Why is the Green River green? an assessment of algal biomass and nutrient levels along a longitudinal gradient in the upper Green River

Nitrogen and phosphorus are common limiting nutrients in aquatic ecosystems. The availability of each nutrient influences organism growth, and ecosystem productivity is affected by the form and intensity of nutrient limitation. We assessed if gradients of nutrient levels and algal biomass occurred longitudinally along a regulated river. Sestonic algal biomass and water nutrient levels were quantified during summer 2008 at 10 sites along the Green River in central Kentucky, beginning directly downstream of the Green River Lake and proceeding to Mammoth Cave National Park. Water samples were tested for total nitrogen (TN), ammonia, nitrate, soluble reactive phosphorus (SRP), and total phosphorus (TP). Sestonic and benthic algal biomass were quantified with a standard chlorophyll extraction procedure. Sestonic and benthic algal biomass increased in a downstream manner, as did both TP and SRP. Ammonia declined in a downstream manner, but nitrate experienced a near three-fold increase along this same gradient. Both TN and TN:TP levels declined, then sharply increased upon entering a karst region 50 km downstream from the Green River Lake. Overall, TN:TP and both algal biomass levels displayed nearly identical longitudinal patterns of increase and decrease.