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We review the state of the art of limnological studies in Lake Iseo and provide updated data concerning long-term investigations (from 1993 to 2018) carried out on chemical and physical parameters (e.g., oxygen, phosphorus, silicon). Changes observed in Lake Iseo were compared with those reported in other Deep South alpine Lakes (DSLs) to highlight analogies and differences of long-term chemical, physical, and biological patterns. Until the 1960s, Lake Iseo and other DSLs were oligotrophic. The increase of anthropogenic pressure and global warming has led to a progressive and unrecovered process of eutrophication. Moreover, the decrease in frequency of full mixing episodes has induced a state of temporary meromixis. Other changes have been identified over the last two decades, especially concerning the phytoplankton and zooplankton communities, and new emerging chemical pollutants were detected. Given the important ecological and socioeconomic role of Lake Iseo, long-term investigations are of paramount importance to understand the response of the lake ecosystem to climatic and anthropogenic stressors. These two factors can also act coupled with new combined and synergic effects.

A combination of field observations and 3-D hydrodynamic simulations were used to identify the phytoplankton species and to estimate the various time scales of the dominant physical and biological processes in Lake Iseo, a deep subalpine lake located in northern Italy, during a stratified period (July 2010). By ordering the rate processes time scales, we derive a phytoplankton patch categorization and growth interpretation that provides a general framework for the spatial distribution of phytoplankton concentration in Lake Iseo and illuminates the characteristics of their ecological niches. The results show that the diurnal surface layer was well mixed, received strong diurnal radiation, had low phosphorus concentrations and the phytoplankton biomass was sustained by the green alga Sphaerocystis schroeterii. The vertical mixing time scales were much shorter than horizontal mixing time scales causing a depth-uniform chlorophyll a concentration. The horizontal patch scale was determined by horizontal dispersion balancing the phytoplankton growth time scale, dictating the success of the observed green algae. The strongly stratified nutrient-rich metalimnion had mild light conditions and Diatoma elongatum and Planktothrix rubescens made up the largest proportions of the total phytoplankton biomass at the intermediate and deeper metalimnetic layers. The vertical transport time scales were much shorter than horizontal transport and vertical dispersion leading to growth niche for the observed phytoplankton. The study showed that time-scale hierarchy mandates the essential phytoplankton attributes or traits for success in a particular section of the water column and/or water body.

Lake Monger (Perth, Western Australia) is a highly eutrophic lake, characterised by very low species richness of macrophytes with the dominance of Potamogeton crispus. Mesocosm experiments were performed using water and plants collected from the lake to determine the effects of vegetation decay on the phosphorus (P) concentrations in the overlying waters. After 2 weeks of experimental incubation of mesocosms with and without re-oxygenation, P concentrations in the water column were significantly higher, showing a quite similar effect of P. crispus on the phosphorus release in different mesocosms. The results of our study provide clear evidence that the P concentrations in overlying waters mainly depend upon the plant P content and developmental stage. Although many sources contribute to the nutrient load of Lake Monger, macrophyte harvesting, prior to its senescence, might constitute a significant in-lake measure for reducing the internal P load.

This paper reports the results of long-term research carried out in the deep southern subalpine lakes (DSL: Maggiore, Lugano, Como, Iseo and Garda). The measurements performed since the 1970s at the time of the maximum overturn documented an increase of water temperature around 0.0 12-0.028 degrees C yr(-1), closely resembling the warming rate found in other lakes in Europe and North America.. The DSL showed an increase of total phosphorus after the 1960s and a complete recovery after the 1990s of Lake Maggiore, which reached mean values of TP close to those of the pristine conditions (

The main focus of this study was to investigate the effects of lime treatment with Ca(OH)(2) on a hypereutrophic hardwater lake (Lake Alserio, Northern Italy), focusing on its impact on plankton communities. We performed a mesocosm experiment using two large enclosures, one for treatment and one control, in the lake. The addition of Ca(OH)(2) proved to be effective in reducing phosphorus concentration, but the effect was limited in time. Redissolution of the precipitating calcium carbonate as it came into contact with the deep water determined a phosphorus increase about 2 weeks after the liming. The liming had a strong impact on the plankton assemblage, as was demonstrated by the sharp decrease in phytoplankton and zooplankton biomass and density, and 1 month after the treatment we observed a different taxonomic composition inside the enclosure from that of the lake. The result of the experiment showed that liming with Ca(OH)(2) can be regarded as a suitable treatment for accelerating recovery of the lake. Multiple treatments will be necessary to improve the trophic level over a longer period.

