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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.

This work investigated the combined effects of nutrient availability and temperature on phytoplankton in large and deep lakes south of the Alps (lakes Garda, Iseo, Como, Lugano and Maggiore). The more eutrophic basins (Lugano and Iseo) showed a higher presence of cyanobacteria, green algae (Chlorophyta and Charophyta) and dinoflagellates (Dinophyta). Besides these two water bodies, high biomasses of cyanobacteria were recorded also in the oligo-mesotrophic Lake Garda. The development of these algal groups during the growing season showed a strong dependence on the surface spring availability of SRP, which, in turn, was related to winter climatic oscillations, deep mixing dynamics, and trophic status. A specific analysis carried out by applying additive mixed modelling, generalized least squares and mixed modelling, allowed investigation of the direct, seasonal effects of water temperature variations and trophic status on different algal groups. The dominant cyanobacteria (Oscillatoriales) showed only a partial relationship with temperature, while Nostocales and Chroococcales, which did not appear to have a close relationship with the trophic status of the lakes, were characterised by abrupt increases during the warmer months. High positive relationships with temperature were found for a few other algal groups (e.g., Chlorophyta, Charophyta and Dinophyta). Overall, the results indicated a positive relationship between the seasonal development of the more abundant and eutrophic-sensitive algal groups and the concurrent effect of trophic status and water temperature. Nevertheless, it was stressed that specific differences could be interpreted taking into account the different autoecological characteristics and susceptibilities of different species and functional groups to other stressing factors favouring losses, including, e.g., vertical sinking and grazing.

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 (

This paper reports the results of investigations carried out on the chemical characteristics and phytoplankton community of Lake Iseo. Samplings were performed on a monthly basis from 1998 to 2000. At least three main algal groups dominated the community throughout the study period. The large Bacillariophyceae were dominant mainly during late winter and early spring (Aulacoseira spp., Melosira varians, Asterionella formosa), with few species able to maintain occasional positive growth also during mid summer and/or autumn (Fragilaria crotonensis and Diatoma elongatum). The thermal stability of the water column and silica depletion were the main factors responsible for the decline of the large spring diatoms. The subsequent growth of Mougeotia sp. (Conjugatophyceae) was favoured by its lower sinking rate and resistance to increasing grazing pressure by the dominant copepods (Copidodiaptomus steueri) and cladocerans (Daphnia hyalina × galeata). Among the cyanobacteria, the greater development of Planktothrix rubescens in the autumn months, with conditions of vertical homogenisation and decreasing Zeu/Zmix ratios, was favoured by its ability to survive at low light irradiances. The temporal replacement of these three groups constitutes the main sequence of the annual phytoplankton succession in Lake Iseo. A large development of other algal groups was recorded only in one or two of the three study years (e.g. Dinophyceae and Chlorococcales). The changes observed in the annual phytoplankton development are discussed in the light of differences in the spring fertilisation of the waters, caused by differences in the depth of the layer involved in the late winter and spring vertical mixing.

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.