LExplore Platform

Vertical Mixing and Basin-scale Energetics in Lake Geneva, Switzerland

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LExplore

L’Explore floating observational platform on Lake Geneva, Switzerland.

This project was developed during my sabbatical at the École Polytechnique Fédérale de Lausanne (EPFL) in Lausanne, Switzerland, where I was hosted by Dr. Johny Wüest. The objectives of the project were to quantify seasonal variations in vertical mixing in the lake, making use of the L’Explore Platform, and to link these observations of vertical mixing with the overall energetics of the lake. This one-of-a-kind floating observational platform is a moored 10m x 10m instrumented platform that contains continuously recording instruments and also serves as a deployment platform for episodic work. During my time there, we performed regular weekly microstructure sampling from the platform, as well as some intensive 24 hour experiments to examine biological variability.

Collaborators

Johny Wüest, Bieito Fernandez, Hugo Ulloa (EPFL); Damien Bouffard (EAWAG)

Inflatable Midlake

The Role of Near-inertial Poincaré Waves in Lake Michigan Mixing and Dispersion

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Inflatable Midlake

Microstructure profiling in the middle of Lake Michigan

Near-inertial internal waves are ubiquitous features in both large lakes and oceans. In large thermally-stratified lakes such as Lake Michigan, these basin-scale waves create strong near-surface currents that rotate clockwise (in the northern hemisphere) over a near-inertial period (~17.5 hours for Lake Michigan). The influence of these basin-scale waves, for which the thermocline movement represents a spinning coin, is particularly strong in the offshore waters, where the spiraling near-surface currents can have tide-like regularity, with magnitudes exceeding 50 cm/s. While these waves are readily observed in velocity and temperature measurements taken in large lakes, their influence on vertical mixing and lateral dispersion is not well-understood. This project examines two related hypotheses: (1) strong near-inertial shear drives cross-thermocline mixing; (2) lateral dispersion is enhanced by vertical shear associated with near-inertial wave currents. Our approach for this project is to carry out a set of field measurements involving moored instruments and microstructure cruises, as well as a large-scale dye and drifter release carried out in the center of Lake Michigan’s southern basin.

Funding

National Science Foundation, Division of Ocean Sciences, Physical Oceanography Program

Collaborators

Nathan Hawley, NOAA Great Lakes Environmental Research Laboratory

Quagga Bottom

The Role of Turbulence in Regulating Quagga Mussel Effects

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Since their arrival at the turn of this century, invasive quagga mussels have dramatically altered the food web in Lake Michigan. Quagga mussels now cover the bottom of much of Lake Michigan, particularly deeper waters, with densities exceeding 10,000 mussels per square meter in certain locations. As prolific filter-feeders, quagga mussels can filter up to several liters of water per day. The increased water clarity of Lake Michigan may seem like a good thing (Lake Michigan’s water clarity now rivals Lake Tahoe), but these hardy “ecosystem engineers” have had a dramatic effect on Lake Michigan’s ecosystem, causing species extinction, alterations to nutrient cycling, and benthic substrate changes. Quagga mussels have invaded all of the Great Lakes except for Lake Superior, and are currently colonizing deeper reservoirs in the Western U.S. as well as lakes in Western Europe (among other places).

The ability of quagga mussels to clear the water column depends strongly on the turbulent mixing characteristics of the overlying water, Our ability to model the effects of mussels on aquatic systems depends then in turn on our abilities to model turbulent mixing in large lakes and oceans. When mixing is weak, mussels can only filter a small volume of water around them; when turbulence is energetic, new water is continually delivered to the lake bed, and mussels effectively have access to the entire water column. Our role in this collaborative project, which focuses on quagga mussel dynamics in the deeper waters of Lake Michigan, is to quantify deep water mixing rates and to link these rates to mussel effects on the food web. To address these objectives, we are carrying out and analyzing a set of Lake Michigan field experiments, in a range of locations and conditions, where we measure water column temperatures, currents, and turbulence using a range of instrumentation and analysis techniques. The mixing prescriptions elucidated from these observations will then serve as the basis for a mixing model onto which we layer nutrient, plankton, and zooplankton models (NPZD).

Funding

National Science Foundation, Division of Ocean Sciences

Project home page – NSF BCO-DMO

Collaborators

Harvey Bootsma and Qian Liao, University of Wisconsin-Milwaukee

David Cannon, University of Michigan

Drifter Trajectories

Lateral dispersion of dye and drifters in the center of a very large lake

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To better understand lateral dispersion of buoyant and nonbuoyant pollutants within the surface waters of large lakes, two lateral dispersion experiments were carried out in Lake Michigan during the stratified period: (1) a dye tracking experiment lasting 1 d; and (2) a drifter tracking experiment lasting 24 d. Both the dye patch and drifters were surface-released at the center of Lake Michigan’s southern basin. Near-surface shear induced by near-inertial Poincaré waves partially explains elevated dye dispersion rates (1.5–4.2 m2 s−1). During the largely windless first 5 d of the drifter release, the drifters exhibited nearly scale-independent dispersion ( K ∼ L0.2), with an average dispersion coefficient of 0.14 m2 s−1. Scale-dependent drifter dispersion ensued after 5 d, with K ∼ L1.09 and corresponding dispersion coefficients of 0.3–2.0 m2 s−1 for length scales L = 1500–8000 m. The largest drifter dispersion rates were found to be associated with lateral shear-induced spreading along a thermal front. Comparisons with other systems show a wide range of spreading rates for large lakes, and larger rates in both the ocean and the Gulf of Mexico, which may be caused by the relative absence of submesoscale processes in offshore Lake Michigan.

