Modelling the biogeochemical cycle of mercury in the marine environment

Modelling the biogeochemical cycle of mercury in marine environment has recently become a challenge of paramount importance for the scientific community. In the last years some authors reproduced the mass balance of Hg in the marine ecosystems by using biogeochemical models based on interconnected zero dimensional boxes (e.g. WASP models). In particular, we used this approach in previous studies, to calculate the mass balance of mercury in the highly contaminated AugustaBay (southern Italy). This work aims to reproduce the biogeochemical cycle of different mercury species (elemental, inorganic, methyl-Hg) in 3D domain of the Augusta Bay by applying innovative mathematical tools. Specifically, an advection-diffusion-reaction model for Hg in seawater was coupled with a diffusion-reaction model for Hg in sediment pore water, in which the metal partition coefficients between dissolved (water and pore water) and particulate phases (Suspended Particulate Matter and sediment particles) were taken into account. Also, the effects on mercury concentration induced by seasonal variations of the most important environmental variables and the dynamics of de-adsorption of total mercury in the solid phase of the sediment were intensively explored. In order to better simulate the Hg chemical-physical processes, we calibrated the model parameters on the basis of both the experimental data collected in the Augusta Bay from May 2011 to October 2017, and the data reported for other polluted areas. Hence, we reproduced the spatio-temporal behaviour of (i) the concentrations of mercury species dissolved in the seawater and in pore water, (ii) the total mercury concentration in seawater, (iii) the total mercury adsorbed in sediments, and (iv) the horizontal and vertical mercury fluxes at the boundaries of the Augusta basin. A quantitative and statistical analysis between the theoretical results and field observations was made for the mercury concentration both in seawater and in pore water, and for the vertical and horizontal mercury fluxes at the boundaries of the 3D domain. The results showed a good agreement between the numerical and the experimental data. Finally, theoretical results evidenced a key role played by seasonal changes of environmental variables in terms of Hg geochemical dynamics and fluxes at the various interfaces.