• Energy Research

The long-term terrestrial and aquatic ecosystem dynamics spanning between approximately 6200 and 4800 cal BP were investigated using pollen, diatoms, pigments, charcoal, and geochemistry from varved sediments collected in a large stratified perialpine lake, Lago Grande di Avigliana, in the Italian Alps. Marked changes were detected in diatom and pigment assemblages and in sediment composition at similar to 4900 cal BP. Organic matter rapidly increased and diatom assemblages shifted from oligotrophic to oligo-mesotrophic planktonic assemblages suggesting that nutrients increased at that time. Because land cover, erosion, and fire frequency did not change significantly, external nutrient sources were possibly not essential in controlling the lake-ecosystem dynamics. This is also supported by redundancy analysis, which showed that variables explaining significant amounts of variance in the diatom data were not the ones related to changes in the catchment. Instead, the broad coincidence between the phytoplankton dynamics and rising lake-levels, cooler temperatures, and stronger spring winds in the northern Mediterranean borderlands possibly points to the effects of climate change on the nutrient recycling in the lake by means of the control that climate can exert on mixing depth. We hypothesize that the increased P-release rates and higher organic-matter accumulation rates, proceeded by enhanced precipitation of iron sulphides, were possibly caused by deeper and stronger mixing leading to enhanced input of nutrients from the anoxic hypolimnion into the epilimnion. Although we cannot completely rule out the influence of minor land-cover changes due to human activities, it may be hypothesized that climate-induced cumulative effects related to mixing regime and P-recycling from sediments influenced the aquatic-ecosystem dynamics.

The long-term terrestrial and aquatic ecosystem dynamics spanning between approximately 6200 and 4800 cal BP were investigated using pollen, diatoms, pigments, charcoal, and geochemistry from varved sediments collected in a large stratified perialpine lake, Lago Grande di Avigliana, in the Italian Alps. Marked changes were detected in diatom and pigment assemblages and in sediment composition at similar to 4900 cal BP. Organic matter rapidly increased and diatom assemblages shifted from oligotrophic to oligo-mesotrophic planktonic assemblages suggesting that nutrients increased at that time. Because land cover, erosion, and fire frequency did not change significantly, external nutrient sources were possibly not essential in controlling the lake-ecosystem dynamics. This is also supported by redundancy analysis, which showed that variables explaining significant amounts of variance in the diatom data were not the ones related to changes in the catchment. Instead, the broad coincidence between the phytoplankton dynamics and rising lake-levels, cooler temperatures, and stronger spring winds in the northern Mediterranean borderlands possibly points to the effects of climate change on the nutrient recycling in the lake by means of the control that climate can exert on mixing depth. We hypothesize that the increased P-release rates and higher organic-matter accumulation rates, proceeded by enhanced precipitation of iron sulphides, were possibly caused by deeper and stronger mixing leading to enhanced input of nutrients from the anoxic hypolimnion into the epilimnion. Although we cannot completely rule out the influence of minor land-cover changes due to human activities, it may be hypothesized that climate-induced cumulative effects related to mixing regime and P-recycling from sediments influenced the aquatic-ecosystem dynamics.

Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition ( chironomids, pigments) or changes were small and independent of temperature ( cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly. Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenkollesee ( Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a. s. l.) and the geographical position (47degrees13'46"N, 11degrees00'51"E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20 30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.

Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition ( chironomids, pigments) or changes were small and independent of temperature ( cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly. Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenkollesee ( Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a. s. l.) and the geographical position (47degrees13'46"N, 11degrees00'51"E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20 30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.

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.