• Energy Research

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.

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.

AbstractGroundwater is a vital ecosystem of the global water cycle, hosting unique biodiversity and providing essential services to societies. Despite being the largest unfrozen freshwater resource, in a period of depletion by extraction and pollution, groundwater environments have been repeatedly overlooked in global biodiversity conservation agendas. Disregarding the importance of groundwater as an ecosystem ignores its critical role in preserving surface biomes. To foster timely global conservation of groundwater, we propose elevating the concept of keystone species into the realm of ecosystems, claiming groundwater as a keystone ecosystem that influences the integrity of many dependent ecosystems. Our global analysis shows that over half of land surface areas (52.6%) has a medium‐to‐high interaction with groundwater, reaching up to 74.9% when deserts and high mountains are excluded. We postulate that the intrinsic transboundary features of groundwater are critical for shifting perspectives towards more holistic approaches in aquatic ecology and beyond. Furthermore, we propose eight key themes to develop a science‐policy integrated groundwater conservation agenda. Given ecosystems above and below the ground intersect at many levels, considering groundwater as an essential component of planetary health is pivotal to reduce biodiversity loss and buffer against climate change.

AbstractGroundwater is a vital ecosystem of the global water cycle, hosting unique biodiversity and providing essential services to societies. Despite being the largest unfrozen freshwater resource, in a period of depletion by extraction and pollution, groundwater environments have been repeatedly overlooked in global biodiversity conservation agendas. Disregarding the importance of groundwater as an ecosystem ignores its critical role in preserving surface biomes. To foster timely global conservation of groundwater, we propose elevating the concept of keystone species into the realm of ecosystems, claiming groundwater as a keystone ecosystem that influences the integrity of many dependent ecosystems. Our global analysis shows that over half of land surface areas (52.6%) has a medium‐to‐high interaction with groundwater, reaching up to 74.9% when deserts and high mountains are excluded. We postulate that the intrinsic transboundary features of groundwater are critical for shifting perspectives towards more holistic approaches in aquatic ecology and beyond. Furthermore, we propose eight key themes to develop a science‐policy integrated groundwater conservation agenda. Given ecosystems above and below the ground intersect at many levels, considering groundwater as an essential component of planetary health is pivotal to reduce biodiversity loss and buffer against climate change.

