The Ohio River is a vital resource for drinking water, recreation, and ecological diversity, yet it is subject to nutrient enrichment from a vast and varied watershed. Nutrient inputs and seasonal periods of low flow affect phytoplankton community dynamics and foster the development of harmful algal blooms (HABs). The 2015 and 2019 blooms of Microcystis underscored the need to better understand the drivers of seasonal changes in phytoplankton communities in the Ohio River. Here we present riverine phytoplankton composition and nutrient conditions along a 50-mile segment of the Ohio River, from Cannelton, IN, to Evansville, IN. Samples of water chemistry and phytoplankton were collected at three locations during twelve sampling events throughout the 2023 and 2024 water years to analyze temporal and spatial patterns. This study focuses on nutrient concentrations, including nitrogen, phosphorus, and dissolved silica (DSi), to evaluate their influence on the relative abundance of phytoplankton including diatoms, cyanobacteria, and other non-siliceous algae. Nutrient ratios frequently indicated favorable conditions for diatom growth, with ample DSi relative to nitrogen and phosphorus (e.g., molar ratios of DSi:SRP were >20 on all but three dates). However, during late summer, the ratios shifted significantly, likely driven by diatom uptake of silica, with DSi:SRP of 8 and DSi:DIN of 0.13 by October 2024. Additionally, this depletion of DSi coincided with an increase in the relative abundance of cyanobacteria and HAB taxa, particularly between August and October, suggesting that silica limitation may contribute to the seasonal shift in phytoplankton dominance. Concurrent with the changes in nutrient ratios and relative abundance of phytoplankton, was reduced river discharge and increased water residence time, a set of conditions conducive to cyanobacteria proliferation. Initial analyses suggest correlations among seasonal phytoplankton composition, flow conditions, and nutrient ratios, offering valuable insights for mitigating HABs and preserving ecological balance in the Ohio River.