layer, with reflection coefficient decaying exponentially away from the ice interface. Reflection is the strongest at the pancake/ice-free and frazil/brash interfaces and the weakest at the brash/ice-free interface. These high resolution observations measured by ATSIS demonstrate the spatially variable nature of waves propagating through ice.

Effects of changing climate on ice cover in three morphometrically different lakes

A one-dimensional hydrodynamic lake model (DYRESM-WQ-I) is employed to simulate ice cover and water temperatures over the period 1911-2014. The effects of climate changes (air temperature and wind speed) on ice cover (ice-on, ice-off, ice cover duration, and maximum ice thickness) are modeled and compared for the three different morphometry lakes, Fish Lake, Lake Wingra, and Lake Mendota, located in Madison, Wisconsin, United States of America. It is found that the ice cover period has decreased due to later ice-on dates and earlier ice-off dates, and the annual maximum ice cover thickness has decreased for the three lakes during the last century. Based upon simulated perturbations of daily mean air temperatures across the range of -10 °C to +10 °C of historical values, Fish Lake has the most occurrences of no ice cover and Lake Wingra still remains ice covered under extreme conditions (+10 °C). Overall shallower lakes with larger surface areas appear more resilient to ice cover changes caused by climate changes.

An entropy-based surface velocity method for estuarine discharge measurement

An entropy-based method is developed to estimate estuarine river discharge from surface velocity measurements. A two-dimensional velocity profile based on the principle of maximum entropy is employed to express the mean velocity as a function of average surface velocity. The entropy-based flow profile is parameterized by the location of maximum velocity in the channel and the shape of the velocity distribution. The entropy parameters are quantified over the tidal cycle to account for the unsteady nature of estuarine flow. The method was tested using experiments conducted at the Danshui River, the largest estuarine system in Taiwan. Surface velocities were measured using an Automated River-Estuary Discharge Imaging System (AREDIS), and full-channel velocity profiles were measured with a moving-boat ADP survey. Entropy parameters were calibrated over the tidal cycle and linearly correlated with the average surface velocity to facilitate estimation from AREDIS measurements. The discharge calculated from average surface velocity and entropy relationships exhibits a 7.7% relative error compared to the ADP velocity profiles. The error nearly doubles when the mean values for entropy parameters are used instead of the variable parameters, indicating the importance of accounting for the unsteady nature of estuarine flows. Furthermore, the effects of measurement coverage area, types of entropy distribution, and wind-induced drift current on the surface velocity-based discharge measurement are evaluated and discussed. Overall, surface velocity measurements in conjunction with the entropy profiles well represent the flow in a complex estuarine environment to provide a reliable estimate of discharge.