Lakes are significant sources of greenhouse gases to the atmosphere globally (DelSontro et al. 2018). The magnitude of these fluxes greatly depend on a lake’s trophic status that is sensitive to environmental change. To address considerable gaps in our understanding regarding how forecasted shifts in the environment may affect greenhouse gas dynamics of lake ecosystems, we investigated the underlying biogeochemical mechanisms that control the origins and fate of greenhouse gases methane (CH4) and carbon-dioxide (CO2) in the oligotrophic Lake Maggiore in Italy. Lake Maggiore is a deep oligomictic lake belonging to the LTER Italian and European networks (DEIMS ID: https://deims.org/f30007c4-8a6e-4f11-ab87-569db54638fe). It is located in Northern Italy’s deep subalpine Lake District that includes Lugano, Como, Garda and Iseo lakes. Studies on physical, chemical and biological features of Lake Maggiore have been conducted continuously since the 1980s. These efforts documented how the lake recovered from eutrophication due to remediation measures, and reached the current oligotrophic status. Long-term data also demonstrate how, in the oligotrophication phase, climate change became a significant factor impacting hydrodynamics, oxygen status, and nutrient levels (Rogora et al. 2021). Sampling for this study took place jointly with the regular monitoring schedule during the day in the summer, August 2023 and August 2024, at two locations in Lake Maggiore: Ghiffa, corresponding to the deepest point of the lake (370 m depth), and Pallanza, a semi-pelagic station (100 m depth). In addition to the regularly monitored parameters—such as chlorophyll, dissolved oxygen, and phosphorus, and nitrogen compounds—we measured the concentrations and stable carbon isotope values of dissolved CH4 and CO2, as well as major carbon pools, including dissolved and particulate organic carbon (DOC and POC), across depth profiles. Dissolved CO2 concentrations varied greatly from 7 µM in the surface layers to as high as 205 µM in the deeper waters, which is consistent with carbon-fixation by photosynthesizing algae or other microorganisms near the surface and/or the accumulation of CO2 from heterotrophy at depth. Methane concentrations along the depth profile in Ghiffa ranging from 7 nM to 357 nM (Fig. 1) were within the reported range for other oligotrophic lakes and, thus, were considerably lower than source methane concentrations in typical eutrophic lake ecosystems. The slightly elevated CH4 concentration near the bottom water at 360 m was indistinguishable based on its δ13C from the samples collected between 50 m to 300 m depths, demonstrating that methane sourced from the sediments is efficiently oxidized in the aerobic water column. Surprisingly, however, higher CH4 levels were detected in the surface waters (0-50 m) in both years than in the deeper layers (Fig. 1). The highest CH4 concentration (2026 nM) with a δ13C value as negative as −61.0 ± 0.2‰, which is consistent with a biogenic methane source, was recorded at 7 m depth in Pallanza, in correspondence with the chlorophyll maximum (8.5 µg/l). Collectively, these observations suggest the presence of lateral source(s) in the surface layers above 50 m from (a) methane imported laterally from the shoreline and inlets (Khatun et al. 2024), or (b) production by photosynthesizing microorganisms in oxic conditions (Bižić-Ionescu et al. 2018), or a combination of both. To facilitate a mechanistic understanding of critical cellular and environmental thresholds that drive the production of CH4 in the oxic surface layer, we conducted microcosm experiments using water samples from both locations (Ghiffa and Pallanza) in conjunction with the regular sampling campaign in August 2024. Methane production significantly reduced within one hour of inorganic phosphorus supplementation in samples collected from the surface chlorophyll maximum layer (8-10 m) at both locations. In contrast, no methane production was observed in deeper samples from below the chlorophyll maximum layers at 50 m. Consistent with the findings of Bižić-Ionescu et al. (2018), our observations indicate that CH4 levels at the oxic surface layer originate—at least in part—from photosynthetic microorganisms and that oxic methanogenesis is driven by enzymatic processes occurring in response to inorganic phosphorus limitation (high nitrogen to phosphorus ratios). This observation, however, does not exclude the possibility of additional contributions from other near surface sources, such as methane imported laterally from the shoreline. Our first-order observations of dissolved gases in Lake Maggiore add to the growing evidence that CH4 is produced at the oxic surface layer in aquatic ecosystems—a surprising finding, given CH4 production has been empirically associated with anoxic conditions. Furthermore, findings from our study demonstrate that oligotrophic lakes provide an opportunity to investigate secondary natural gas sources in aquatic ecosystems that would be difficult or impossible to study in other lake environments. For example, eutrophic lakes are exposed to higher levels of nutrient loads and different nitrogen to phosphorus ratios, which could inhibit oxic methanogenesis observed here. In addition, eutrophic lakes are often associated with overwhelmingly high methane inputs from anoxic sub-habitats, making it difficult to detect, distinguish, and quantify additional contributions even if they are present. Understanding the drivers and delineating the sources of dissolved gases, such as CH4 and CO2, in oligotrophic lakes will allow for their long-term monitoring, better forecasting if trophic changes may occur, and eventually estimating their broader contributions to methane and carbon budgets.

