Long-Term Loads of Nutrients and Sediment from Non-Tidal Regions of the Chesapeake Bay Watershed: An Assessment of Seasonal Trends and Progress

Abstract:

To assess historical loads of nitrogen (N), phosphorus (P), and suspended sediment (SS) from the non-tidal Chesapeake Bay watershed (NTCBW), we have analyzed long-term seasonal trends of flow-normalized loads at the fall-line of nine major tributaries that account for over 90% of NTCBW streamflow. Seasonal comparisons revealed that (1) nutrient and sediment loads have been generally highest in Jan-Mar and lowest in Jul-Sep, and (2) decadal-scale trends have generally followed similar patterns in all seasons, with some important exceptions. Generally, total N load from NTCBW has dropped since the late 1980s, but particulate nutrients and SS have risen since the mid-1990s. The majority of this rise was from the Susquehanna River and relates to diminished trapping of SS during extreme events at Conowingo Reservoir. Substantial rising trends in SS were also observed, however, in the other rivers. Moreover, the summed magnitudes of rise in particulate P loading in other tributaries are of similar magnitude as those for the Susquehanna. Dissolved nutrient loadings have dropped in the upland (Piedmont-and-above) rivers, but dissolved nutrients loadings have risen in two small rivers that are within the Coastal Plain and affected by lagged groundwater input. These two rivers may be most representative of Coastal Plain watersheds that feed some major tidally influenced regions that are not otherwise accounted by this study. Finally, analysis of fractional contributions revealed that the seven upland rivers yielded surprisingly steady fractions of N, suggesting consistency of N input and N-modulation across the watershed. Overall, this study demonstrates the value of integrated and comparative evaluations of historical data and highlights the importance of maintaining long-term water-quality monitoring at multiple watershed locations.