Size‐fractionated phytoplankton biomass and its implications for the dynamics of an oligotrophic tropical lake

Summary 1. Size‐fractionated phytoplankton biomass was examined in relation to the hydrodynamics of tropical Lake Alchichica from 1999 to 2002. 2. Alchichica is a warm monomictic lake, in which mixing takes place from late December to early March. The lake is oligotrophic (mean total chlorophyll‐a concentration 4.2 ± 4.2 μg L−1) and its phytoplankton biomass is dominated (72.3 ± 16.4%) by large individuals (>2 μm). The degree of dominance of the large size class (nano‐ and microplankton) over the small size class (picoplankton) throughout the year is mainly determined by the availability of silicate and the Si/N ratio in the hypolimnion prior to the mixing period. 3. This is the first record of an oligotrophic tropical lake dominated by large size fractions of phytoplankton. Because of this dominance, the fate of most primary productivity is rapid sedimentation to the bottom followed by decomposition that promotes an anoxic hypolimnion. 4. Our findings in tropical Lake Alchichica challenge the idea that oligotrophic waters are dominated by small phytoplankton, as has been well established for the oligotrophic ocean and temperate lakes.