The diversity that we observe in nature is surprising, given that theories and experiments predict that competitively superior species outcompete others. In this talk, I will introduce some mechanisms that promote the coexistence of phytoplankton in a lake. First, I will revisit an underappreciated theoretical prediction as to how the community dynamics change with the reduction in the number of limiting factors. The theoretical prediction was applied to long-term observational data (provided by Lake Biwa Environmental Research Institute), which showed that the opportunities for coexistence are reduced with lake eutrophication because the interspecific trade-offs collapsed. A negative correlation between the degree of phosphorus limitation and species’ body size was detected, suggesting the existence of a trade-off among species. Next, I will show how much the seasonal heterogeneity of the environment contributes to the coexistence of phytoplankton. Empirical models of phytoplankton community dynamics, which were informed by the observational data, indicated that the contributions of seasonal variance in the growth rates to the stability of the coexistence were small (generally less than 10%) and that the coexistence was possible without temporal niche differentiation. These results collectively suggest that, counter-intuitively, the phytoplankton diversity is primally maintained due to interspecific trade-offs that render strong self-regulation.