Vertical and horizontal distribution of the microcystin producer Planktothrix rubescens (Cyanobacteria) in a small perialpine reservoir

Among cyanobacteria, Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis & Komárek is a species that is well adapted to develop in moderately nutrient rich and deep lakes. In this typology of waterbodies, the competitive abilities of this species rely in its capacity to stand and growth in the dimly illuminated metalimnetic layer during the warmer months. In this work, we have studied the seasonal development and distribution of this species in Lake Ledro, a meso-oligotrophic reservoir located in the Eastern Alps. During the last decade, this species has given rise to numerous and extended surface bloom episodes, causing the reddening of vast areas of the lake. In summer, the light intensities in the zone of greater development of this cyanobacterium (the metalimnion, between the euphotic depth and the layer of maximum development of the species) were between 2 and 20 µmol m –2 s –1 , i.e. values that were well within the light intensities required to sustain the optimal growth of filaments. The formation of the autumn and winter blooms was triggered by the cooling of surface waters and the deepening of the mixed layer, which, eroding the metalimnion, entrained the fil- aments of P. rubescens in the surface mixed layers. The formation of the surface blooms was associated with the presence of high amounts of microcystins, which in a few occasions reached concentrations between 10 and 22 µg L –1 , posing potential problems for the exploitation of water resources.