Microplastic pollution is a growing concern globally due to the risks they may pose to ecological communities. Phytoplankton are key ecological community in aquatic ecosystems providing both energy to food webs and have critical roles in ecosystem functions such as carbon cycling. To date studies on how microplastics effect phytoplankton have largely been limited to laboratory exposure studies using monocultures of algae. It remains unknown how the structure of phytoplankton communities will be influenced by growing microplastic pollution. The aim of this study was to determine how different concentrations microplastic fibers influence phytoplankton community structure. Two six-day microcosm studies were conducted testing the response of the phytoplankton community to low, medium, and high microplastics concentrations on the Georges River, Australia. The results showed the highest concentrations of microplastics significantly altered the structure phytoplankton community. These differences were largely driven by increased abundances of cyanobacteria taxa Aphanocapsa and Pseudanabaena, and to a lesser extent reduced abundances of taxa including Crucigenia and Chlamydmonas. There were no significant differences between controls and the low and medium treatments in either experiment. The high concentrations used in this experiment whilst likely rare in the environment are environmentally relevant and equivalent to some of more polluted ecosystems. The results highlight the potential risk to food webs and ecosystem functioning through altering the dynamics of primary production and provide evidence for further study examining the response of ecological communities to microplastics in the environment.