Cannon Ice Free Radiative Convection

Ice-free radiative convection drives spring mixing in a large lake

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In this work we highlight the importance of radiative convection as a mixing mechanism in a large, ice-free lake (Lake Michigan, USA), where solar heating of waters below the temperature of maximum density drives vertical convection during the vernal turnover. Measurements taken over a 2-week period at a 55-m deep site demonstrate the ability of radiative convection to mix the entire water column. Observations show a diurnal cycle in which solar heating drives a steady deepening of the convective mixed layer throughout the day (dHCML/dt = 12.8 m/hr), followed by surface-cooling-induced restratification during the night. Radiative convection is linked to a dramatic enhancement in turbulence characteristics, including both turbulent kinetic energy dissipation (ϵ: 10−9–10−7 W/kg) and turbulent scalar diffusivity (Kz: 10−3–10−1 m2/s), suggesting that radiative convection plays a major role in driving vertical mixing throughout the water column during the isothermal spring.

Observations of turbulence and mean flow in the low‐energy hypolimnetic boundary layer of a large lake

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Near-bed measurements are reported for both mean flow and turbulence structure in the deep hypolimnetic waters of Lake Michigan (55 m depth) during stratified and unstratified periods to determine validity and restrictions of the expected law-of-the-wall (LOW) behavior. Near-bed currents were weak (U50 = 3, 16 cm s−1 for mean, maximum currents respectively at 50 cm elevation), dominated by subinertial energy across all seasons, and showed little seasonal variation in spite of the strong seasonality to wind forcing. Velocity structure for wave-free conditions showed strong log-linear trends within 1 mab, with over 98% of the 2152 velocity profiles producing significant log-linear fits within the bottom meter and a strictly logarithmic velocity profile extending to only 66 cmab on average (Cd 50 = 0.0052; zo = 0.0015 m). Stratification was dynamically unimportant to mean flow and turbulence, but fitted log-linear length scales suggest that deviations from strictly logarithmic velocity structure may be explained by flow unsteadiness. Turbulent quantities measured within 1 m of the bed including dissipation, turbulent kinetic energy, and turbulent length scales followed LOW expectations in the mean, but individual estimates deviated by several orders of magnitude. The observed deviations from LOW turbulent structure were found to be correlated with the log-linear length scales fit to mean velocity profiles and were consistent with the effects of flow unsteadiness.

Physicochemical characteristics of a southern Lake Michigan river plume

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Riverine inputs are a major source of nutrients to the Laurentian Great Lakes and have important effects on nearshore biological processes, where mixing between river and lake water leads to formation of heterogeneous river plumes. We examined the physical and chemical characteristics of the St. Joseph River plume in southern Lake Michigan between May and October 2011, and in October 2012, June 2013 and April 2014. Specific electric conductivity and stable isotopes of water were used to quantify the fraction of river water (FRW) at sampling sites in Lake Michigan. Both tracers predicted similar patterns of FRW among sites; however, there was a systematic offset between the two methods, and specific electric conductivity method under-predicted the FRW by ~5%. We observed a distinct, seasonally varying river plume, with plume size correlated with flow rate of St. Joseph River. Within the plume, sediments and nutrients were non-conservative and exhibited significant and seasonally varying losses that we attribute to settling of particle-bound nutrients and/or nutrients in particulate phase below the plume. The characteristics and the spatiotemporal heterogeneity of the river plume documented here may have important implications for the nearshore biogeochemistry of the Great Lakes and for understanding the roles of these features in ecological processes in nearshore areas.

Logarithmic velocity structure in the deep hypolimnetic waters of Lake Michigan

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The characteristics of the bottom boundary layer are reported from a Lake Michigan field study carried out in deep hypolimnetic waters (55 m depth) during the stratified period (June–September 2012). The sandy substrate at the measurement site was densely covered with invasive quagga mussels (mean size: 1.6 cm; mean density: 10,000 mussels m−2). The measurements reveal a sluggish, compact bottom boundary layer, with flow speeds at 1 mab less than 5 cm s−1 for most of the period, and a dominance of subinertial energy. A downwelling event caused the largest currents observed during the deployment (10 cm s−1 at 1 mab) and a logarithmic layer thickness of 15 m. In spite of the weak flow, logarithmic profile fitting carried out on high-resolution, near-bed velocity profiles show consistent logarithmic structure (90% of profiles). Flow was dominated by subinertial energy but strong modified by near-inertial waves. Fitted drag coefficients and roughness values are Cd1m = 0.004 and Z0 = 0.12 cm, respectively. These values increase with decreasing flow speed, but approach canonical values for 1 mab flow speeds exceeding 4 cm s−1. The estimated vertical extent of the logarithmic region was compact, with a mean value of 1.2 m and temporal variation that is reasonably described by Ekman scaling, 0.07 u*/f, and the estimated overall Ekman layer thickness was generally less than 10 m. Near-bed dissipation rates inferred from the law of the wall were 10−8−10−7 W kg−1 and turbulent diffusivities were 10−4−10−3 m2s−1.