Matteo De Vincenzi

Shallow lakes are dynamic ecosystems where water circulation is governed by the balance between wind and through-flowing discharge. In these lakes the aquatic vegetation both shapes and is shaped by water circulation, as a strong interaction exists between biological and physical processes. Hence, the understanding of lakes hydrodynamics is essential to plan effective safeguard and remediation measures in such environments. An interesting case study for this topic is the Mantua Lake system, where all the above-cited features are present. Mantua Lakes are a shallow fluvial lake system located in Northern Italy that was created in the 12th century by the damming of the Mincio River. The river drains a watershed that is intensively exploited by agriculture. This resulted in a high nutrient input load that since the 1970s has caused the lakes’ eutrophication, eventually worsened by summer low discharges due to water diversion for irrigation purposes. In addition, macrophytes of the species Nelumbo nucifera (lotus flower) were introduced in 1921 as a food source and now represent a problem for their high invasiveness which requires massive cutting efforts. Until now, restoration strategies have been missing due to the deficit of hydrodynamic studies.

The aim of this study is to better understand the interactions between the aquatic vegetation and hydro-thermodynamic processes in shallow lakes. This increased knowledge will also provide support to water management authorities.

We started our investigation from a bathymetric survey of the Superior Lake of Mantua (area 4 km2, mean depth 3.3 m) using a “DeeperSonar CHIRP+” instrument. Basing on the acquired bathymetry, we set up a three-dimensional hydrodynamic model (Delft3D FLOW), where the presence of aquatic macrophytes was simulated through the “(Rigid) 3D vegetation model” implemented in Delft3D. The model was calibrated with in-situ temperature profiles acquired with a multiparameter probe and water velocity data measured with an ADCP along lake transects. We simulated different scenarios to assess the impact of variables such as discharge, wind and vegetation on lake hydrodynamics.

Preliminary results obtained by numerical simulations showed the establishment of an area with speeds of the order of a few millimeters per second inside the main patch of emergent vegetation, which can potentially lead to sediment deposition and oxygen depletion. We are planning further investigations to evaluate for example the effects of water diversion from agricultural canals to the lake, or other possible remediation measures for limiting water stagnation.