Lakes are significant sources of greenhouse gases to the atmosphere globally (DelSontro et al. 2018). The magnitude of these fluxes greatly depend on a lake’s trophic status that is sensitive to environmental change. To address considerable gaps in our understanding regarding how forecasted shifts in the environment may affect greenhouse gas dynamics of lake ecosystems, we investigated the underlying biogeochemical mechanisms that control the origins and fate of greenhouse gases methane (CH4) and carbon-dioxide (CO2) in the oligotrophic Lake Maggiore in Italy. Lake Maggiore is a deep oligomictic lake belonging to the LTER Italian and European networks (DEIMS ID: https://deims.org/f30007c4-8a6e-4f11-ab87-569db54638fe). It is located in Northern Italy’s deep subalpine Lake District that includes Lugano, Como, Garda and Iseo lakes. Studies on physical, chemical and biological features of Lake Maggiore have been conducted continuously since the 1980s. These efforts documented how the lake recovered from eutrophication due to remediation measures, and reached the current oligotrophic status. Long-term data also demonstrate how, in the oligotrophication phase, climate change became a significant factor impacting hydrodynamics, oxygen status, and nutrient levels (Rogora et al. 2021). Sampling for this study took place jointly with the regular monitoring schedule during the day in the summer, August 2023 and August 2024, at two locations in Lake Maggiore: Ghiffa, corresponding to the deepest point of the lake (370 m depth), and Pallanza, a semi-pelagic station (100 m depth). In addition to the regularly monitored parameters—such as chlorophyll, dissolved oxygen, and phosphorus, and nitrogen compounds—we measured the concentrations and stable carbon isotope values of dissolved CH4 and CO2, as well as major carbon pools, including dissolved and particulate organic carbon (DOC and POC), across depth profiles. Dissolved CO2 concentrations varied greatly from 7 µM in the surface layers to as high as 205 µM in the deeper waters, which is consistent with carbon-fixation by photosynthesizing algae or other microorganisms near the surface and/or the accumulation of CO2 from heterotrophy at depth. Methane concentrations along the depth profile in Ghiffa ranging from 7 nM to 357 nM (Fig. 1) were within the reported range for other oligotrophic lakes and, thus, were considerably lower than source methane concentrations in typical eutrophic lake ecosystems. The slightly elevated CH4 concentration near the bottom water at 360 m was indistinguishable based on its δ13C from the samples collected between 50 m to 300 m depths, demonstrating that methane sourced from the sediments is efficiently oxidized in the aerobic water column. Surprisingly, however, higher CH4 levels were detected in the surface waters (0-50 m) in both years than in the deeper layers (Fig. 1). The highest CH4 concentration (2026 nM) with a δ13C value as negative as −61.0 ± 0.2‰, which is consistent with a biogenic methane source, was recorded at 7 m depth in Pallanza, in correspondence with the chlorophyll maximum (8.5 µg/l). Collectively, these observations suggest the presence of lateral source(s) in the surface layers above 50 m from (a) methane imported laterally from the shoreline and inlets (Khatun et al. 2024), or (b) production by photosynthesizing microorganisms in oxic conditions (Bižić-Ionescu et al. 2018), or a combination of both. To facilitate a mechanistic understanding of critical cellular and environmental thresholds that drive the production of CH4 in the oxic surface layer, we conducted microcosm experiments using water samples from both locations (Ghiffa and Pallanza) in conjunction with the regular sampling campaign in August 2024. Methane production significantly reduced within one hour of inorganic phosphorus supplementation in samples collected from the surface chlorophyll maximum layer (8-10 m) at both locations. In contrast, no methane production was observed in deeper samples from below the chlorophyll maximum layers at 50 m. Consistent with the findings of Bižić-Ionescu et al. (2018), our observations indicate that CH4 levels at the oxic surface layer originate—at least in part—from photosynthetic microorganisms and that oxic methanogenesis is driven by enzymatic processes occurring in response to inorganic phosphorus limitation (high nitrogen to phosphorus ratios). This observation, however, does not exclude the possibility of additional contributions from other near surface sources, such as methane imported laterally from the shoreline. Our first-order observations of dissolved gases in Lake Maggiore add to the growing evidence that CH4 is produced at the oxic surface layer in aquatic ecosystems—a surprising finding, given CH4 production has been empirically associated with anoxic conditions. Furthermore, findings from our study demonstrate that oligotrophic lakes provide an opportunity to investigate secondary natural gas sources in aquatic ecosystems that would be difficult or impossible to study in other lake environments. For example, eutrophic lakes are exposed to higher levels of nutrient loads and different nitrogen to phosphorus ratios, which could inhibit oxic methanogenesis observed here. In addition, eutrophic lakes are often associated with overwhelmingly high methane inputs from anoxic sub-habitats, making it difficult to detect, distinguish, and quantify additional contributions even if they are present. Understanding the drivers and delineating the sources of dissolved gases, such as CH4 and CO2, in oligotrophic lakes will allow for their long-term monitoring, better forecasting if trophic changes may occur, and eventually estimating their broader contributions to methane and carbon budgets.