Modelling nitrogen fluxes during tidal cycles in a salt marsh in the northern Venice Lagoon

The Venice Lagoon is a biodiverse, valuable and delicate environment which is currently threatened by several factors, including progressive erosion, eustatism and other anthropogenic pressures, for example pollution and eutrophication. The salt marshes system of the Lagoon, which provides a series of relevant ecosystem services, is one of the most endangered habitats of this ecosystem. Mathematical modelling of the biochemical processes occurring at these sites is recognised as one of the key activities that are needed to gain more solid knowledge on their influence on the nutrient balance of the Lagoon and on the interventions that are most suitable for protecting its ecosystem. In this thesis work, fluxes of nitrogen dissolved forms were modelled for a specific salt marsh located in the northern part of the lagoon, for which a set of experimental data collected in several sampling campaigns, spanning from year 2015 to 2020, was already available. A CSTR approach was adopted to represent the hydrological and biogeochemical dynamics occurring on the aboveground part of the salt marsh at the timescale of the monitored tidal events. The concentrations of nitrogen dissolved forms, categorised as ammonium ion (NH4+ ), nitrates (NOx) and dissolved organic nitrogen (DON), were set as state variables for the model, and their mass balances were described through a system of ordinary differential equations, to be solved numerically. A selection of the processes and of the forcing factors that are relevant for the aforementioned balances were carried out with the use of a global sensitivity analysis, through the adoption of Latin Hypercube Sampling followed by a multilinear regression procedure; the rate of the relevant processes were adjusted through a parameter calibration, which consisted in the minimization of an objective function with a Particle Swarm algorithm. The calibrated model performance in the simulation of the concentration of different nitrogen forms in the studied tidal events was generally quite satisfying in terms of quantitative accuracy, whereas it showed a lower reliability in terms of matching the temporal patterns of observations. Available data and model results agree in describing the tendency of the studied salt marsh sub-basin to determine a net consumption of dissolved ammonia and nitrates, and a net release of dissolved organic nitrogen. These findings suggest that salt marshes could play an important role in the biogeochemical cycling and ecology of the lagoon areas where they are sited, since dissolved inorganic nitrogen uptake can contribute to limit aquatic primary production in their surrounding waters.