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Even though agricultural land use is a principal source of nonpoint-source pollution, the in-stream movement and transport of nutrients and their interaction with benthic sediments are not well understood. The objectives of this study were to: (1) assess if nutrients were limiting algal growth, (2) assess equilibrium between sediments and water column phosphorus (P), (3) evaluate spatial and temporal variability in P buffering capacity and easily exchangeable P, and (4) evaluate variations in stream nutrient retention efficiency in Moore's Creek, a pasture-dominated watershed in northwest Arkansas. This study used nutrient limitation bioassays, sediment nutrient extractions, and short-term nutrient injections to accomplish the study objectives. Results suggested that light, not nutrients, was limiting algal growth in Moore's Creek. Sediment equilibrium P concentration measurements suggested that sediments may be releasing dissolved inorganic P during winter and spring; however, sediments were a potential temporary sink of dissolved inorganic P during summer and fall. Exchangeable P concentrations varied between 0.4 and 1.0 mg kg-1 of dry sediments, and P sorption index varied between 3.2 and 5. Concentrations of ammonia nitrogen (NH4-N) and ortho P (PO4-P) generally decreased between sampling stations during nutrient injections at all three sites; however, nitrate N (NO3-N) concentrations increased downstream. A significant retention of NO3-N was not observed in any of the experiments, indicating that agricultural headwater streams may be a source of downstream NO3-N transport. The results indicated that a riparian cover should be maintained in nutrient-rich headwater streams in order to minimize algal production in the streams. Even nutrient-rich streams may continue to assimilate, to some extent, increased loads of P, altering the timing and magnitude of downstream transport of P.

Nutrient Retention, Nutrient Limitation, and Sediment-Nutrient Interactions in a Pasture-Dominated Stream