Phytoplankton dynamics in a deep, tropical, hyposaline lake

The annual variation of the phytoplankton assemblage of deep (64.6 m), hyposaline (approximate to8.5 g l(-1)) Lake Alchichica, central Mexico (19 degrees N, 97degrees W), was analyzed in relation to thermal regime, and nutrients concentrations. Lake Alchichica is warm monomictic with a 3-month circulation period during the dry, cold season. During the stratified period in the warm, wet season, the hypolimnion became anoxic. N-NH3 ranged between non detectable (n.d.) and 0.98 mg l(-1), N-NO2 between n.d. and 0.007 mg l(-1), N-NO3 from 0.1 to 1.0 mg l(-1) and P-PO4 from n.d. to 0.54 mg l(-1). Highest nutrient concentrations were found in the circulation period. Chlorophyll a varied from <1 to 19.8 mug l(-1) but most values were <5 mug l(-1). The euphotic zone (>1% PAR) usually comprised the top 15-20 m. Nineteen algae species were identified, most of them are typical inhabitants of salt lakes. Diatoms showed the highest species number (10) but the small chlorophyte Monoraphidium minutum, the single-cell cyanobacteria, Synechocystis aquatilis, and the colonial chlorophyte, Oocystis parva, were the numerical dominant species over the annual cycle. Chlorophytes, small cyanobacteria and diatoms dominated in the circulation period producing a bloom comparable to the spring bloom in temperate lakes. At the end of the circulation and at the beginning of stratification periods, the presence of a bloom of the nitrogen-fixing cyanobacteria, N. spumigena, indicated nitrogen-deficit conditions. The well-stratified season was characterized by low epilimnetic nutrients levels and the dominance of small single-cell cyanobacteria and colonial chlorophytes. Phytoplankton dynamics in tropical Lake Alchichica is similar to the pattern observed in some deep, hyposaline, North American temperate lakes.