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with CB, riverine inputs of nutrients to the NAS are much higher (Table 2.1), with riverine inputs of anthropogenic nutrients to the western NAS (dominated by the Po River) accounting for 80%, 83%, 84%, and 73% of annual inputs of TN, NO_x, TP, and DIP, respectively. This reflects the higher population density in river watersheds of the western shore. Notably, nutrient inputs to the NAS are much lower than those to CB when normalized to the volume of the receiving water bodies (~1812 × 10³ kg N km⁻³ year⁻¹ for CB vs. ~285 × 10³ kg N km⁻³ year⁻¹ for the NAS). In addition, the TN/TP molar ratio for CB is much lower than for the NAS (33:1 vs. 41:1) while the NO_x/DIP molar ratio for CB is much higher than for the NAS (141:1 vs. 72:1).

      2.3.2. Seasonality (2004–2012)

For both CB and the NAS, nutrient and sediment loads show strong seasonal variability (Figure 2.2). For CB, the annual input cycles of freshwater, TN, TP, and suspended sediment (SS) are typically unimodal with maxima during March–April and minima during July–August. This pattern has been reported for TN in all the major tributaries to CB (Zhang et al., 2015). For the NAS, the annual input cycles of freshwater, TN, and TP tend to be more bimodal with high inputs during April–June and October–December and low inputs during January–March and July–September.

      2.3.3. Long‐Term Trends (1985–2015)

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.

2.4. CONTROLS OF NUTRIENT EXPORT

      2.4.1. Nutrient Sources

Watershed export of nutrients is complex due to heterogeneities in their sources, fates, and transports. In terms of sources, agriculture nonpoint sources, atmospheric deposition, urban (storm water) sources, as well as point sources (wastewater treatment plants) account for most inputs to CB and the NAS watersheds (Ator et al., 2011; Palmeri et al., 2005; Salvetti et al., 2006; Viaroli et al., 2018; Volf et al., 2013). Globally, there is a significant linear correlation between net anthropogenic N supplies to coastal watersheds and total riverine nitrogen export to the coastal ocean (Boyer & Howarth, 2008). However, it should be noted that the major sources and their relative contributions can vary significantly both spatially (as a function of watershed characteristics such as land use, climate, and geology) and temporally (as a function of watershed management, urbanization) (Ator et al., 2011; Carpenter et al., 1998).

Figure 2.2 Boxplots showing seasonal loads of (a) total nitrogen (TN), (b) nitrate + nitrite (NO_x), (c) total phosphorus (TP), (d) dissolved inorganic phosphorus (DIP), and (e) suspended sediment (SS) to Chesapeake Bay (four boxes on the left) and the northern Adriatic Sea (four boxes on the right) from tributaries with available data (9 and 13 tributaries, respectively) in 2004–2012.

Figure 2.3 Time series of annual loads of (a) total nitrogen (TN), (b) nitrate + nitrite (NO_x), (c) total phosphorus (TP), (d) dissolved inorganic phosphorus (DIP), and (e) suspended sediment (SS) to Chesapeake Bay and the northern Adriatic Sea from their largest tributaries (i.e., Susquehanna and Po, respectively) in 1985–2015. Mann‐Kendall (MK) trend slope and significance (p) values are shown in legend.

For the CB watershed, riverine export of TN was dominated by agriculture nonpoint sources (fertilizer and manure 54%), followed by contributions from atmospheric deposition (17%), point sources (16%), and urban sources (12%). Riverine export of TP was dominated by agriculture nonpoint sources (43%), followed by point sources (32%) and urban sources (11%) (Ator et al., 2011). Typically, nutrients accumulate in watersheds during dry periods of low flows and are transported to receiving waters via groundwater discharge and surface water runoff during wet periods and storm events (Shields et al., 2008; Tesi et al., 2013). Groundwater can represent a major fraction of riverine load (especially N). Bachman et al. (1998) estimated that base flow (a proxy of groundwater input) accounted for 17–80% (median 48%) of the TN load at 36 CB monitoring sites. In addition, riverine export can be strongly modulated by reservoirs. For example, Conowingo Reservoir and two others in the lower Susquehanna River historically trapped about 2%, 45%, and 70% of annual N, P, and SS load, respectively (Langland & Hainly, 1997).

      Riverine inputs of TN to the NAS were also dominated by agriculture nonpoint sources (40%) followed by point and urban sources (27%), and groundwater (29%). The TP load mainly originated from point sources and urban sources (43%) and agriculture nonpoint sources (36%) (Volf et al., 2013). For the Po watershed, TN load comes from point sources (40%), nonpoint sources (20%), and groundwater and springs (40%), whereas TP loads come from point sources (80%) and nonpoint sources (20%) (Salvetti et al., 2006).

      2.4.2. Controlling Factors

      In the CB watershed, several controlling factors have been identified for nutrient export in the Susquehanna River (Zhang, Ball, et al., 2016). First, river flow dominates the interannual variability of constituent export. Second, land‐use patterns strongly affect the relative contribution of the subwatersheds. Specifically, long‐term median yields of N, P, and SS all correlate

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