Modelling the mercury biogeochemical cycle in coastal and marine environments

The alteration of mercury (Hg) biogeochemical cycle induced by anthropic activities has caused widespread concern, owing to methylmercury (MeHg) toxicity that poses a human health hazard. Due to its speciation among oxidized (HgII), reduced (Hg0) and organic compounds (MeHg, Me2Hg) mercury has a complex cycle that causes its global distribution in the biosphere and its biomagnification along marine food webs, eventually resulting in human exposure through fish consumption. Significant advances have been made over the past decades that have improved our knowledge of biogeochemical mechanisms controlling environmental levels of Hg species. However, synergistic and antagonistic effects among processes and environmental variability often act as confounding factors in the understanding of such mechanisms. Mathematical models provide a thorough framework for the investigation of biogeochemical cycles from a holistic perspective, taking into account the most relevant processes while allowing discerning among the contributions of individual factors affecting modelled concentrations. The goal of this dissertation is to gain insight into the processes that affect Hg cycle, by analyzing case studies of coastal and marine environments. Available observational data were used to simulate Hg dynamics at the study sites by means of a dynamic box model that simulates Hg fate and transport (WASP7). Here is presented the model implementation for 4 case studies, which overall highlights how different processes dominate the Hg cycle across ecosystems, depending on hydrodynamic features, local pollution sources and other biogeochemical forcing. Chapter 1 provides an overview of the Hg cycle and of the most recent findings, with a comparative perspective on coastal and ocean dynamics. Chapter 2 presents a model implementation in a 2D domain, aimed at representing a shallow coastal lagoon of the Northern Mediterranean Sea affected by historical Hg pollution (Marano – Grado Lagoon). The model is used to estimate present-day concentrations and fluxes of Hg and MeHg, evaluating the contribution of the lagoon to the wider cycle of Hg in the Mediterranean area. In Chapter 3, the implementation of the lagoon developed in Chapter 2 is extended to perform long-term scenario analysis (100 years), with the aim of evaluating possible outcomes of both climate change and a management action such as removal of riverine Hg input. Chapter 4 investigates the dynamics of Hg and MeHg under changeable redox conditions, in the permanently stratified water of the Black Sea, focusing on the open question of the occurrence of Hg methylation in sulfidic environments. In Chapter 5 the model is applied to a heavily polluted harbor of the Southern Mediterranean Sea (Augusta Bay) subjected to fishing ban. The model is used to estimate MeHg concentrations and, together with measurements of HgT in the trophic web, it is employed to derive bioaccumulation factors (BAF) and investigate biomagnification.