*4.3. Surface Sediments*

Organic carbon content in the silt and clay fraction (<63 μm) of the surficial sediments of two inlets was low and ranged from 0.26% to 0.59%, whereas at the inner part and the outlet of the stream it ranged between 3.97%−8.20%, and 10.40%, respectively (Table 4). Organic-rich sediments are common in coastal lagoons [14], mainly due to the high productivity of these systems in relation to other coastal marine and estuarine systems.


Sd 3.3 17400 5800 5631 19.0 25.2 1.17 1.08 9.4 4.15 113 62.9 28.6 8.38

Total metals contents in the surface sediments followed the order (median values): Al (33100 mg·kg−1) > Fe (18200 mg·kg−1) > Mn (200 mg·kg−1) > Zn (75 mg·kg−1) > Cu (37.0 mg·kg−1) > Pb (25.3 mg·kg−1) Cd > 0.69 mg·kg−1), and varied widely on the spatial scale (Table 4). Aluminum, the structural component of clay minerals, ranged from 6790 to 62,500 mg·kg−1. The lower Al values were determined at the two inlets, due to the high abundance of carbonates and quartz at these sediments. The higher ones were determined at the central part of the lagoon (stations A7, A8), as well as the outlet of the stream (station A10), indicating the preferable accumulation of terrigenous aluminosilicates at these sites.

The large variation of Al contents suggests that granulometric and mineralogical differences exist among the sediment samples. This is despite the fact that sieving out the sand fraction provides theoretically a more homogeneous fraction of the sediment for metal levels estimation and distribution patterns analysis. Thus, normalization to Al was employed in order to further minimize grain-size effects [15,28].

Figure 6 shows the spatial variation of metal to Al (Me/Al) ratios. The highest Fe/Al values were observed at the outlet of the stream, whereas the lowest ones at the two inlets. A similar distribution pattern is observed for Cu. Copper correlated with Al (r = 0.936; < 0.0005) and Fe (r = 0.982; < 0.0005), suggesting their common transport pathway through land run-off and a similar deposition pattern in the surface sediments. Cadmium to Al, as well as Pb/Al ratio values were higher at the eastern part of the lagoon and exhibited maxima at the sediments of station A4. Values of Zn/Al were rather homogeneous throughout the lagoon, however, a local maximum was observed at station A4. Zinc correlated with Al (r = 0.918; < 0.0005) and Fe (r = 0.918; < 0.0005), which is indicative of their common origin from terrestrial sources.

**Figure 6.** Spatial distribution of (**a**) Fe/Al, (**b**) Mn/Al, (**c**) Cd/Al, (**d**) Cu/Al, (**e**) Pb/Al, and (**f**) Zn/Al values in the surface sediments of the Antinioti Lagoon.

The potential mobility of metals under changing environmental conditions, thus their (bio)availability, can be estimated by extracting the sediment samples with dilute HCl (0.5 N) [54]. The procedure extracts the fraction of metals retained in sediments by adsorption, complexation and precipitation processes [55].

The extractability of metals (i.e., the percentage of 0.5 HCl extracted contents in relation to total contents) in the surface sediments followed the order (median value): Fe (40%) < Zn (48%) < Cu (56%) < Mn, Cd (65%) < Pb (77%), suggesting that Fe and Zn were the most inert metals, whereas Pb, Cd and Mn the potentially more mobile metals under changing environmental conditions. The extractability of Mn at the sediments of the two inlets and the stream accounted for more than 90%. This substantial increase of the extractability in the inlets and the stream compared to the inner part of the lagoon (median value: 51%) suggests that in the former areas Mn is found in more labile phases, which are related to the flocculation processes discussed previously.

Dilute HCl extractable Cd, Cu, Pb and Zn contents were significantly correlated with OC content (r = 0.629–0.827; < 0.05), which is consistent with the findings of Yuan et al. [42]. The association of this fraction of metals with OC may result in the mobilization of metals along with the oxidation of OC. [8,18]. More insights into diagenetic processes are gained through the examination of core sediments.
