Ecological and taxonomic observations on the flagellate algae characterising four years of enclosure experiments in Lake Tovel (Southern Alps)

Lake Tovel (Italian Alps, Brenta Dolomites, 1178 m a.s.l.), with a dimictic mixolimnion, is a meromictic and oligotrophic mountain lake, once notorious for the summer reddening of its waters due to dinoflagellate blooms that ceased suddenly in 1964. Since 1997 the Museo Tridentino di Scienze Naturali has been carrying out in situ experiments in order to understand the factors regulating the blooms. Experiments were carried out in different types of enclosures to test the reaction of phytoplankton to phosphorus enrichment and light reduction, with special reference to Glenodinium sanguineum Marchesoni, the dinoflagellate responsible for the reddening. The aim of the present contribution is to study the ecology of other flagellate algae typical of Lake Tovel, by analysing the data from the enclosure experiments. Particular attention is given to competition for resources, the comparison with G. sanguineum and some interesting taxonomic observations. The four flagellate taxa selected for this investigation were Gymnodinium uberrimum (Allman) Kofoid & Swezy, Dinobryon cf. sociale var. americana (Brunnthaler) Bachmann, Campylomonas sp. and Tetraselmis sp. Number of individuals and biovolume calculated for each taxon during the experiments were statistically related to NO3–N and TP concentrations, water temperature, pH and oxygen saturation. A prompt and positive response to P-enrichment was found in all experiments for both G. sanguineum and G. uberrimum. However, the first taxon showed a pronounced development only when nutrients were coupled with high light intensities, while the latter appeared to be a better competitor in low light intensities and low water temperature. The behaviour of Tetraselmis sp. was similar to that of G. sanguineum, while the development pattern of Campylomonas sp. was similar to that of G. uberrimum. D. sociale did not show a clear relationship either to nutrient enrichment or weather conditions.