Water temperatures and stratification are important drivers for ecological and water quality processes within lake systems, and changes in these with increases in air temperature and changes to wind ...speeds may have significant ecological consequences. To properly manage these systems under changing climate, it is important to understand the effects of increasing air temperatures and wind speed changes in lakes of different depths and surface areas. In this study, we simulate three lakes that vary in depth and surface area to elucidate the effects of the observed increasing air temperatures and decreasing wind speeds on lake thermal variables (water temperature, stratification dates, strength of stratification, and surface heat fluxes) over a century (1911–2014). For all three lakes, simulations showed that epilimnetic temperatures increased, hypolimnetic temperatures decreased, the length of the stratified season increased due to earlier stratification onset and later fall overturn, stability increased, and longwave and sensible heat fluxes at the surface increased. Overall, lake depth influences the presence of stratification, Schmidt stability, and differences in surface heat flux, while lake surface area influences differences in hypolimnion temperature, hypolimnetic heating, variability of Schmidt stability, and stratification onset and fall overturn dates. Larger surface area lakes have greater wind mixing due to increased surface momentum. Climate perturbations indicate that our larger study lakes have more variability in temperature and stratification variables than the smaller lakes, and this variability increases with larger wind speeds. For all study lakes, Pearson correlations and climate perturbation scenarios indicate that wind speed has a large effect on temperature and stratification variables, sometimes greater than changes in air temperature, and wind can act to either amplify or mitigate the effect of warmer air temperatures on lake thermal structure depending on the direction of local wind speed changes.
Flash rips are episodic bursts of water jetting offshore, which can lead to drowning incidents by sweeping swimmers offshore without warning, thus posing a hidden and unrecognized danger to ...beachgoers. This study reveals hazards of flash rips by investigating a series of drowning incidents along coasts of Lake Michigan during a series of storm events on July 18–21, 2019. Occurrences and causes of flash rips were depicted through webcam image observations, storm features of atmospheric disturbances, hydrodynamic circumstances of wind waves and meteorologically induced water level fluctuations, and model-reconstructed nearshore circulations. Results shows that flash rips were generated during or after storms through nearshore processes of storm-induced wind waves and meteorologically induced water level fluctuations. With small wind waves, low water level fluctuations, and a timing delay of rip occurrences relative to the causative convective storms, flash rips pose a hidden hazard to unaware swimmers. Historical observations for incidents in Lake Michigan between 2002 and 2019 further show that dry conditions or fair weathers and a calm water signature at the beach can likely generate unexpected hidden flash rips, resulting in the highest drowning risks. There is an urgent need for communication, education, and prediction/forecast of hidden flash rips to the Laurentian Great Lakes and worldwide coastal communities.
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•Flash rips as the cause for drowning incidents in Lake Michigan were examined.•Flash rips occur during and after fast-moving convective storms.•Many drowning incidents occur under hydrodynamic circumstances with small waves and low water level fluctuations.•Timing delay of storms to generate hidden flash rips elevates drowning risks.•Conditions of unexpected hidden flash rips at the beach are depicted.
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, USA. 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.
Lake Analyzer is a numerical code coupled with supporting visualization tools for determining indices of mixing and stratification that are critical to the biogeochemical cycles of lakes and ...reservoirs. Stability indices, including Lake Number, Wedderburn Number, Schmidt Stability, and thermocline depth are calculated according to established literature definitions and returned to the user in a time series format. The program was created for the analysis of high-frequency data collected from instrumented lake buoys, in support of the emerging field of aquatic sensor network science. Available outputs for the Lake Analyzer program are: water temperature (error-checked and/or down-sampled), wind speed (error-checked and/or down-sampled), metalimnion extent (top and bottom), thermocline depth, friction velocity, Lake Number, Wedderburn Number, Schmidt Stability, mode-1 vertical seiche period, and Brunt-Väisälä buoyancy frequency. Secondary outputs for several of these indices delineate the parent thermocline depth (seasonal thermocline) from the shallower secondary or diurnal thermocline. Lake Analyzer provides a program suite and best practices for the comparison of mixing and stratification indices in lakes across gradients of climate, hydro-physiography, and time, and enables a more detailed understanding of the resulting biogeochemical transformations at different spatial and temporal scales.
