Algal blooms are increasing globally, but research on the impact of cultural eutrophication on algal communities and blooms in freshwater ecosystems is more limited in lotic systems compared to lentic systems. This study investigates algal community composition across streams and reservoirs in the upper Ohio River Basin, encompassing seven catchments with degrees of agricultural and urban land uses in Ohio, Indiana, and Kentucky, USA. Here, we aimed to examine how river conditions mediate the relationship between land use patterns and phytoplankton communities in downstream reservoirs. Agricultural land use in watersheds influenced water chemistry in both streams and reservoirs, leading to increased conductivity, nitrogen, and turbidity. In streams, phytoplankton biomass increased with temperature, conductivity, phosphate and ammonium, while showing a negative relationship with nitrate. In reservoirs, phytoplankton biomass showed positive relationships with conductivity, dissolved oxygen, pH, and turbidity, but decreased with phosphate concentrations. These relationships suggest that agricultural land use affects phytoplankton communities through multiple pathways, including altered nutrient availability and physical conditions, with somewhat different patterns emerging in streams versus reservoirs. Ongoing community analyses will further explore how these environmental patterns couple phytoplankton assemblages between reservoirs and the streams that feed them. These findings will make valuable contributions to understanding the distribution and drivers of phytoplankton, while also providing insights into the effects of nutrient enrichment and broader aspects of stream and watershed ecology.