Monitoring Mesoscale to Submesoscale Processes in Large Lakes with Sentinel-1 SAR Imagery: The Case of Lake Geneva

As in oceans, large-scale coherent circulations such as gyres and eddies are ubiquitous features in large lakes that are subject to the Coriolis force. They play a crucial role in the horizontal and vertical distribution of biological, chemical and physical parameters that can affect water quality. In order to make coherent circulation patterns evident, representative field measurements of near-surface currents have to be taken. This, unfortunately, is difficult due to the high spatial and temporal variability of gyres/eddies. As a result, few complete field observations of coherent circulation in oceans/lakes have been reported. With the advent of high-resolution satellite imagery, the potential to unravel and improve the understanding of mesoscale and submesoscale processes has substantially increased. Features in the satellite images, however, must be verified by field measurements and numerical simulations. In the present study, Sentinel-1 SAR satellite imagery was used to detect gyres/eddies in a large lake (Lake Geneva). Comparing SAR images with realistic high-resolution numerical model results and in situ observations allowed for identification of distinct signatures of mesoscale gyres, which can be revealed through submesoscale current patterns. Under low wind conditions, cyclonic gyres manifest themselves in SAR images either through biogenic slicks that are entrained in submesoscale and mesoscale currents, or by pelagic upwelling that appears as smooth, dark elliptical areas in their centers. This unique combination of simultaneous SAR imagery, three-dimensional numerical simulations and field observations confirmed that SAR imagery can provide valuable insights into the spatial scales of thus far unresolved mesoscale and submesoscale processes in a lake. Understanding these processes is required for developing effective lake management concepts.

Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin, Rafael Sebastian Reiss

Convective winter cooling is an important process in the annual cycle of a lake ecosystem, because it brings oxygen to the deeper layers. However, in deep lakes, convective processes are often not strong enough to extend the mixing down to the deepest layers. This may become even more problematic in the future considering climate change and potentially milder winters. In large deep lakes, two other processes originating in the coastal zone can significantly contribute to winter deep water renewal: cold water density currents on the lateral slopes caused by differential cooling between the shallow literal zone and the deep off-shore region, and upwelling of deep water masses over the lateral slopes which results from wind forcing and may be influenced by the Coriolis force. Previous field experiments in Lake Geneva, the largest freshwater lake in Western Europe (max. depth 310 m), have shown that cold-water density currents are a common phenomenon during wintertime, frequently reaching the thermocline at O(100m) depth. As part of a project investigating winter mixing dynamics in Lake Geneva, we examine wind-driven, Coriolis-influenced upwelling in order to better understand its role in the deep-water exchange during the weakly stratified period (December-March). The study is based on field observations taken on the northern shore of the lake, 20 km west of Lausanne in the winters of 2017-19. Near-bottom water temperatures are inferred from a fiber-optic based Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1m) and O(0.1°C), respectively. Temperature and current velocities in the water column are recorded by several ADCP and thermistor chain moorings. The data set is completed by regular CTD campaigns following wind events favoring upwelling. These field measurements were combined with a validated 3D hydrodynamic model (MITgcm) in order to analyze and quantify the exchange between the littoral zones and deeper parts of the lake. It was observed that following medium to strong northeast-bound wind events (i.e. winds parallel to the main axis of the central lake) lasting for at least one day, near-shore temperatures at all instruments suddenly dropped to the same temperatures as those in the deep open waters and remained low for several days. This indicates that, triggered by persistent wind-stress and under the influence of the Coriolis force, hypolimnetic water reached the surface in the near shore zone, where it is subject to heat and gas exchange with the atmosphere, before returning to the deep layers. Numerical modeling of these events showed comparable results and allowed to trace the origin of this cold water to the deep layers. Our field observations and the numerical results suggest that in addition to the previously observed cold-water density currents, wind-driven upwelling is a common phenomenon in Lake Geneva during the weakly stratified winter period and potentially a significant mechanism for deep-water renewal.

