The present work reviews our current understanding of the limnology of Lake Zirahue´n and extends this knowledge through an analysis of the phytoplankton, illustrating phenomena that typify monomictic tropical lakes. The analysis reinforces the postulate that atelomixis determines the variation in phytoplankton composition of deep tropical lakes with a monomictic mixing regime. Similarly, it is proposed that an incomplete or partial atelomixis, generating a highly dynamic mixing layer, is a force that possibly drives the selection of algal groups, like the Desmidiaceae, with a high surface area to volume ratio. These organisms, characteristic of tropical lakes, though found in the deep epilimnetic layer, receive sufficient irradiance to support their high photosynthetic rate; their high population density survive thanks to a constant supply of nutrients. Lake Zirahue´n is a particularly relevant case for Mexico, since the low calcium concentration and the trophic level of the lake provide an explanation for the otherwise uncommon presence of desmids in other Mexican lakes, even in lakes that exhibit atelomictic phenomena.

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Monthly collections of phytoplankton were supported by physical-chemical data and measures of chlorophyll a concentrations in the search for particular environmental factors that could explain the constant presence of desmids in a Mexican tropical lake, a characteristic not common among the phytoplanktic communities of Mexican lakes. Samplings were taken from the water column in the deepest part of the lake (40 m) and intensified in the metalimnetic zone, whose establishment was monitored by observations of temperature and oxygen profiles. The general behavior of Lake Zirahue´n was typical of warm monomictic tropical lakes at high elevation: a short mixing phase during the hemispheric winter. The depth of Z MIX and Z eu revealed a well-illuminated epilimnion, suggesting that phytoplankton communities are not likely to be light-limited. The oligotrophic nature of the lake is indicated by discrete concentrations of inorganic nutrients, P TOT in the interval of 0.01-0.03 mg l À1 and chlorophyll a between 0.23 and 3.98 mg l À1 . These characteristics together with a low concentration of calcium, define a lacustrine environment different from other Mexican lakes, and one that could be suitable for desmids communities.

This is a further contribution to the knowledge of the desmid flora of Cuba. One hundred and sixty samples collected from the most important regions in regard to occurrence of desmids and diversity of mi croalgae species in Cuba (Pinar del Río, Isla de la juventud, Ciénaga de Zapata, Matanzas and San Felipe, Camagüey), were examined. Emphasis was made on the taxonomic survey of those  cylindrical" representative genera. 22 infrageneric taxa belonging to 17 species were identified, 18 of which are new records from the country. Some populations examined showed a wide morphological variability. Each taxon is briefly described based on Cuban material. Original drawings and comments about distribution and synonymy according to the available taxonomic literature are included.