**5. Discussion**

Antinioti Lagoon is a remote, shallow, non-industrialized and not particularly urbanized system that has a high biodiversity and is of major geochemical interest. The present assessment of heavy metals in the various compartments of this lagoonal system provides a series of conclusions and raises some critical issues to be addressed in forthcoming research projects that will be discussed hereafter.

The freshwater spring of groundwater origin that flows as a stream into the lagoon, is the major source of dissolved Mn, Cd, and Pb, as well as particulate (w/w) Mn, Cd, and Zn (Table 2). The importance of groundwater discharges as a transport pathway of nutrients, carbon, and trace metals has been acknowledged in other coastal lagoon settings [7,70] and they are considered as a rival contributor to riverine inputs of land-derived material into the ocean [45,71,72]. Groundwater seepage in the Antinioti Lagoon is a field for future research, in order to elucidate whether the enrichment ascribes to natural geochemical processes, i.e., diagenesis, or to anthropogenic activities (e.g., the use of Cd- bearing phosphate fertilizers) throughout its catchment area.

Trace metals participate in a series of critical, physical and geochemical processes that define their mobility and fate within the transitional fresh-saline water interface and beyond. Precipitation of Al, Fe, Mn oxyhydroxides, and flocculation are probably the most important ones. These processes are considered to be responsible for the rather unusual enrichment of large (> 8 μm) particles, both at the stream and within the lagoon (Figure 3). Precipitates of clays and Fe/Mn oxyhydroxides on large suspended solids (>8 μm), probably as composite coatings with organic matter, strongly affect the ability of this fraction of SPM to bind trace metals. Scavenging of Pb predominantly by Fe oxyhydroxides, Cd and Zn by Mn oxyhydroxides and Cu by Al and Fe/Mn phases explains the enrichment of the coarser particles (>8 μm) with metals in relation to the finer ones (<8 μm). Furthermore, the large particles were found to be less prone to desorption processes than the smaller ones. With increasing salinity, the partition coefficient of the small particles for Cd, and Zn decreased, as a result of desorption. In contrast, the variability of KD-L values was much smaller (Figure 5).

Scavenging of trace metals by large particles, with higher settling velocities than the finer ones, has serious implications on their transport and dispersion patterns across the system. Although the density of flocs is unknown and particle settling rates cannot be accurately estimated, in the grain-size range of 1–100 μm, the settling velocity of grains of ~100 μm is in the order of 10<sup>4</sup> larger than the settling velocity of particles of 1 μm [73]. Thus, the enrichment mechanism of large particles with metals, combined with the irreversible adsorption of Cd and Zn onto Mn oxyhydroxides may explain the efficient entrapment of Mn, Cd, and Zn within the boundaries of the lagoon, evidenced by their significantly lower particulate concentrations (w/w) in the inlets than in the inner part. The behavior of Cu was differentiated from Cd, and Zn in the way that desorption occurred from both fractions of SPM, causing the dispersal of Cu in the dissolved phase beyond the system. According to Roussiez et al., [53] the removal of Cu into solution was not restricted in the fresh-saline water interface but instead, continued well after the deposition of fluvial material through the degradation of organic matter with which Cu was originally bound.

Resuspension of bottom sediments is expected to cause a decrease of particulate trace metal contents due to the mixing of enriched brackish water particles with high trace metal contents with coarse particles with low metal contents of marine origin [74]; this was not the case in our study. Aluminum, Fe, and Mn contents of the coarser fraction of the SPM in the bottom saline water were higher than in the surface layer, despite the resuspension of sediments (Figure 4). Such an enrichment is attributed to flocculation processes occurring at high salinity regimes. Sholkovitz [75], with a series of laboratory experiments, showed that although the amount of flocculated Al is maximized at salinity 12, at higher salinities the removal of Al from solution reaches a constant value. In the case of Mn, its removal from the solution increases and levels off at salinities 15–25, whereas an additional removal occurs at salinities 27–30. At the same time, the finer fraction of SPM increased at the surface layer. The presence of fine-grained particles in suspension has been described by Eisma [73] as re-flocculation observed in several estuarine systems. According to the latter author, at the saline part of the estuary, flocs of fluvial origin are broken up by organisms that consume the organic matter gluing the particles together. Re-suspension of organic matter will then result in newly formed flocs with organic matter of estuarine origin. Although the exact mechanism is not known, flocculation at the seaward boundary of transitional water systems may have serious implications on the transport and dispersion of pollutants across a system, as suggested by previous studies [53,76–78].

In the sediments, the occasionally elevated values of EF for Cd and Pb, falling within the range of moderate modification in the surface sediments to severe modification in the core sediments, prioritize these elements in terms of environmental concern and pollution abatement measures. The labile fractions of Cu, Cd, Pb and Zn in the surface sediments were found to be associated with organic matter. Future studies should consider the analysis of planktonic samples to further elucidate the role of organic matter in the transport and accumulation of trace elements in the sediments. Nevertheless, the high organic carbon content in the core sediments (up to 12%) has been shown to influence the post-depositional distribution of total and labile fractions of metals. Accumulations of Cd coinciding with Mn depletions in gray-black sub-surface sediments, sugges<sup>t</sup> that the profiles of these elements are greatly influenced by diagenetic processes.

Looking to the future, decision making should balance the sustainability of values and functions of the lagoon and the risks associated to the remobilization potential of toxic trace elements from the sediments. Dredging of sediments for the maintenance of the fish overwintering trench could result in the oxidation of sub-surface sediments, with high potential of releasing significant amounts of metals that would pose consequent environmental risks for the biota as well as economic and health risks for the end consumers.

**Author Contributions:** Conceptualization, F.B. and M.S.; methodology, F.B.; software, F.B. and A.K.; validation, F.B. and V.P.; formal analysis, F.B.; investigation, F.B., V.P.; resources, M.D. and M.S.; writing—original draft preparation, F.B.; writing—review and editing, F.B., A.K., M.S.; visualization, F.B. and A.K.; supervision, M.S.; project administration, F.B.; funding acquisition, M.S. and M.D.

**Funding:** This research received no external funding.

**Acknowledgments:** We wish to thank Eleftheria Aperi, Olga Chalkiadaki and Paraskevi Skourti for their valuable help in the laboratory. We also thank people in the Kerkyra Island for providing assistance in the field and useful information about the study area.

**Conflicts of Interest:** The authors declare no conflict of interest.





**Table 2.** Spearman correlation coefficient (r) of particulate elements (w/w) in the > 8 μm fraction of particulates and significance level ( )

\*\* Correlation is significant at the 0.01 level (2-tailed); \* Correlation is significant at the 0.05 level (2-tailed).