2019

Micropollutants in Large Lakes: From Potential Pollution to Risk Assessments

David Andrew Barry, Florence Bonvin, Neil Graham, Luca Rossi

This book is divided into six additional chapters: Chapter 2 deals with the agricultural source of micropollutants; Chapter 3 with the urban source of micropollutants; Chapter 4 with lake circulation; Chapter 5 with the influence of a wastewater treatment plant on a bay; Chapter 6 with the input of sediments and their transport in a bay; and Chapter 7 with the risk assessment of the mixture of the compounds present in lake water. A last chapter (8) synthesizes the main findings and presents perspectives for future studies. More specifically: Chapter 2 is dedicated to the micropollutants that enter lakes through agriculture. These are mainly pesticides. After a review of the different types of compounds, the authors will put forth the main pathways these compounds follow to enter lakes. The case of glyphosate, an herbicide used worldwide, will be discussed. Chapter 3 illustrates the urban sources of pollutants with pharmaceuticals and biocides. Two models will be covered that allow estimating the load and dynamics of these chemicals that may exit from an urban catchment and therefore reach lakes. Special attention will be paid to the “end of pipe” removal of these compounds at a WWTP. Chapter 4 is dedicated to lake circulation. The aim of this chapter is to present an overall description of lake hydrodynamics, which is driven by three factors: wind, temperature and Coriolis forces. The main inflow to Lake Geneva is provided by the Rhône River. In addition, the topography surrounding the lake varies rapidly, for example, with mountains in the eastern part. As a consequence, the lake’s currents can vary markedly. To achieve our objective, we will utilise hydrodynamic modelling. A model will be proposed that permits describing the behavior of the top layer of the lake based on wind direction. The importance of stable hydrogen and oxygen isotopes for characterising the sources of water and mixing processes in the lake will be described and illustrated with the example of the Rhône river interflow to it. The measured variations of the isotopic compositions within the lake can serve as verification of the lake hydrodynamic models. However, the emphasis of the chapter will be on explaining the major current features and how they relate to wind forcing, which is one of the main drivers. Chapter 5 and 6 are devoted to the Vidy Bay in the Lausanne area of Lake Geneva. This bay receives the treated effluents of the largest wastewater treatment plant of the lake catchment. The latter represents therefore one of the major point sources of contaminants for the lake. These two chapters will focus on the various processes that control the transfer of contaminants (associated with particles or in a dissolved state) discharged into the bay and transported to the lake’s main water body. More specifically, Chapter 5 describes the WWTP plume dynamics. In relation to hydrodynamics (Chapter 4), specifically the behaviour of the plume formed through the treatment plant effluent release, field measurements are utilised to attempt to determine the plume behavior under various lake conditions. Furthermore, it investigates the degradation of organic pollutants. Photodegradation and other degradation processes are studied with theoretical, experimental and modelling approaches. Chapter 6 focuses on sediment path tracing. The pollutants associated with particles (trace metals, hydrophobic components, etc.) settle to the lakebed, but can be remobilized and transported to deeper parts of the lake. Bottom currents, and other physical forces, can cause resuspension and advection. Tracers used to identify this advection comprise natural and anthropogenic radioisotopes along with mercury as a major toxic trace element. Chapter 7 concentrates on one major issue of micropollutants in aquatic systems, like lakes, i.e., the risk of the mixture. The evaluation of the risk of mixture of micropollutants is not trivial and we will first present critically the models that allow doing so. We will also demonstrate their validity as predictive tools with the example of herbicide mixtures in Lake Geneva. Finally, we will map the risk of micropollutants around the lake. Finally in Chapter 8, we synthesize the main findings of all chapters and discuss recommendations for the management of mid-sized lakes regarding micropollution.

CRC Press & EPFL Press2018

Current variability in a wide and open lacustrine embayment in Lake Geneva (Switzerland)

Roham Bakhtyar, David Andrew Barry, Nicolas Le Dantec, Ulrich Lemmin, Amir Mehdi Razmi, Alfred Johny Wüest

Field measurements and numerical simulations were used to determine the effects of dominant meteorological conditions on the hydrodynamics of a wide (aspect ratio ~2), relatively deep (seasonally stratified) and open lake embayment (Vidy Bay, Lake Geneva). A three-dimensional hydrodynamic model (Delft3D-FLOW) was employed to simulate flow in the lake. High-resolution maps of wind, temperature and humidity (over the lake) were applied as input to drive the model. Because wind was the main force driving flow in the lake, currents in the embayment were investigated systematically for different wind conditions and seasonal stratification. Satisfactory model validation was achieved using drifter and moored measurements within the embayment. Markedly different circulation patterns were measured within the embayment, with the transition from one pattern to another occurring abruptly for small changes in wind direction. These distinct patterns resulted from relatively small changes in the large gyre of Lake Geneva’s main basin, especially the angle between the current in front of the embayment and the embayment shoreline. The boundary between the embayment and the pelagic zone was defined by the largest gyre within the embayment. This study shows that, (i) in a large lake, complex current patterns can occur even within a minor embayment, and (ii) that these patterns can transition rapidly over a small range of wind directions. Near-shore gyre can occur for lengthy periods, which has implications for flushing of discharges within the embayment.