Phosphorus Source – Sink Relationships of Stream Sediments in the Rathbun Lake Watershed of Southern Iowa.

Stream sediments often play an important role in regulating phosphorus (P) concentrations in stream water. The purpose of this study was to evaluate sediment – water column P relationships in streams within the Rathbun Lake watershed in southern Iowa, specifically targeting four representative creeks. A second objective was to evaluate the relationship between sediment properties and calculated indicators of P loss risk to determine whether environmental risk can be predicted. Stream water samples collected from the watershed indicated that both median dissolved P (DP) (0.07 mg L^-1) and total P (TP) (0.24 mg L^-1) concentrations were high relative to proposed nutrient standards, suggesting that some form of remediation is necessary to improve water quality in Rathbun Lake. When evaluated by Mehlich-3 extractable P and degree of P saturation (DPS), stream bank and bed sediments had low risk of P loss. However, equilibrium P concentration (EPC), which ranged from 0.02 to 0.12 mg L^-1, indicated that some bed sediments could release P to water depending on DP concentrations in the stream water column and the time of the year. The likelihood of P desorption from the sediments increased with increasing pH (r = 0.92, p < 0.01) and sand content (r = 0.78, p < 0.05), but decreased with Fe (r = -0.93, p < 0.001) associated with organic matter. Based on  proposed nutrient criteria value for streams (0.04 mg P L^-1), results suggest that changes in land use within the riparian areas may, at least initially, have little effect on P loads in the streams leading to Rathbun Lake.