Long-term trends in species composition and diurnal migration of dinoflagellates in Lake Tovel (Trentino, Italy)

Lake Tovel, an oligotrophic mountain lake (Trentino, N. Italy) is famous for its past bright red summer blooms of the dinoflagellate Glenodinium sanguineum Marchesoni. In 1938 and 1941 Baldi studied the lake and bloom phenomenon in detail. In particular, he found that the dinoflagellate showed high morphological plasticity and that it was present in the whole lake, but concentrated in the Red Bay, where it exhibited an active vertical migration. In 2001 the presence of G. sanguineum in the whole lake, with higher densities in the Red Bay could be confirmed. However the cell densities found in the Red Bay are up to 10³ lower than those reported by Baldi. During two diurnal studies in August 2001 almost identical diurnal depth variations of maximum density of G. sanguineum were found, likely to be related to positive phototaxis. Although in the Red Bay the patterns of vertical variations were less clear, potential influential factors (wind, sun, temperature) are analysed and discussed. Dinoflagellate species composition from this and earlier studies show an increase in the number of taxa recorded from the lake in recent years and a shift in species composition possibly related to changes in the trophic status of the lake.