Results/Conclusions Sestonic and filamentous biomass (= chlorophyll-α) levels increased monthly along the longitudinal gradient. In contrast, periphyton biomass levels increased minimally monthly yet displayed no longitudinal pattern. Nitrate and soluble reactive phosphorus levels exhibited distinct longitudinal increases, whereas total phosphorous displayed minimal change and ammonia levels decreased in the downstream direction. Total nitrogen (TN) levels spiked longitudinally but decreased sharply in the well-developed downstream karst sites. Results from the nutrient limitation assays revealed that the highest periphyton levels were with N + P treatments at the most upstream sites, indicating nutrient limitation in the weak karst bedrock region. Overall, in Kentucky's Green River, algal growth appears to be mainly P-limited but likely also by TN availability during late summer.