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.
High‐frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (u*) and convection (w*) to turbulence in the surface mixed layer. Seasonal ...patterns of u* and w* were dissimilar; u* was often highest in the spring, while w*increased throughout the summer to a maximum in early fall. Convection was a larger mixed‐layer turbulence source than wind shear (u*/w* < 0.75) for 18 of the 40 lakes, including all 11 lakes <10 ha. As a consequence, the relative contribution of convection to the gas transfer velocity (k, estimated by the surface renewal model) was greater for small lakes. The average k was 0.54 m day−1 for lakes <10 ha. Because u* and w*differ in temporal pattern and magnitude across lakes, both convection and wind shear should be considered in future formulations of lake‐air gas exchange, especially for small lakes.
Key Points
Convection was consistently a larger turbulence source than wind for small lakes
Convection and wind shear have different diurnal seasonal and geographic pattern
Wind‐only methods for gas exchange are not appropriate for small lakes
•A new type of localized vertical grid (LSC2) is implemented in a 3D model.•It significantly reduces pressure gradient error, diapycnic mixing.•It is free of staircases.•Results show improved ...representation of stratification in lakes and oceans.
We present a new vertical coordinate system for cross-scale applications. Dubbed LSC2 (Localized Sigma Coordinates with Shaved Cell), the new system allows each node of the grid to have its own vertical grid, while still maintaining reasonable smoothness across horizontal and vertical dimensions. Furthermore, the staircase created by the mismatch of vertical levels at adjacent nodes is eliminated with a simple shaved-cell like approach using the concept of degenerate prisms. The new system is demonstrated to have the benefits of both terrain-following and Z-coordinate systems, while minimizing their adverse effects. We implement LSC2 in a 3D unstructured-grid model (SELFE) and demonstrate its superior performance with test cases on lake and ocean stratification.
The performance of a new type of flow energy harvester based on oscillating foils is investigated through numerical modeling by using two methods, a 2D thin-plate model and a 3D nonlinear ...boundary-element model. The fluid–structure interaction problem involved in the dynamics of a heaving/pitching foil coupled with an actuation/energy harvesting system in this device is examined. The 2D analysis allows us to simulate dynamics of the flapping-foil system over a large range of parameters and to identify areas of special interests (e.g., high energy output or high efficiency). In the vicinity of these areas the 3D model can accurately predict the performance of the system. By examining the power extraction capacity and efficiency of the system at various geometric, mechanical, and kinematic parameters, the optimal performance of the system is determined. In addition, the performance is found to be enhanced by the presence of a solid ground, as well as the thickness of the foil (at certain frequencies).
Summary
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.
Two meteotsunamis were unexpectedly observed along the eastern coast of Japan coincidentally with atmospheric pressure disturbances prior to two typhoons, respectively. The maximum wave height of ...meteotsunamis was up to ~ 0.5 m in Aburatsubo Bay with the pressure fluctuation of ~ 2 hPa before Typhoon Wipha. This study reveals the causes of the unexpected meteotsunamis in association with atmospheric forcings of the typhoons. The possible origin is identified from typhoon activities at the outer circulation that produced traveling pressure disturbances as a driving force to generate meteotsunamis. The causality of meteotsunamis is revealed by time–frequency analysis of sea level, atmospheric pressure, and wind speed. Results show that a combination of atmospheric forcings, pressure and wind stress, is responsible for the onset of meteotsunamis. The pressure fluctuation is found to be an indicator to determine the origin of atmospheric forcings and the formation of corresponding meteotsunamis when the meteorological condition is relatively calm before a typhoon.
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