EFFECT OF A POINT SOURCE INPUT ON STREAM NUTRIENT RETENTION¹

Abstract

ABSTRACT: 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 quantify the impact of nutrient inputs in the receiving stream. We used an artificial injection upstream of the PS inputs to estimate background S 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 (SRP) in the upstream reference section was O.75 km, but S net ranged from 9.0 to 31 km for SRP and 3.1 to 12 km for NO 3 ‐N in the reach below the PS. S net ‐SRP was significantly correlated with discharge whereas S net ‐NO 3 ‐N was correlated with the amount of NO 3 ‐N enrichment from the PS. In order to examine specific mechanisms of P retention, loosely exchangeable P and P Sorption Index (PSI) of stream sediments were measured. Sediments exhibited little natural P buffering capacity (low PSI) above the PS, but P loading from 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 regulated P retention in Spavinaw Creek, whereas the level of N enrichment and possibly biotic uptake and denitrification influenced N retention. Regardless of the mechanism, Spavinaw Creek demonstrated little ability to retain PS‐added nutrients because net nutrient uptake lengths were in the km range.