• Energy Research

Hypolimnetic withdrawal has been applied as a restoration measure in lakes subject to eutrophication together with external load reduction, to decrease internal load by removing limiting nutrient phosphorus (P) from anoxic deep waters and contributing to the unloading of bottom sediments from previously deposited nutrients and organic matter. The aim of this study is to evaluate the effect of hypolimnetic withdrawal on Lake Varese, a 24 m-deep and 14.8 km2-large subalpine lake in North-Western Italy. The lake suffered from extended eutrophication in the second half of the 20th century due to uncontrolled delivery of untreated urban sewage. Several restoration measures have been implemented during the years, including hypolimnetic withdrawal. In 2019, a cooperative programme for the protection and management of the lake and its surroundings was launched, establishing a systematic annual hypolimnetic withdrawal in the stratified season since 2020. In this research, we calibrated a one-dimensional (1D) coupled ecological-hydrodynamic model (General Lake Model/Aquatic EcoDynamics - GLM/AED2) of Lake Varese with data surveyed in the lake in 2019-2021. Model simulations of the period 2020-2021 with and without the performed withdrawal proved the effectiveness of this measure on hypolimnetic P concentration reduction. Then, future simulations of 2023-2085 were carried out to predict the future efficiency of hypolimnetic withdrawal and of reductions in external nutrient loads under climate change scenarios. Results show that the prescribed withdrawal increases hypolimnetic temperatures. This effect, coupled with thermocline deepening due to global warming, will possibly lead to decreasing water mass stability in autumn and shorter stratification in the moderately deep Lake Varese, with an eventual decrease of P concentrations in the water column. The future effectiveness of hypolimnetic withdrawal is further discussed considering the possible role of dry periods.

Hypolimnetic withdrawal has been applied as a restoration measure in lakes subject to eutrophication together with external load reduction, to decrease internal load by removing limiting nutrient phosphorus (P) from anoxic deep waters and contributing to the unloading of bottom sediments from previously deposited nutrients and organic matter. The aim of this study is to evaluate the effect of hypolimnetic withdrawal on Lake Varese, a 24 m-deep and 14.8 km2-large subalpine lake in North-Western Italy. The lake suffered from extended eutrophication in the second half of the 20th century due to uncontrolled delivery of untreated urban sewage. Several restoration measures have been implemented during the years, including hypolimnetic withdrawal. In 2019, a cooperative programme for the protection and management of the lake and its surroundings was launched, establishing a systematic annual hypolimnetic withdrawal in the stratified season since 2020. In this research, we calibrated a one-dimensional (1D) coupled ecological-hydrodynamic model (General Lake Model/Aquatic EcoDynamics - GLM/AED2) of Lake Varese with data surveyed in the lake in 2019-2021. Model simulations of the period 2020-2021 with and without the performed withdrawal proved the effectiveness of this measure on hypolimnetic P concentration reduction. Then, future simulations of 2023-2085 were carried out to predict the future efficiency of hypolimnetic withdrawal and of reductions in external nutrient loads under climate change scenarios. Results show that the prescribed withdrawal increases hypolimnetic temperatures. This effect, coupled with thermocline deepening due to global warming, will possibly lead to decreasing water mass stability in autumn and shorter stratification in the moderately deep Lake Varese, with an eventual decrease of P concentrations in the water column. The future effectiveness of hypolimnetic withdrawal is further discussed considering the possible role of dry periods.

