The Anacostia River, Washington D.C., is a freshwater ecosystem that historically received high concentrations of nutrients from sewage and stormwater outfalls. Restoration efforts have been implemented recently that may improve water quality and alter the relative abundance of different phytoplankton taxa in the river. To determine the effects that environmental shifts may have on diatom abundance and phytoplankton community composition in the Anacostia River, a mesocosm experiment and laboratory studies were conducted. The results of the mesocosm study revealed that diatoms were consistently outcompeted by cyanobacteria. Additionally, phosphorus enrichment led to a 50% increase in cyanobacterial abundance and decreased the abundance of diatoms. In the culture study, shifts in water temperature and nutrient availability altered diatom growth rates, photosynthesis, silica deposition, and NO3- reduction. Together, these studies highlight the interactive effects that nutrient availability and temperature may have on the physiology and subsequent growth of diatoms in the Anacostia River.