Picophytoplankton, including the marine cyanobacteria Prochlorococcus and Synechococcus, are dominant primary producers in oligotrophic oceans and play a fundamental role in global carbon cycling and marine biogeochemical processes. Their ecological success is closely linked to their extensive genetic diversity, niche partitioning, and adaptive responses to environmental variability. Here, both field-based and experimental approaches were employed to investigate the population dynamics, ecological interactions, and thermal responses of picocyanobacterial communities. We examined the diversity, distribution, and growth dynamics of Prochlorococcus and Synechococcus in the oligotrophic Argo Basin, a key region influenced by the Indonesian Throughflow. By integrating flow cytometry, dilution experiments, and high-throughput sequencing, we characterized the vertical distribution of major clades and quantified their growth and grazing rates. The results revealed strong depth-dependent shifts in community composition, with distinct clades occupying different ecological niches. Following the exploration on correlations between picophytoplankton and environmental variables, we assessed the temperature sensitivity of in situ phytoplankton communities across a large-scale transect in the eastern Pacific Ocean. Using temperature-modulated dilution experiments, we quantified growth and grazing responses under multiple temperature treatments. We expect phytoplankton communities from different regions exhibit distinct thermal responses in growth estimates, reflecting variations in community composition, intrinsic properties, and environmental conditions. To further investigate adaptive responses to environmental change, we conducted long-term thermal adaptation experiments on Synechococcus strains isolated from Hong Kong coastal waters, evaluating changes in thermal performance curves and physiological traits. The results demonstrated that different Synechococcus clades exhibit distinct thermal adaptation strategies, suggesting that thermal adaptation may alter competitive dynamics among lineages and contribute to shifts in community structure under future ocean warming. Together, these results demonstrate that picophytoplankton dynamics are governed by the combination of environmental forcing, trophic interactions, and physiological adaptation. This integrated approach is expected to provide new insights into how phytoplankton communities respond to environmental variability and contribute to predicting changes in marine primary production under ocean warming.