The MAGIC model was used to evaluate the relative sensitivity of several possible climate-induced effects on the recovery of soil and surface water from acidification. A common protocol was used at 14 intensively studied sites in Europe and eastern North America. The results show that several of the factors are of only minor importance (increase in pCO2 in soil air and runoff, for example), several are important at only a few sites (seasalts at near-coastal sites, for example) and several are important at nearly all sites (increased concentrations of organic acids in soil solution and runoff, for example). In addition changes in forest growth and decomposition of soil organic matter are important at forested sites and sites at risk of nitrogen saturation. The trials suggest that in future modelling of recovery from acidification should take into account possible concurrent climate changes and focus specially on the climate-induced changes in organic acids and nitrogen retention

The MAGIC model was used to evaluate the relative sensitivity of several possible climate-induced effects on the recovery of soil and surface water from acidification. A common protocol was used at 14 intensively studied sites in Europe and eastern North America. The results show that several of the factors are of only minor importance (increase in pCO2 in soil air and runoff, for example), several are important at only a few sites (seasalts at near-coastal sites, for example) and several are important at nearly all sites (increased concentrations of organic acids in soil solution and runoff, for example). In addition changes in forest growth and decomposition of soil organic matter are important at forested sites and sites at risk of nitrogen saturation. The trials suggest that in future modelling of recovery from acidification should take into account possible concurrent climate changes and focus specially on the climate-induced changes in organic acids and nitrogen retention

Investigating relation between meteo-climatic indices and between-year variation in Daphnia population density and phenology is crucial for e.g. predicting impact of climate change on lake ecosystem structure and functioning. We tested whether and how two teleconnection indices calculated for the winter period, namely the East Atlantic pattern (EADJF) and the Eastern Mediterranean Pattern (EMPDJF) were correlated with Daphnia population growth in two Italian subalpine lakes, Garda and Maggiore. We investigated between-lake temporal coherence in: i) water temperature within the water layer in which Daphnia is distributed; ii) timing of Daphnia initial and spring maximum population density peak and iii) the level of Daphnia spring maximum population density peak over an eleven-year period (1998-2008) of unchanged predation pressure by fish and invertebrates, and of common oligotrophy. Between-lake temporal coherence was high for an earlier start, an earlier, and lower, Daphnia population spring density peak after milder winters. Peak density level was coherently, positively correlated with soluble reactive phosphorus (SRP) concentration. We hypothesized that Daphnia peak densities were related to atmospheric modes of variability in winter and to the degree of late winter mixing promoting replenishment of algal nutrients into upper water layers and phytoplankton growth, enhancing food availability and Daphnia fecundity, promoting Daphnia peak. 

Investigating relation between meteo-climatic indices and between-year variation in Daphnia population density and phenology is crucial for e.g. predicting impact of climate change on lake ecosystem structure and functioning. We tested whether and how two teleconnection indices calculated for the winter period, namely the East Atlantic pattern (EADJF) and the Eastern Mediterranean Pattern (EMPDJF) were correlated with Daphnia population growth in two Italian subalpine lakes, Garda and Maggiore. We investigated between-lake temporal coherence in: i) water temperature within the water layer in which Daphnia is distributed; ii) timing of Daphnia initial and spring maximum population density peak and iii) the level of Daphnia spring maximum population density peak over an eleven-year period (1998-2008) of unchanged predation pressure by fish and invertebrates, and of common oligotrophy. Between-lake temporal coherence was high for an earlier start, an earlier, and lower, Daphnia population spring density peak after milder winters. Peak density level was coherently, positively correlated with soluble reactive phosphorus (SRP) concentration. We hypothesized that Daphnia peak densities were related to atmospheric modes of variability in winter and to the degree of late winter mixing promoting replenishment of algal nutrients into upper water layers and phytoplankton growth, enhancing food availability and Daphnia fecundity, promoting Daphnia peak. 

