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Status and Trends of Total Nitrogen and Total Phosphorus Concentrations, Loads, and Yields in Streams of Mississippi, Water Years 2008-18
Proceedings of the 2022 Mississippi Water Resources Conference

Year: 2022 Authors: Hicks M.B.

To assess the status and trends of nutrient conditions of surface waters throughout Mississippi, the U.S. Geological Survey, in cooperation with the Mississippi Department of Environmental Quality, summarized concentrations and estimated loads, yields, trends of total nitrogen (TN) and total phosphorus (TP) between 2008 and 2018 water years for 22 streams in Mississippi.

Relation of streamflow to concentrations of TN and TP varied among sites and were generally related to land use: sites with high agriculture land use in the drainage basin generally had positive correlations between streamflow and nutrient concentration, suggesting influence of event-driven nonpoint-source runoff; sites near urban (developed) areas generally had negative correlations, suggesting chronic point-source influences during low-flow conditions; sites with high forest land use and lower agriculture and urban land use had little to no association between streamflow and concentration.

Seasonal distributions of concentrations of TN and TP also corresponded closely with differences in land use among sites. Sites near urban (developed) land had the highest nutrient concentrations in late summer and fall, whereas nutrients were highest during the spring among sites with a high percentage of agricultural land. However, seasonal patterns in nutrient concentrations were not apparent among sites that were primarily forested or with little developed land

Trend analyses of TN loads between 2008 and 2018 water years indicated that eight sites had statistical likelihoods for upward trends of TN loads, seven sites had statistical likelihoods for downward trends, and six sites had no statistical trend. Trend analyses of loads of TP resulted in 16 sites with upward trends, 3 sites with downward trends, and 2 sites considered "about as likely as not."

Results of estimated nutrient yields calculated for sites across MS in this study varied in consistency compared to predicted regional-scale nutrient yields generated by 2012 SPAtially Referenced Regressions on Watershed attributes (SPARROW) model. Notably, yields of TN and TP at four sites in the highly agricultural area of northwest Mississippi (Delta) were underestimated by SPARROW model by an average of 25 percent and 44 percent, respectively.

Overall, data indicate yields of TN may have slightly decreased over the last 20-30 years, but TP yields remain constant or are increasing and SPARROW model estimates for Mississippi streams may be improved with additional calibration sites and data, especially in the Delta.

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