## 2 Study Areas and Historical Information  
### The Augusta Harbor  
The Augusta Bay (Figure 1a) is a semi-enclosed natural basin occupying around 30 km of the eastern coast of Sicily (Ionian Sea, southern Italy) and covering an area of approximately 4000 ha [29; 30]. The Augusta Harbor is the northern sector of the bay and was sheltered in the early 1960s with the construction of artificial breakwaters. Since then, the harbor is connected to the bay and the open sea only through two artificial inlets, called Scirocco (in the south, 300 m wide and 13 m deep) and Levante (in the east, 400 m wide and 40 m deep), respectively [30]. The Augusta Harbor is about 24 km\({}^{2}\) and the average water depth is approximately 15 m, with the maximum depth (40 m) in correspondence of the Levante inlet [31]. Exchanges with the open sea are mainly driven by tidal fluctuations and are consequently correlated with the entry/exit of tidal flows and relative amplitudes. Open sea waters enter predominantly from the Levante inlet (mean speed of 18 cm s\({}^{-1}\)), then flow northward, parallel to the dam, slow and constantly with their pace and exit at the Scirocco inlet at a speed of 5-6 cm s\({}^{-1}\)[32]. The north-western sector of the harbor is locally influenced by the poor seasonal outflows of the Mulinello, Marcellino, and Cantera streams but their contribution is generally poor, with localized hydrodynamic effects [31]. Due to its confinement, the hydrodynamic regime of the Augusta Harbor is scarce, favoring the accumulation of nutrients and pollutants in sediments and the occurrence of eutrophication episodes [31; 33].  
The Augusta Harbor hosts one of the most important ports of the Mediterranean Sea, and many industries are located along its western part, including petroleum and petrochemical plants. The industrial development started in the 1950s with the construction of oil refineries and proceeded in the 1960s with the onset of a Mercury Cell Chlor-Alkali Plant (MCCAP), almost contemporary to the construction of the outer breakwaters [34]. Sediment dredging, disposal, and partial nourishment of the western coast have further modified the pristine environment of the bay. Indeed, 3/5 of the port surface (approximately 14.4 km\({}^{2}\)) have been dredged so far while the dredged material might have been released over a large area of the bay on the way to offshore dumping sites (yellow areas; Figure 1a) [27].  
Since the 1970s the area has become internationally recognized as a polluted environment from industrial and petrochemical plants, but also from agriculture and urban wastes, with consequent risks to the ecosystem and human health [30; 34; 35]. Specifically, mercury contamination from the MCCAP in the southern sector of the Augusta Harbor (Figure 1a) has produced sediment concentrations beyond international levels in parts of the area and is by far the most important environmental issue [27; 34; 36]. It is estimated that \(\sim\)500 t of Hg have been discharged to the Augusta Harbor [30; 34]. In the 1980s the democverization and waste treatment plants started working in response to the Italian anti-pollution law 319 of 1976 (aka \"Legge Merli\", in Italian), and discharged treated wastewater outside the harbor. Owing to the high state of environmental degradation, this area was included in the 2002 National Remediation Plan by the Italian Environmental Ministry, then the MCCAP closed in 2005 with the removal of the Hg cells [32; 34].  
In the early 2000s, scientific evidence became necessary to support legal investigations against the industries that caused the pollution and who could potentially be expected to contribute to remediation costs. In 2003, researchers from CNR-ISMAR Bologna started work to provide such evidence.