Factors affecting the seasonality and distribution of cyanobacteria and chlorophytes: a case study from the large lakes south of the Alps, with special reference to Lake Garda

Chlorophytes and cyanobacteria are among the most typical algal groups, con tributing to the aesthetic appearance and quality of the epilimnetic waters of the deep (251-410 m) and large (6.5-49x10(9) m(3)) lakes located on the southern edge of the Alps (from oligo-mesotrophy to meso-eutrophy: Maggior e, Garda, Como, Iseo and Lugano). The results obtained from monthly surveys carried out in the largest of these lakes (Garda) have been reported in de tail. The thermal stability of the water column and silica depletion were t he main factors responsible for the decline of the great spring diatoms. Th e successive growth of Mougeotia sp. was favoured by its lower sinking velo city and resistance to the increasing grazing pressure. During summer, the maximum stability of the water column, with high vertical nutrient concentr ation gradients, determined a major algal differentiation with a typical in crease, among others, of Chlorococcales at the surface and metalimnetic str atification of various Oscillatoriales. The development of oligotrophic blo oms, caused by a rapid thickening at the surface of Anabaena in the eastern , sheltered basin, was also discussed in light of the trophic characterisat ion of Lake Garda. From autumn to spring, the decreasing light, the increas ing mixing depth and nutrient availability favoured a progressive dominance of vertical homogeneous populations of Planktothrix (autumn) and colonial diatoms. The same functional groups of chlorophytes and cyanobacteria have been recognised in the studies published so far on the phytoplankton of dee p southern subalpine lakes. In this respect, their morphometric and physica l properties appear to constitute a sort of standardising factor, reducing the range of possible dominants. With increasing TP concentrations and biom ass, filaments of Planktothrix and Planktolyngbya, along with the Chlorococ cales, became important. The dominance of Mougeotia (one of the most charac teristic features of these large lakes) appears restricted to a medium trop hic range, whereas the distribution of the Chroococcales and other filament s ascribed to Pseudanabaena and/or Limnothrix is more irregular. The conspi cuous presence of Aphanizomenon in Lake Lugano is typical and characteristi c of a more productive lake. Among the Nostocales, a clear interpretation o f the Anabaena blooms in lakes Garda and Iseo is complicated by the peculia r behaviour of filaments concentrating at the surface, which is apparently restricted, within the medium trophic range, to stable water columns.