LONG-TERM EXPORT OF NITROGEN, PHOSPHORUS, AND SEDIMENT IN THE SUSQUEHANNA RIVER BASIN: ANALYSIS OF DECADAL-SCALE TRENDS AND SUB-BASIN MASS BALANCES

Reduction of nitrogen (N), phosphorus (P), and suspended sediment (SS) export has long been a management focus for Chesapeake Bay. Here we present a comprehensive evaluation of nutrient and sediment loads at multiple locations in the Bay’s largest tributary, Susquehanna River Basin (SRB), over the last three decades. Our analyses included trend analysis at seven sites and mass-balance analysis for the SRB’s sub-basins. Trend analysis reveals that nutrient and sediment loads, including both dissolved and particulate species, have generally declined at all sites except Conowingo (non-tidal SRB outlet). These downward trends are generally consistent in timing and magnitude across sites, indicating watershed-wide progress after decades of management on atmospheric, point, and agricultural sources. Comparisons between temporal trends in riverine yield and source input reveal consistently smaller slopes for the former, suggesting the possibility of legacy sources from surface and sub-surface stores. Mass-balance analysis reveals that net contributions from all six sub-watersheds upstream of Conowingo Reservoir follow similar patterns, with concurrent peaks and troughs for riverflow and solute loads, suggesting similar conditions of rainfall and material processing across the SRB. The relative contributions of each sub-basin to total non-tidal SRB load are consistent with expectations based on relative drainage area and dominant land use. Long-term median yields of N, P, and SS have all correlated positively with the area fraction of agricultural or urban lands but negatively with that of forested land, and these patterns were observed under different hydrological conditions. Moreover, riverflow was observed to have affected the relative importance of dissolved vs. particulate species at all sites, with high-flow years showing smaller proportions of dissolved nutrient fractions and decreased annual ratios of particulate P/sediment and total N/total P. Overall, these results provide insights on constituent export from different areas of the SRB including information about effects of land use and riverflow. The findings have significant implications on management of this large watershed and illustrate the value of long-term monitoring at multiple locations across the watershed.