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.

Habib UAV

Evaluation of UAV LiDAR for mapping coastal Environments

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Unmanned Aerial Vehicle (UAV)-based remote sensing techniques have demonstrated great potential for monitoring rapid shoreline changes. With image-based approaches utilizing Structure from Motion (SfM), high-resolution Digital Surface Models (DSM), and orthophotos can be generated efficiently using UAV imagery. However, image-based mapping yields relatively poor results in low textured areas as compared to those from LiDAR. This study demonstrates the applicability of UAV LiDAR for mapping coastal environments. A custom-built UAV-based mobile mapping system is used to simultaneously collect LiDAR and imagery data. The quality of LiDAR, as well as image-based point clouds, are investigated and compared over different geomorphic environments in terms of their point density, relative and absolute accuracy, and area coverage. The results suggest that both UAV LiDAR and image-based techniques provide high-resolution and high-quality topographic data, and the point clouds generated by both techniques are compatible within a 5 to 10 cm range. UAV LiDAR has a clear advantage in terms of large and uniform ground coverage over different geomorphic environments, higher point density, and ability to penetrate through vegetation to capture points below the canopy. Furthermore, UAV LiDAR-based data acquisitions are assessed for their applicability in monitoring shoreline changes over two actively eroding sandy beaches along southern Lake Michigan, Dune Acres, and Beverly Shores, through repeated field surveys. The results indicate a considerable volume loss and ridge point retreat over an extended period of one year (May 2018 to May 2019) as well as a short storm-induced period of one month (November 2018 to December 2018). The foredune ridge recession ranges from 0 m to 9 m. The average volume loss at Dune Acres is 18.2 cubic meters per meter and 12.2 cubic meters per meter within the one-year period and storm-induced period, respectively, highlighting the importance of episodic events in coastline changes. The average volume loss at Beverly Shores is 2.8 cubic meters per meter and 2.6 cubic meters per meter within the survey period and storm-induced period, respectively.

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.

Assessment and characterization of writing exercises and core engineering textbooks

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Writing has been identified as a critical skill and element of the engineering profession, yet it is rarely included in sophomore-level and junior-level courses. Textbooks often influence how courses are structured, and reading assignments and homework problems are frequently assigned directly from textbooks. In this project, textbooks are systematically searched for writing-based problems in four core engineering courses: fluid mechanics, thermodynamics, statics, and circuits. The authors focused efforts on identifying learning activities that could potentially allow students to practice writing, learn through writing, and use writing to relate course content to broader applications and contexts. Results included the total number of end-of-chapter questions with writing components and the classification of types of writing prompts. Analysis showed a limited availability of questions with writing components in textbooks and a missed opportunity to incorporate important writing education within the context of technical engineering concepts.

Engineering instructors on writing: Perceptions, practices, and needs

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Background- With communication skills deemed increasingly important for engineering graduates, we wanted to understand how writing is currently included in engineering classes, what challenges are caused by including writing in such classes, and what resources would be most useful to help engineering instructors more easily include writing in engineering classes. Literature review- Writing is a necessary skill for engineering graduates and has received increased attention in engineering classes. However, despite many instructors’ beliefs that writing is an important skill for engineers, it is not typically taught in a systematic and comprehensive way across the engineering curriculum. Research questions- 1. What perceptions of writing, and specifically writing in engineering, do engineering instructors hold? 2. To what extent do engineering instructors report incorporation of writing activities and assignments in their classes? 3. What barriers do engineering instructors perceive as inhibiting the inclusion of more writing in engineering courses? 4. What resources do engineering instructors desire to expand and improve the inclusion of writing in engineering courses? Research methods: A survey was completed by engineering instructional staff (n = 190 respondents, 10.7% response rate) from seven institutions as well as by some members of the Big10+ Engineering Deans Mailing List. Instructors were asked about their general perceptions about writing in engineering and were also asked to consider the most recent engineering course that they taught and reflect on how they included (or did not include) writing in their course. Findings and conclusions- As expected, we found that most engineering instructional staff agree that writing skills are very important in engineering. Yet, we found that constraints on time and resources kept instructors from including more writing in their courses. This paper concludes with a discussion of our efforts to develop resources, such as rubrics, graded writing examples, and strategies for developing writing prompts, to help instructors include more writing in their engineering courses.

Regulation of plankton and nutrient dynamics by profundal quagga mussels in Lake Michigan: a one-dimensional model

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Invasive dreissenid mussels have altered plankton abundance and nutrient cycling in the Great Lakes. In this study, a 1-D hydrodynamic-biogeochemical coupled model is developed to investigate their effects at a mid-depth offshore site in Lake Michigan. Model simulation shows that water surface temperature and vertical thermal structure can be well reproduced. Driven by the simulated vertical mixing, the biological model solves the transport and transformation of nutrients, plankton and detritus in the water column. Mussel grazing and excretion are added at the bottom boundary. The biological model predicts a notable decline of phytoplankton biomass and considerable increase of dissolved phosphorus (DP) in the entire water column at the end of spring. However, the reduction of phytoplankton and the increase of DP are limited to the bottom 20 m in summer as a result of the strong stratification. Model results also show that mussels can maximize particle delivery to the benthos, as the modeled benthic diffusive flux of particulate phosphorus exceeds the passive settling rate by 4.2× on average. Model simulation over a 10-month period indicates that profundal mussels have the potential to significantly change the distribution of energy and nutrients in the water column, even in a deep and stratified environment.