Effect of a point source input on stream nutrient retention

We examined the effect of a point source (PS) input on water chemistry and nutrient retention in Spavinaw Creek, Arkansas, during summer baseflows in 1998 and 1999. The nutrient uptake length (S-W) concept was used to quantif y the impact of nutrient inputs in the receiving stream. We used an artific ial injection upstream of the PS inputs to estimate background Sw and used the natural decline in nutrient concentrations below the PS to estimate the net nutrient uptake length (S-net). S-W for soluble reactive phosphorus (S RP) in the upstream reference section was 0.75 km, but S-net ranged from 9. 0 to 31 km for SRP and 3.1 to 12 kin for NO3-N in the reach below the PS. S -net-SRP was significantly correlated with discharge whereas S-net-NO3-N wa s correlated with the amount of NO3-N enrichment from the PS. In order to e xamine specific mechanisms of P retention, loosely exchangeable P and P Sor ption Index (PSI) of stream sediments were measured. Sediments exhibited li ttle natural P buffering capacity (low PSI) above the PS, but P loading fro m the PS further reduced PSI. Loosely exchangeable P in the sediments also increased three fold below the PS, indicating sediments removed some water column P. The physical process of flow and sediment sorption apparently reg ulated P retention in Spavinaw Creek, whereas the level of N enrichment and possibly biotic uptake and denitrification influenced N retention. Regardl ess of the mechanism, Spavinaw Creek demonstrated little ability to retain PS-added nutrients because net nutrient uptake lengths were in the kin rang e.