A growing concern exists about the effects of chloride (Cl) on freshwater systems. Increasing Cl concentrations have been observed in the last few decades in several rivers and lakes, mainly in northern countries. In Italy, present levels and temporal changes of sodium (Na) and Cl in water bodies have rarely been assessed. Based on long-term data for the lakes of the subalpine district in Italy (Maggiore, Lugano, Como, Iseo, Garda), we analyzed trends affecting Cl and Na concentrations during the last 25years, with the aim of identifying temporal changes and assessing possible causes. An in-depth analysis is presented for Lake Maggiore. Positive temporal Na and Cl trends were evident in all studied lakes, with the trends increasing since early 2000s. Data for Lake Maggiore tributaries showed a clear seasonality (higher values in winter and early spring). The NaCl used as road de-icing agent, together with Cl discharge from wastewater treatment plants, were identified as the main causes for the observed trends. Chloride concentrations in the lakes are below the threshold limit for reduced water quality and below concentrations known to harm aquatic biota. However, considering the relevance of deep subalpine lakes, representing almost 80% of the total freshwater volume in Italy, these trends indicate an important chemical change, which warrants further analysis.

This paper offers a synoptic account of studies on the phytoplankton communities in the deep southern subalpine lakes (DSL) Garda, Iseo, Como, Lugano and Maggiore. The main cause of the degradation of the water quality in the DSL is eutrophication. The euphotic layers of these lakes are trophically different, ranging from the oligo-mesotrophy of lakes Maggiore and Garda to the meso-eutrophy of lakes Iseo and Lugano. The trophic status as estimated by using total phosphorus and chlorophyll a has provided consistent results in agreement with the models proposed by OECD (1982. Eutrophication of Waters. Monitoring, Assessment and Control, OECD, Paris). Though related with chlorophyll a and TP, the Secchi disk depths have significantly underestimated the trophic status of the DSL. Two trophic indices using the algal orders (PTIorders) and species (PTIspecies) were drawn up on the basis of the distribution of phytoplankton along a trophic gradient defined by the application of multivariate methods; the scores emerging from these indices were used to make a definitive ecological classification of water bodies on a scale from 1 to 5, in accordance with the Water Framework Directive. A third index (PTIOE) was computed as the ratio between the annual mean values of the cumulative biovolumes of two groups of algal orders with opposite trophic characteristics. The three PTI indices were highly correlated, providing a consistent classification of the water bodies. The indices proposed in this work were specifically adopted for use in the DSL. However, the criteria for their implementation constitute a robust and impartial tool for assessing similar indices in other lake typologies and for evaluating the degree of specificity of the trophic indicator values assigned to the single phytoplankton orders and species.

The spatial distribution of phytoplankton in the surface waters of typical deep sub-alpine lake was investigated by analyzing water samples collected at three depth ranges (0-1 m, 9-13 m, 15-19 m) at seven sites. A total of 21 samples were analyzed by a Generalized Linear Model. The biovolume of different phytoplanktonic taxa significantly differed between depth ranges. Indeed, different algal taxa showed maximum population density at different depths, as a result of different optima of light, physical conditions and nutrients. The layer of 9-13 m depth showed the greatest abundance of phytoplankton, mainly represented by Bacillariophyceae and Cyanobacteria. Conversely, Chlorophyceae, which require few nutrients and are not inhibited by the high light energy, were most abundant at the surface. The vertical heterogeneity in phytoplankton distribution closely paralleled that of nutrients, which in turn depends on the stable thermal stratification of the lake water column. Biovolume of phytoplankton taxa and chemical parameters did not differ significantly between sampling sites that were some kilometers far away from each other. This is due to large horizontal movements of water that occur in the lake and that determine rather uniform conditions throughout the basin.

The lake Iseo has been recently contaminated by DDT residues, originated from the melting of a glacier that released the pollutants accumulated in the past. Because of this recent input, DDT residues concentrations rose more quickly in zooplankton than in fish during 2009. In autumn 2010 the ratio drastically dropped to one-two for all the compounds indicating that the glacial DDT load should have been ceased. The situation was different for PCBs that were released to a much lower extent from glaciers. The PCB 138 ratio between zooplankton and fish was always around one-two in both years. As the zooplankton response to pollution changes resulted particularly prompt, our research highlights the importance of this component as an early warning bioindicator of hydrophohydrophobic pollutants.

