Long term picoplankton dynamics in a warm-monomictic, tropical high altitude lake

Long term analyses of the microbial loop, centred on the picoplankton dynamics, were carried out over a fiver-year (1998 to 2002) period in Lake Alchichica (Puebla, Mexico), a high altitude athalassohaline lake. The hydrodynamics of the lake followed a warm-monomictic pattern with mixing at a minimum temperature during the early dry season while the stratification was pronounced in the late dry season and throughout the rainy season; anoxic conditions in the hypolimnion lasted <9 months. The annual mean concentrations of chlorophyll-a were below 4 mu g L-1 in 1998, 1999 and 2001, however, 6.1 and 5.2 mu g L-1 in 2000 and 2002, respectively. Total picoplankton, TPP, displayed a temporal pattern that followed the mixing-stratification cycle. The highest TPP values (the whole water column >= 5 x 10(6) cells mL(-1)) were found during mixing and early stratification (January-March). The minimum numbers were present during late stratification (October-November). The maximum TPP numbers were observed within the layer 0-20 m, which corresponded to the epilimnion during the stratification period. Neither the thermocline nor the deep chlorophyll maximum showed an elevated TPP concentration. In the hypolimnion, TPP numbers were low (frequently <1 x 10(6) cells mL(-1)) apparently as a result of the long period of anoxia. Nothwithstanding autotrophic picoplankton (APP) contributed even >= 30% of TPP (2001 to 2002); no significant correlation was found between TPP and chlorophyll-a.