Lake Maggiore is a site of the Italian and European Long-Term Ecological Research (LTER) network. It belongs to deep subalpine Lake District in Northern Italy, including lakes Lugano, Como, Garda and Iseo. Lake Maggiore has been monitored for physical, chemical, and biological features since the 1980s in the framework of the limnological campaigns funded by the International Commission for the Protection of Italian-Swiss Waters (CIPAIS). Starting from the 1990s, the lake recovered from eutrophication thanks to remediation measures and reached the present oligotrophic condition. In the last two decades, climate change turned out to be the main driving factor for the long-term evolution of the lake, affecting thermal and hydrodynamical features, oxygen status, nutrient levels and distribution and biological communities (Rogora et al. 2021). In 2020 a high frequency monitoring (HFM) system consisting of a limnological buoy (LM1) equipped with sensors for meteorological and limnological variables and algal pigments was developed and tested in the framework of an EU Interreg project between Italy and Switzerland focusing on lake quality monitoring as a critical input for successful lake management (Tiberti et al. 2021). The buoy was deployed in the Pallanza basin of Lake Maggiore, anchored at a depth of about 40 m. The system was complemented in 2024 by a second monitoring buoy (LM2) in the Ispra basin of the lake. Present activities of HFM data collection, validation and management are continued under the PNRR-ITINERIS (Italian Integrated Environmental Research Infrastructures System) funded by Next Generation EU. Both LM buoys are equipped with a weather station, a thermistor chain (13 and 11 thermistors for LM1 and LM2, respectively) to measure the water temperature profile and sensors for pH, conductivity, dissolved oxygen, and algal pigments (chlorophyll a (Chl-a), phycocyanin (PC) and phycoerythrin (PE)) at about 1.5 m depth. LM1 buoy has an additional Chl-a sensor at about 8 m depth and a live webcam. All sensors are connected to the electronic control unit, which has been specifically designed within the project for the signal acquisition, data storage, basic data elaboration and a wireless data transfer. For further details on the system see Tiberti et al. (2021). Data gathered by the sensors are subject to quality control, also through a regular comparison with discrete data collected by long-term monitoring. During the first two years, we tested the performance of the fluorometric sensors by comparing HFM data with those obtained by traditional methods for the assessment of algal pigments and phytoplankton biomass (Rogora et al. 2023). The test results and the data collected in the following years confirmed in-situ sensors as reliable systems to describe the short-term variability of algal pigments and the use of these data as a proxy of the seasonal pattern of phytoplankton biovolume. As an example, sensor data provided insights into the length and intensity of short lived events, such as the regularly occurring spring diatom blooms or the rapid algal bloom events that cannot easily be captured by the monthly sampling. A further example of the usefulness of the HFM system in Lake Maggiore was the chance to get data when field monitoring was not allowed for technical or logistic constraints, e.g. unfavourable weather conditions, malfunctioning or unavailability of the boat or other equipment. In 2020, during the pandemic period, the long-term monitoring program was forced to stop for a few months; however, some basic but important limnological data were guaranteed by the HFM system, avoiding significant gaps in the time series. Data collected through the HFM system proved to be fundamental in the assessment of climate change impact on Lake Maggiore, particularly of extreme weather conditions. Surface water temperature measured by the buoys in the last few years reached values as high as 30 °C. Even if a direct comparison of the buoy data with those collected in previous years by different systems (e.g., discrete profiles with multiparameter probe) must be done with caution, the extreme temperatures measured in recent years are presumably the highest values ever recorded in Lake Maggiore surface water. The drought of 2022 in Northwestern Italy provided a tremendous example of a condition affecting water resources and the services they provide. In Lake Maggiore area, a combination of scarce snow accumulation in winter and lack of precipitation in spring resulted in an unusual low water level in spring and summer. HFM data put in evidence an unprecedented increase of conductivity in surface water, due to solute concentration. The seasonal pattern of Chl data from HFM in 2022, compared with the previous years, showed low concentration throughout the summer period (June-Aug; Fig. 1). Data from discrete monitoring indicated a higher than average water transparency in 2022 and confirmed low phytoplankton biomass in late spring and summer, and a limited seasonality overall. We hypothesized that scarce precipitation caused a reduced nutrient influx from the watershed, which was indeed confirmed by the monitoring of catchment loads. This condition, coupled with the lack of nutrient replenishment from the deep water during winter because of the increasing stability of the water column, fostered an enhanced oligotrophic condition in summer. These examples demonstrate how HFM, used in conjunction with discrete monitoring, represents an important support to long-term studies on aquatic ecosystems, providing useful insights into ecological processes in response to global change.