Winter air and spring surface water temperatures, spring epilimnetic phosphorus, and hypolimnetic oxygen in the deep lakes south of the Alps (Garda, Iseo, Como, Lugano, and Maggiore) showed a high degree of temporal coherence. The common temporal patterns were originating from the effects caused by winter climate, and from corresponding synchronisms in the interannual variations in the extent of the spring water renewal and replenishment of nutrients. In turn, the sequence of linked causal events was triggered by two atmospheric modes of variability relevant for the Mediterranean region, i.e. the East Atlantic pattern (EA) and the Eastern Mediterranean Pattern (EMP). In contrast, there were no significant relationships of air and water temperatures with the North Atlantic Oscillation. In oligotrophic lakes, which were characterised by weak vertical nutrient gradients, the spring replenishment of P was negligible and difficult to detect (Maggiore), or detectable but of minor importance (Garda and Como) compared to that measured in more enriched lakes (Iseo and Lugano). The applicability of EA and EMP in the study of the impact of climate on aquatic ecosystems will require to be tested by expanding the number, typology, and geographical location of water bodies in the Alpine and Mediterranean regions.

The identification of the factors and mechanisms determining a particular lake’s typology is crucial for the correct interpretation of trophic evolution. Nutrient concentrations are not the only properties which determine the trophic characteristics of lakes; others, such as morphometry, hydrology and climatic conditions, also have a major influence on the development of algal biomass and species composition. Large, deep lakes belong to a well defined typology. Their trophic status depends not only on algal nutrient loads, but also on the extent of the spring vertical mixing and renewal time. These factors are closely interrelated, for the actual renewal time approaches the theoretical renewal time only during the complete overturn. This paper compares the influence of different circulation patterns on the trophic status of two deep subalpine lakes with contrasting mixing characteristics. Lake Garda is the largest lake in Italy (S=368 km2, V=49 km3, Zmax=350 m). It is characterised by irregular circulation events (oligomixis); during the 1990s, complete homogenisation of the water column occurred in 1991 and 1999-2000. The years between these episodes showed an increase of hypolimnetic temperatures and a progressive vertical stratification of nutrients. Full overturn episodes were characterised by a sudden decrease of temperature and complete homogenisation of the chemical and physical variables along the water column, with a corresponding increase of nutrients and algal biomasses at the surface. In contrast, the last mixing involving the deepest waters in Lake Iseo (S= 60,9 km2, V=7,6 km3, Zmax=251 m) occurred at the beginning of the 1980s, after which the reduced water renewal and the high trophic level resulted in a marked decrease in oxygen concentrations and in the establishment of conditions of anoxia during the 1990s. At present the lake is meromictic. In 1999 and 2000, when a complete overturn was observed in lakes Maggiore and Como as well as in Lake Garda, Lake Iseo experienced a spring mixing which was still limited, but able to determine an increase in the algal biomass.

Lake Iseo is undergoing a progressive deterioration in water quality, mainly due to eutrophication. This paper, based on measurements performed on the main tributaries and the outflow, considers the phosphorus, nitrogen and silica budget of the lake, related to its trophic evolution. The chief conclusion of the study is that the significant reduction in P load over the last few years following the construction of treatment plants is not enough to reverse the trend to eutrophication and the progressive deterioration in water quality. An attempt to use the OECD statistical approach to quantify the relationships between P load and the trophic condition of the lake yielded poor results, because the characteristics of the lake did not fulfil the hypothesis of the model. The reduced tendency to a complete overturn of the water is one of the most important aspects, leading to hypolimnion anoxia and P release from the sediments.

Lake Alserio is located in the subalpine area of Northern Italy, in the Brianza region near Lake Como. In November 1999 an EU funded project was started with the aim of devising and implementing interventions leading to the recovery of the lake and the surrounding area, which is characterised by a high degree of biodiversity as regards species and natural habitats. Since the seventies, Lake Alserio has been affected by eutrophication, which has resulted in high phosphorus (P) concentrations (annual average in 1999: 80-100 μg P l-1). The EU project envisaged liming the lake with Ca(OH)2 to reduce P concentrations in the water. This paper presents and discusses the results of chemical analyses performed on the lake and its tributaries in the first year of the study (November 1999-November 2000). The efficiency of liming with Ca(OH)2 to reduce P concentrations in laboratory and enclosure experiments is also discussed. The present condition of Lake Alserio is between eutrophic and hypereutrophic. Anoxic conditions characterise the hypolimnic waters for a prolonged period (May - October) during summer stratification, with a consequent high P release from the sediments (summer P concentrations in the deep water are close to 1 mg P l-1). Inflowing waters are characterised by high contents of ammonium and phosphorus, showing the importance of domestic sewage in the lake catchment; external load is still too high to permit the water to reach mesotrophic condition, the ultimate goal of recovery. Results of the liming experiments suggest that the treatment, if applied to Lake Alserio, could accelerate its recovery, although the specific procedures to be employed are still to be defined. However, before these or any other remedial operations can be undertaken, the external P load must be reduced by completing the diversion of sewage from the lake.