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This study presents the first record of a living dinoflagellate cyst with a hypocystal, antapical archeopyle. It is also the first detailed account of the archeopyle of a living freshwater cyst from the genus Peridinium. The cysts were isolated from sediment traps deployed in Lake Nero di Cornisello, a low-alkalinity high mountain lake of the Adamello mountain range (2233 m above sea level, South Eastern Alps, Italy). The archeopyle is large, clearly hypocystal, polygonal, and slightly peanut-shaped. The species producing this cyst belongs to the Peridinium umbonatum group and is described based on scanning electron microscopy and light microscopy. Partial sequences of SSU rDNA were obtained and compared with previously published sequences from the P. umbonatum group. The taxonomic position of the species is discussed.

This paper documents the discovery of peridinioid organic walled dinoflagellate cysts in Holocene sediments of the freshwater, low-alkalinity, high mountain Lake Nero di Cornisello (Adamello mountain range, Trentino, Italy). Among the three main cyst morphotypes found in the samples, the dominant one is acavate, with a smooth and light brown colored wall, with a clear conical shape, elongated with a pointy to rounded end in the antapical-ventral part, with the epicyst broader than the hypocyst and displays a typical peridinioid archeopyle. Comparison with data available in literature, besides pointing out the general scarcity of observations on dinocysts in lakes sediments, suggested a strong morphological similarity with the marine genus Brigantedinium. This finding pinpoints the need for more detailed studies on cysts in freshwater environments offering a new tool for paleoenvironmental interpretations.

Lake Tovel (Trentino, Brenta Dolomites, NE Italy) was renowned for red blooms due to the dinoflagellate Glenodinium sanguineum. These blooms suddenly ceased in 1964. Responses of the summer phytoplankton communities to nutrient enrichments, analysed experimentally in enclosures, identified phosphorus as the factor limiting both G. sanguineum and a dominant Fragilaria tenera. Competitive interactions suggested that the dinoflagellates were favoured by higher temperatures, whereas the diatoms were limited by silica reduction. The ratio between the biovolumes of the two algae was an important parameter for predicting which species would benefit from the phosphorus additions and outcompete the other one under the experimental condition.

Freshwater red tides due to dinoflagellates are less common than their marine analogues, which are usually a serious problem and can even be toxic. This was not the case for Lake Tovel in the Adamello-Brenta Natural Park (Southern Alps, Italy), where the dinoflagellate Glenodinium sanguineum Marchesoni accumulated carotenoids (astaxanthin-like compound) and caused a spectacular and regular "summer reddening", which suddenly ceased in 1964. Today Lake Tovel is a temperate, meromictic (with dimictic mixolimnion), oligotrophic mountain lake, characterised by marked water level fluctuations. G. sanguineum is still present in the lake, although with markedly lower densities in comparison to the pre-1965 period. Enclosure studies were carried out to identify the main factors regulating the blooms. In 1998, by means of phosphorus enrichments, it was possible to obtain a marked increase in numbers of G. sanguineum. Phosphorus additions in similar enclosures in 1999, when weather conditions during the summer were not optimal (mostly cloudy with frequent rainfalls), did not have the same effect, since species known to thrive in spring or under icecover developed. In summer 2000, by attenuating light in one of the enclosures, it was shown that irradiance conditions strongly interacted with phosphorus availability in determining the species that dominated the phytoplankton. In summer 2001, G. sanguineum increased again in the enclosure enriched with phosphorus. Nitrogen was observed to become a limiting factor only in conditions forced by phosphorus additions. From the first four years of in situ experiments, we concluded that phosphorus and light conditions were among the key factors controlling the proliferation and the dominance of G. sanguineum.

The analysis and identification of a wide range of secondary metabolites biosynthesized by different algal taxa and cyanobacteria has been performed through a selective and sensitive methodology, mainly based on reversed-phase HPLC coupled both to UV photodiode array detection and to atmospheric pressure mass spectrometric techniques (HPLC-DAD-APIMS). Results are reported here with special attention to the analyses carried out both on the natural phytoplankton (mixed populations) of Lake Tovel (Northern Italy, Brenta Dolomites) and on enclosure-produced biomass of the dinoflagellate Glenodinium sanguineum Marchesoni (1941). This analytical procedure might represent a powerful tool for the fast screening of the taxonomic composition (broad groups, e.g. divisions) of natural mixed populations of phytoplankton, by providing a reliable distribution of accessory pigments extracted from microalgae, such as carotenoids and chlorophyll derivatives. Furthermore, we showed that in the same chromatographic analysis other classes of natural products, such as galactolipids, alkaloids, sterols and mycosporine-like amino acids, can be detected by using combined optical and mass spectrometric techniques. These metabolites represent distinctive biochemical signatures, sometimes even at the species level.

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.