Influence of anthropic pressures and hydrological variability on nitrogen and phosphorus transport throughout watersheds in the Po River District

Human development in the last century has significantly altered biogeochemical nutrient cycles at both global and local scales, leading to increased nutrient inputs to aquatic ecosystems and widespread eutrophication. However, eutrophication risk in aquatic ecosystems is a multifactorial issue, depending not only on the absolute load of N and P, but also on their relative proportions and molecular forms, and on their temporal distribution. Nutrient load generation, transport, and export are influenced by hydrology, human pressures, catchment characteristics, and seasonality, which also contribute to defining nutrient pool composition and stoichiometry. The general aim of this thesis was to improve the understanding of the processes that regulate the generation, transport and transformation of nutrients within watersheds. The analysis was conducted at various spatial and temporal scales to better characterize the ways in which changes in land use and management are changing the magnitude and timing of N and P delivery across gradients of both hydrological characteristics and land use. Specific aims were: 1) to assess the long term evolution of nitrogen and phosphorus loads exported from the Po river to the Adriatic sea in relation to precipitation patterns; 2) to quantify the spatial variability of anthropogenic nutrients input to watersheds and riverine loads and evaluate how different watershed features influence N and P export and their ratios; 3) to examine how artificial reservoirs in Mediterranean streams with intermittent fluvial regimes affect the transport of nitrogen, phosphorus, and silicon, as well as their stoichiometry, in relation to hydrology and to the level of reservoir impoundment. The analysis was conducted in watersheds located in the Po River Hydrographic District a territory, defined under the Water Framework Directive for water management, that is subject to various anthropogenic pressures, including a rapid expansion of urban areas and intensive agricultural production. The District covers an area of approximately 86,000 km2 and includes the catchment area of the Po River, which is the major river that drains into the Adriatic Sea. It is a historically impacted territory, with 47% of the land being anthropized. The population density is 232 individuals per km-2 and the livestock density is 48 livestock units per km-2. The District is composed of six different hydro-ecoregions, and three main climates: Alpine, Continental and Mediterranean. The analysis of the long-term N and P export regime of the Po River in relation to hydrological variability was first estimated to assess the role of precipitation pattern and seasonality in load transport. The study found that precipitation and seasonality affects nutrients export regime differently depending on their form. Additionally, the long-term trend shows an extension of the summer export condition at the expensive of autumn. The contribution of hydrology to nitrogen and phosphorus loads in relation to anthropogenic nutrient sources was then examined. The high spatial diversity of rivers N and P load cannot be solely explained by levels of net anthropogenic nitrogen and phosphorus input. This diversity depends on source type, runoff and, for P, also precipitation. TN as NO3- loads are mainly driven by feed trade and fertilizer application, while P depends on a mix of urban direct input and soil erosion during storms. Reservoirs built to sustain intensive agriculture can affect the characteristics of the hydrographic network, hydrological continuity, and seasonality of water resources in dry watersheds. A one-year experiment on two dammed reservoirs in the Apennines showed that managing the filling and emptying of reservoirs reduce flood effects and increase the release of particulate matter downstream, altering the N:P:Si stoichiometry and pool composition. Nutrient transformation does not necessarily reduce loads on an annual basis. This suggests that the reservoir can act as either a source or a trap, depending on nutrient and hydrologic patterns. Reservoirs managed for irrigation in a Mediterranean climate could change nutrient levels in the hydrographic network and disrupt natural hydrological connectivity, causing nutrient transport to be out of sync with seasonal dynamics. Shifts in seasonality can have significant implications for stream metabolism and nutrient dynamics in downstream waterbodies especially in terms of increased eutrophication risk.