Abstract

Atezca Lake is located in a mountainous zone of North–eastern Mexico. It is a small subtropical monomictic system with a winter circulation period, and stratification in spring and summer. The lake was first studied monthly during two annual cycles in 1981 and 1983; biannual sampling was then conducted in the stratification and circulation periods for 10 years (1984–1994) in order to determinate the seasonal and annual successional patterns of the phytoplankton community.

Thermal behaviour was found to influence both physical and chemical features. A high concentration of nitrates and phosphates occurred at the beginning of the stratification period and decreased toward late stratification. During the overturn period these nutrients increased. These dynamics affected the phytoplankton assemblage because bacillariophyceans and chlorophyceans were dominant in early stratification, dinophyceans and cyanophyceans in late stratification and bacillariophyceans and chrysophyceans were abundant at circulation.

The long term changes showed an increase in the thickness of the anoxic hypolimnion; surface nitrates also diminished, whereas phosphates tended to increase. From 1981 to 1986 the phytoplankton community was dominated by chlorphyceans; however from 1990 to 1994 they were replaced by cyanophyceans, especially Microcystis aeruginosa. The results indicate that the lake has shown severe signs of eutrophication in recent years. The prospects for recovery depend upon sewage input control, hyplomnetic discharges and conservation of the forest in the catchment area.

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