Ventilation mechanisms in deep lakes are crucial for their ecosystem functioning. In this paper we show the relevance of planetary rotation in affecting ventilation processes in relatively narrow, elongated deep lakes. Through a recent field campaign in Lake Garda (Italy), we provide explicit observational evidence for the development of lake-wide wind-driven secondary flows influenced by the Coriolis force in a narrow lake. The interpretation of these observations is supported by results from numerical simulations with a three-dimensional model of the lake. The results add an additional element, often neglected in narrow lakes, to be carefully considered when assessing the response of lakes to external forcing and climate change.

The effects of wind forcing and Earth’s rotation on the transport processes in Lake Garda, Italy, are investigated for the first time under different thermal stratification conditions and typical diurnal wind cycles. Numerical simulations are performed by means of a modeling chain composed of a meteorological (WRF) and a hydrodynamic (Delft3D) model. Transport processes are studied through the combined analysis of the residual (time averaged) flow field and the trajectories of Lagrangian particles. Results show that strong currents develop in winter under the forcing of synoptic northerly Föhn winds, especially in the elongated northern region, where winds are channeled by the steep orography. Significant water volumes are displaced laterally by Ekman transport, producing intense downwelling and upwelling along the steep shores. Instead summer patterns are controlled by the diurnal cycle of local breezes, alternately blowing along the main axis of the lake. The resulting circulation reveals counterclockwise gyres in the northern part, driven by the alternating wind direction and affected by Coriolis force. The analysis suggests that complex circulations can develop in lakes with relatively simple geometries, like the narrow trunk region of Lake Garda, where the effect of Earth’s rotation unexpectedly influences the transport patterns.

Freshwater algae exhibit complex dynamics, particularly in meso-oligotrophic lakes with sudden and dramatic increases in algal biomass following long periods of low background concentration. While the fundamental prerequisites for algal blooms, namely light and nutrient availability, are well-known, their specific causation involves an intricate chain of conditions. Here we examine a recent massive Uroglena bloom in Lake Geneva (Switzerland/France). We show that a certain sequence of meteorological conditions triggered this specific algal bloom event: heavy rainfall promoting excessive organic matter and nutrients loading, followed by wind-induced coastal upwelling, and a prolonged period of warm, calm weather. The combination of satellite remote sensing, in-situ measurements, ad-hoc biogeochemical analyses, and three-dimensional modeling proved invaluable in unraveling the complex dynamics of algal blooms highlighting the substantial role of littoral-pelagic connectivities in large low-nutrient lakes. These findings underscore the advantages of state-of-the-art multidisciplinary approaches for an improved understanding of dynamic systems as a whole.

Deep water circulation and mixing processes are responsible for the transport of matter, nutrients and pollutants in deep lakes. Nevertheless, detailed continuous observations are rarely available. To overcome some of these deficiencies and with the aim of improving our understanding of deep mixing processes, a dedicated yearlong mooring comprising 100 high-resolution temperature sensors and a single current meter were located in the deeper half of the 344 m deepest point of the subalpine Lake Garda, Italy. The observations show peaks and calms of turbulent exchange, besides ubiquitous internal wave activity. In late winter, northerly winds activate episodic deep convective overturning, the dense water being subsequently advected along the lake-floor. Besides deep convection, such winds also set-up seiches and inertial waves that are associated with about 100 times larger turbulence dissipation rates than that by semidiurnal internal wave breaking observed in summer. In the lower 60 m above the lake-floor, however, the average turbulence dissipation rate is approximately constant in value year-around, being about 10 times larger than open-ocean values, except during deep convection episodes.

A calibrated three-dimensional numerical model (Delft3D) and in-situ observations are used to study the relation between deep-water temperature and deep mixing in Lake Garda (Italy). A model-observation comparison indicates that the model is able to adequately capture turbulent kinetic energy production in the surface layer and its vertical propagation during unstratified conditions. From the modeling results several processes are identified to affect the deep-water temperature in Lake Garda. The first process is thermocline tilting due to strong and persistent winds, leading to a temporary disappearance of stratification followed by vertical mixing. The second process is turbulent cooling, which acts when vertical temperature gradients are nearly absent over the whole depth and arises as a combination of buoyancy-induced turbulence production due to surface cooling and turbulence production by strong winds. A third process is differential cooling, which causes cold water to move from the shallow parts of the lake to deeper parts along the sloping bottom. Two of these processes (thermocline tilting and turbulent cooling) cause deep-mixing events, while deep-cooling events are mainly caused by turbulent cooling and differential cooling. Detailed observations of turbulence quantities and lake temperature, available at the deepest point of Lake Garda for the year 2018, indicate that differential cooling was responsible for the deep-water cooling at that location. Long-term simulations of deep-water temperature and deep mixing appear to be very sensitive to the applied wind forcing. This sensitivity is one of the main challenges in making projections of future occurrences of episodic deep mixing and deep cooling under climate change.

The SAR Doppler frequencies are directly related to the motion of the scatterers in the illuminated area and have already been used in marine applications to monitor moving water surfaces. Here we investigate the possibility of retrieving surface water velocity from SAR Doppler analysis in medium-size lakes. ENVISAT images of the test site (Lake Garda) are processed and the Doppler Centroid Anomaly technique is adopted. The resulting surface velocity maps are compared with the outputs of a hydrodynamic model specifically validated for the case study. Thermal images from MODIS Terra are used in support of the modeling results. The surface velocity retrieved from SAR is found to overestimate the numerical results and the existence of a bias is investigated. In marine applications, such bias is traditionally removed through Geophysical Model Functions (GMFs) by ascribing it to a fully developed wind waves spectrum. We found that such an assumption is not supported in our case study, due to the small-scale variations of topography and wind. The role of wind intensity and duration on the results from SAR is evaluated, and the inclusion of lake bathymetry and the SAR backscatter gradient is recommended for the future development of GMFs suitable for lake environments.