## 3 Nitrogen and Phosphorus Conditioning in the Sea  
The period of significant social changes, robust post-World War II industrial development, and the subsequent globalization of trade from the 1950s to the 1990s led to a substantial increase in the consumption of N and P for various purposes. Notably, N was extensively used as a fertilizer in agriculture, and its subsequent mineralization in surface and ground water, rather than industrial discharges into the atmosphere and urban wastewater, resulted in a substantial input of N watercourses eventually reaching the sea [8]. This pattern was also observed in the Northern Adriatic basin. As for P, its primary uses were prevalent in domestic environments and industry [8]. N and P are two elements that exhibit different behaviors once emitted into terrestrial and aquatic environments. The fate of P in the soil is primarily determined by chemical processes, such as adsorption/desorption and dissolution/precipitation. In contrast, the fate of N is primarily governed by biological processes including mineralization, nitrification, and denitrification [59]. Indeed, the extensive use of P and N during the latter half of the 20 th century, along with the effects of climate change, has had significant implications for the Northern Adriatic Sea. The impact of climate change has become particularly evident in this region, starting from the end of the 20 th century [36; 60]. Notably, these changes are characterized by intense precipitation events, especially in spring and autumn, which significantly influence river outflows, leading to the transport of freshwater and suspended sediment into the sea [18; 61; 62]. As a consequence, these freshwater and sediment inputs carry pollutants, including N and P [19; 23; 63; 64]. The combination of nutrient inputs from anthropogenic sources and climate-induced changes in hydrological patterns has contributed to alterations in nutrient concentrations and dynamics within the Northern Adriatic Sea. Figure 3: Simplified description of the seasonal surface circulation in the Northern Adriatic Sea, image adapted from [15] in agreement with [57; 58]; blue (WAC, Western Adriatic current) and light blue (EAC, Eastern Adriatic current) for winter conditions; green for summer conditions, with eddies along the western coast. In Italy, the total use of P in domestic detergents was prevented through Ministerial Decree n. 413 of 13 September 1988, which took effect from January 1989. Previously, from 1985 to 1988, there were other gradual measures implemented to reduce the use of P. Subsequently, in 2012, Europe adopted restrictive measures through Regulation 259/2012/EC, setting limitations for P of 0.5 g/ dose in domestic laundry detergents and 0.3 g/ dose in domestic dishwasher detergents. These regulatory measures have significantly contributed to the reduction of P inflow in surface watercourses up to the sea. As a result of this reduction, observable effects were recorded in the coastal strip with particular evidence in the Western North Adriatic Sea. The predictions made by [PERSON] & Ferrari [9] anticipated a decrease in P levels by the year 2020. Subsequent observations from the late 1990s to the 2000s showed a leveling off of P inputs into the sea, followed by a downward trend in the first two decades of the 21 st century. However, the same trend was not observed for N. Despite the study by [PERSON] & [PERSON] [9] and subsequent works (e.g., [46]), N levels did not exhibit evident decreases, except in the strictly coastal strip around 3 km from the coast, where observations by [PERSON] et al. [23] demonstrated lower river flows and consequently reduced N and P loads. Over the years, the annual runoff of the Po River has exhibited significant fluctuations on a multi decadal time scale, with an overall decrease (\(-33\%\)[65]) observed in all rivers of the Northern Adriatic when comparing recent discharges to those before the 1980s.