Effectiveness of Nitrogen Assimilation in the Non-Tidal Chesapeake Bay Watershed: Evaluations Based on Thirty Years of Data

Control of watershed nutrient input has long been a priority of Chesapeake Bay watershed management for alleviating hypoxia in the Bay. Therefore, systematic evaluations of historical watershed nutrient inputs and responses of river water quality can help managers to assess the effectiveness of nutrient management across the Bay watershed. Toward that end, we conducted a comprehensive comparison of multi-decadal total nitrogen (TN) watershed input and riverine output for the nine major rivers in the non-tidal Bay watershed. Specifically, we (1) compiled available data regarding multi-decadal TN input from four major sources, (2) obtained updated estimates of TN flux at downstream (edge-of-tide) river locations ('output'), and (3) used these inputs and outputs to quantify the watersheds' land/river effectiveness (LRE) factors in regard to TN assimilation, including an investigation of relationships between LRE and TN input. Our compiled data for N sources confirmed known trends regarding the two largest of the four TN sources (atmospheric deposition and fertilizer loadings) - i.e., that the former has declined significantly in all river basins, whereas the latter has decreased in most of the river basins. For the other two sources, point sources have declined most dramatically in the Patuxent River but exhibited various trends in other basins, whereas manure sources have increased statistically significantly in most of the river basins. The riverine output results were observed to follow watershed inputs in a non-linear manner. Finally, the LRE of the various basins were observed to correlate inversely with the watersheds' input loadings, whereas temporal correlations within a given basin were less consistent. In addition, the Susquehanna sub-watersheds show lower LREs compared with other river basins, with riverine output similar to or even larger than input, implying that greater management effort at these locations could be especially fruitful for load reduction.