Why is the Green River green? Nutrient limitation of algal biomass accrual in the Upper Green River, Kentucky

Background/Question/Methods   Nutrient limitation in algal communities frequently results from a lack of nitrogen (N) or phosphorus (P) relative to cellular growth needs. During summer baseflow conditions we assessed relationships between ambient algal biomass and nutrient levels. Experimental nutrient limitation assays were also conducted on periphytic algae along a longitudinal weak to extensive karst (= limestone) bedrock gradient in the Green River, Kentucky.

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.