Abstract
Phytoplankton are vital components of freshwater ecosystems, serving as primary producers and ecological indicators of freshwater health. While substantial research has explored the dynamics and potential drivers of phytoplankton communities in freshwater lakes, the influence of altitude-a crucial geographic factor-remains unelucidated. In this study, we investigated 26 lakes located along the eastern section of the Hu Line (Heihe-Tengchong Line) from China, focusing on how altitudinal gradients shape phytoplankton communities. Our findings reveal that cyanobacteria increasingly dominate community abundance with rising altitude. However, this dominance results in a reduction of community diversity, as cyanobacteria outcompete other taxa, thereby narrowing the ecological niches available. Further analysis indicates that the effects of altitude are mediated primarily by dominant cyanobacteria, which are equipped with gas vesicles that provide an adaptive advantage under low atmospheric pressure conditions associated with higher altitudes. This physiological trait allows cyanobacteria to maintain buoyancy and occupy favorable niches in the water column, enhancing their proliferation at the expense of overall community diversity. The study underscores the critical role of altitude in modulating phytoplankton community structure through its direct influence on cyanobacteria. These findings contribute new insights into the ecological processes that regulate freshwater ecosystems across altitudinal gradients, and highlight the need for targeted management strategies in regions where cyanobacterial dominance (cyanobacterial blooms) may compromise water quality and biodiversity.