Cozzi, 2016; Djakovac et al., 2012; Stachowitsch, 2014). The quality of marine waters was also degraded by toxic dinoflagellate blooms and massive accumulations of mucilaginous aggregates (Djakovac et al., 2012; Giani et al., 2012). The economic impacts of these events (primarily on tourism) resulted in Italian regulations in 1986 to reduce polyphosphates in detergents and in the establishment of the Po Basin Authority in 1989 to manage nutrients inputs to the Po River, the largest tributary of the NAS (Seagle et al., 1999). In 2000, the Water Framework Directive 2000/60/EC (WFD) of the European Union (EU) established a framework for member states to achieve good ecological and chemical status objectives for inland surface waters, estuaries, and coastal waters within 1 nautical mile from shore through watershed management by 2015 (Teodosiu et al., 2003). In 2013, the Management Plan of the Po River (PdGPo 2010) was approved, which opened a new phase for water management and for the reduction of nutrient loads, through the realization of spill basins for agriculture and manure wastes and the implementation of the wastewater collection and depuration systems (Bortone, 2014). However, 15 years after the directive was agreed to, achieving its objectives remains a challenge, with 47% of EU surface waters not reaching good status in 2015 (Voulvoulis et al., 2017). To achieve the objectives of the WFD, a more integrated understanding of the relationships between land‐use practices in coastal watersheds and the status of surface waters is needed.
The main objective of this chapter is to review and compare the current status, seasonality, and long‐term trends of freshwater and nutrient inputs to CB and the NAS. We begin with an overview of the two watersheds followed by a comparison of freshwater inputs in terms of their seasonality and long‐term trends. We then compare nutrient and sediment loads from the watersheds, elucidate the controls of nutrient and sediment export, highlight some of the major challenges to achieving reductions in land‐based inputs, and conclude with recommendations for the management and restoration of CB and the NAS.
2.2. OVERVIEW OF THE WATERSHED AND FRESHWATER INPUTS
Chesapeake Bay is a large estuary in the Mid‐Atlantic region of the United States. Among its many tributaries, nine account for over 90% of river flow into CB (Chanat et al., 2016; Moyer et al., 2012). The watershed of the Susquehanna River, the largest river discharging directly into the mainstem bay, comprises about 43% of total CB watershed and is dominated by forested areas (~65%). The NAS is a shallow, semienclosed arm of the NE Mediterranean Sea. The Po River, the largest river discharging into the NAS, has a watershed that comprises 67% of the total NAS watershed and hosts large urban and industrial settlements, as well as extended areas of intensive cropping and livestock activities (Seagle et al., 1999).
River flows into CB, and associated inputs of nutrients and sediments, are monitored by the US Geological Survey (US Geological Survey, 2018). For the NAS, flow rates of Italian rivers are monitored the Hydrographic and Mareographic National Service of Italy (1917–1990s) and the Regional Environmental Protection Agencies (1990s to today). Nutrient data were obtained from the scientific literature, monitoring programs, and past projects (Cozzi & Giani, 2011; Cozzi et al., 2019). Data for the Istrian Rivers were provided by the Environmental Agency of the Republic of Slovenia, the Croatian Meteorological and Hydrological Service, and the European Environmental Agency.
The volume transports (Q) of major rivers flowing into CB and the NAS are similar and exhibited strong interannual variability during 1985–2015 (Figure 2.1). On average, the Susquehanna accounts for 62% of riverine inputs of freshwater while the Po accounts for 69%. The highest transports in CB watershed occurred during the years 1996, 2004, and 2011 in association with major hurricanes and tropical storms (Figure 2.1a). River flows to the NAS were characterized by a maximum during 2014 and lows during the drought period of 2003–2007 (Figure 2.1b).
Figure 2.1 Time series of annual freshwater input to (a) Chesapeake Bay and (b) northern Adriatic Sea and boxplots of seasonal freshwater input to (c) Chesapeake Bay and (d) northern Adriatic Sea in the period of 1985–2015, including input from the largest river (Susquehanna and Po, respectively), input from all major tributaries, and direct wet precipitation.
The volume transports of major rivers flowing into CB and the NAS show strong seasonal variability. Seasonal peaks tended to occur during January–March and April–June in CB rivers (Figure 2.1c). In comparison, seasonal peaks tended to occur during April–June and October–December in the NAS (Figure 2.1d). Direct precipitation to the sea surface was estimated to account for 14–29% of seasonal freshwater input to CB and for 10–23% of the input to the NAS. For both systems, direct precipitation is more important in July–September relative to other months that have higher Q.
Freshwater inputs to CB and the NAS were further compared on a centennial scale using data from the Susquehanna and Po Rivers. During 1900–2015, the Susquehanna annual Q has an estimated Mann–Kendall (MK) trend slope of ‐0.0063 km3 year−1 (p = 0.79). On a seasonal basis, Susquehanna Q had negative slopes in all four seasons, i.e., ‐0.010, ‐0.038, ‐0.095, and ‐0.025 km3 year−1 in January–March, April–June, July–September, and October–December, respectively, with the latter three trends being statistically significant. During 1917–2015, the Po annual Q has an estimated trend slope of 0.016 km3 year−1 (p = 0.86). On a seasonal basis, Po Q was estimated to have positive slopes in January–March (0.0050 km3 year−1) and October–December (0.0006 km3 year−1) and negative slopes in April–June (‐0.012 km3 year−1) and July–September (‐0.015 km3 year−1), with the July–September trend being statistically significant.
2.3. NUTRIENT INPUTS
2.3.1. Recent Status: 2004–2012
Median annual inputs of nutrients and sediment in 2004–2012 to CB and the NAS are summarized in Table 2.1. NOx (nitrate + nitrite) accounts for most of the TN load, whereas dissolved inorganic P (DIP) is a minor fraction of the TP load. Among the nine major tributaries, the Susquehanna contributes about 65% of TN and 46% of TP (Zhang et al., 2015). Relative to riverine inputs, inputs from direct precipitation to CB are small (7% of TN and 17% of TP).
Table 2.1 Median annual inputs of freshwater (Q, km3) and nutrients (106 kg year−1) to Chesapeake Bay (CB) and the northern Adriatic Sea (NAS) due to river runoff and precipitation (PPT) for the years 2004–2012
| Parameter | CB | NAS | |||||
|---|---|---|---|---|---|---|---|
| Rivers | PPT | Total | Western rivers | Northern rivers | Eastern rivers | PPT |
|
