Monitoring transport processes in slow flow inland waters. The case study of the Mantua wetland-lake system

Physical processes in lentic ecosystems are driven by different forcings than those in lotic environments, where hydrodynamic actions are dominant. As the flow velocity gradually decreases, other processes begin to play a significant role, such as temperature gradients, wind forcing, and the presence of vegetation. This is precisely the case in the Mincio fluvial system near the city of Mantua (Lombardy, Northern Italy), where the morphology changes from a single-thread river to a fluvial lake complex, passing through a multiple-channel transitional wetland. Most of the catchment area of this unique lake system is located within the Po Valley, specifically downstream of a heavily human-altered environment with intensive agricultural use. As a result, it suffers from many environmental problems, including eutrophication, siltation, hypoxia, and the spread of invasive macrophytes, further exacerbated by low discharge. In fact, the flow regime in the Mincio River is regulated by a complex system of hydraulic infrastructures, which have been designed to guarantee flood protection, but often in conflict with environmental conservation. The present study aims to investigate different processes (morphological evolution, dissolved oxygen dynamics, temperature-induced circulations) that characterise this lentic ecosystem, on the basis of an extensive dataset of field measurements. All of the studied aspects are related to transport processes that, given the low-energy environment, are highly sensitive to small variations in the drivers and to external forcings. For this reason, a complete understanding of these dynamics is crucial for better management of the system. First, an analysis was conducted to identify the most effective methodology for achieving good-quality velocity measurements in slow currents (< 10 cm/s). The performance of different Acoustic Doppler Current Profilers (ADCP) was tested and compared, helping to fill the literature gap on ADCP application in lakes. Technical solutions were also suggested to resolve instrument-related problems. Another series of instrumentation tests was conducted as part of the setup of a low-cost sonar device designed to investigate the morphological evolution of the lake system. With this instrument, a bathymetric survey of the lake and wetland was acquired and compared with available historical maps dating back to 2006. The evidence of a deposition trend was clear, with depth differences of the order of 1m and a possible risk of terrestrialization. Twenty monthly field campaigns, including discharge, total suspended solids concentration and dissolved nutrients measurements, were conducted at five sections of the Mincio River and in its two main tributaries (Goldone and Osone channels). These two channels, which mainly drain agricultural areas, were identified as responsible for high nutrient and suspended solids loads after rainfall events. In addition, dissolved oxygen sensors were deployed at different depths, inside and outside vegetation patches, both in the lake and in the wetland. As a result of unfavourable ecological and hydrodynamic conditions, severe bottom anoxia problems were observed during the summer, especially under emerging macrophyte cover. Following the testing phase, an ADCP was used to monitor water circulation in Mantua’s Lake Superiore. The occurrence of density-driven currents, which had been hypothesised to be due to the presence of vegetation and a sloping bottom, was confirmed by field measurements. The velocity measurements were coupled with a 2-year continuous temperature monitoring with logger chains at different locations in the lake. This allowed the quantification of the effect of dense vegetation patches on water temperature. The present work offers a twofold contribution: it provides evidence-based management recommendations for local authorities and also makes available to the scientific community an extensive field study on key topics of freshwater lentic ecosystems, such as sedimentation, anoxia, water mixing, and the role of aquatic vegetation, which can be extended to other similar low-energy water environments.