## 4 Discussion  
The source of SOM, FHA, BHA, and FA was identified based on their \(\delta^{13}\)C values and TOC/TN molar ratios. Figure 10 shows the cross plots of \(\delta^{13}\)C values against TOC/TN for the four components, superimposed to the fields corresponding to the geochemical signatures of different contributors to the aquatic organic matter, namely C3 and C4 plants, and freshwater and marine DOC, POC, and algae. Their characteristic \(\delta^{13}\)C vs. TOC/TN fields are after [18]. Marine compounds are \({}^{13}\)C-enriched relative to their freshwater counterparts, whereas TOC/TN ratios in plants are higher than in algae. Overall, SOM and BHA showed a mixed (freshwater-marine) signature, the latter displaying a slightly higher marine contribution (Figure 10). FHA showed a dominant marine signature, with the exception of a few samples characterised by a freshwater signature. One of these samples was collected in Mar de Cubatao and showed a typical freshwater-algal signature (\(\delta^{13}\)C\({}_{\rm FHA}=-28.0\)%), whereas the others were collected in the NE sector of the study area (Mar Pequeno), thus suggesting the influence of the freshwater input from Valo Grande Canal. FA showed a mixed \(\delta^{13}\)C signal and the highest TOC/TN ratios, reflecting the geochemical signature of C3 plants. This indicated that FA likely originated from the decay of organic matter derived from local mangroves, in line with previous data of [48], pointing out that mangrove plants leaved a significant isotopic imprint (\(\delta^{13}\)C \(\sim-25.6\)%) in the sedimentary organic matter of the study area. Regardless their source, both SOM and HS exhibited a distinct SW-NE \({}^{13}\)C-depletion trend of the order of 1.5 to 2.0% (Figure 5). This \({}^{13}\)C-depletion trend clearly reflected the increasing contribution of terrestrial organic matter linked to the freshwater input from the Valo Grande Canal. This behavior was also confirmed by the statistical correlation between the \(\delta^{13}\)C values of SOM, FHA, BHA, and FA. \(\delta^{13}\)C\({}_{\rm SOM}\), \(\delta^{13}\)C\({}_{\rm FHA}\), \(\delta^{13}\)C\({}_{\rm BHA}\), and \(\delta^{13}\)C\({}_{\rm FA}\) showed Pearson's correlation coefficients (\(r\)) of 0.89, 0.91, and 0.91, respectively (Table 1). The positive correlation was even higher (\(r\) = 0.90-0.98) among \(\delta^{13}\)C\({}_{\rm FHA}\), \(\delta^{13}\)C\({}_{\rm BHA}\), and \(\delta^{13}\)C\({}_{\rm FA}\). \(\delta^{13}\)C\({}_{\rm FHA}\) showed a negative correlation (\(r\) = \(-0.82\)) with clay, which reinforced the conclusion drawn from their geochemical signature that FHA were dominantly of marine source. The concentration of FHA showed negative correlation (\(r=-0.92\)) with sand and positive correlations with silt and clay (\(r=0.92\)). Since FHA exhibited a dominant marine geochemical signature (Figure 10), the positive correlation with fine sediment may suggest that FHA underwent adsorption onto mud particles. Interestingly, the concentration of FHA was negatively correlated (\(r=-0.83\)) with \(\delta^{13}\)C\({}_{\rm FHA}\) values (Table 1), suggesting that \({}^{13}\)C-depleted FHA (i.e., FHA with a weaker marine signature) were more abundant. In terms of metal content, FHA was the HS fraction with the maximum concentrations of bound Cu, Zn, and As (Figures 6,8,9 and Section 3.4.1). These metals showed no correlation with one another, nor were they correlated with Pb, Cr, V, and Mn, with the exception of Cu and Pb (\(r=0.77\)) (Table 2). This suggests a common origin for Cu and Pb, which was likely derived from mining residues. In turn, a positive correlation (\(r=0.76\)-\(0.94\)) was observed among Cr, V, and Mn (Table 2), which was possibly derived from a common binding mechanism. In fact, contrary to Cu, Zn, and Pb; As, Cr, and V exist in the environment as oxyanions, whereas soluble Mn species include ionic couples with bicarbonate and sulphate ions [49].