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Interannual variations in Susquehanna annual loads of TN and NO_x have negative MK slopes while TP and SS have positive slopes (Figure 2.3). Although these slopes are not statistically significant, their directions are consistent with results of flow‐normalized loads (Zhang et al., 2015) that account for interannual variability in river flow (Hirsch et al., 2010). Declines in TN and NO_x were partially due to upgraded wastewater treatment (Boynton et al., 2008) and decreases in atmospheric deposition due to the Clean Air Act (Eshleman et al., 2013; Linker, Dennis, et al., 2013). By contrast, TP and SS loads have increased since the late 1990s, likely due to declining trapping efficiency of the Conowingo Dam in the lower Susquehanna (Hirsch, 2012; Langland, 2015; Zhang, Ball, et al., 2016; Zhang et al., 2013).

Interannual variations in the Po River annual load of TN had a positive MK slope while loads of NO_x, TP, DIP, and SS had negative slopes (Figure 2.3). The only statistically significant trend for Po is with DIP, which shows a long‐term reduction after the peak in the 1980s. This pattern can be attributed to a reduction of P content in fertilizers and detergents, as well as improved management of wastewaters (Cozzi & Giani, 2011; Viaroli et al., 2018). By contrast, N loads have been driven by interannual oscillations from persistent anthropogenic N emission in the watershed and by interannual changes in river flow, particularly during the extreme drought of 2003–2007 (Cozzi et al., 2019). Current transport of SS by the Po is high compared to river flow, due to large SS contributions by the Apennine tributaries and the absence of dams in the lower river (Tesi et al., 2013). Transport of SS to the NAS is critical for the maintenance of delta and along‐shore habitats, as well as for sedimentation processes in the western Adriatic Sea (Frignani et al., 2005). Despite SS transport decreases during the previous century, the present estimates suggest that SS loads have not changed significantly since the 1980s.