OCCURRENCE OF TRACE AND RARE EARTH ELE-MENTS (REE) IN MARINE FAUNA

This doctoral thesis investigated the distribution, bioaccumulation, and ecological effects of trace elements and rare earth elements (REEs) in three Mediterranean ecosystems character-ized by different hydrodynamic conditions: the Tyrrhenian Sea (open environment), the Venice Lagoon (semi-enclosed environment), and the Estany dels Ponts lagoon in Mallorca (confined environment). Due to the pronounced ecological and hydrodynamic differences among these sys-tems, different biological indicators were selected for each study site according to their ecological relevance and methodological suitability. Integration among the three case studies was achieved at the level of environmental processes and contaminant behavior, rather than through a direct comparison of biological indicators, which differ across the studied environments. In the Tyrrhenian Sea, phytoplankton samples showed variable concentrations of essential (Fe, Zn, Cu, Mn, Ni) and non-essential (Al, As, Cd, Pb) elements, distributions dependent on depth and seasonality. Rare earth elements were more concentrated in the uppermost 30 m of the water column with light REEs (LREE–La, Ce, Nd) predominating over heavy REEs (HREE), indicating both natural and anthropogenic influences. REE concentrations were significantly higher in samples collected at 30 m depth compared to 50 m (one-way ANOVA, p < 0.001; Table 2), with ΣREE values ranging from 24 to 70 μg kg⁻¹ at 30 m and 7.0–11 μg kg⁻¹ at 50 m. The distinction between natural and anthropogenic contributions is based on the observed vertical and seasonal distribution patterns, the predominance of LREEs over HREEs, and the occurrence of gadolinium enrichment, which is widely recognized as a marker of anthropogenic inputs related to medical applications. In the Venice Lagoon, analyses of zooplankton (Acartia clausii and Acartia tonsa) revealed greater contaminant retention compared to the open environment, with significantly higher LREE concentrations in sediments and copepods than in water. Seasonal and spatial differences indicate that limited water circulation and strong anthropogenic pressure favor the accumulation and bioavailability of these elements. In the hyper-confined system of Estany dels Ponts, benthic foraminiferal assemblages re-flected localized metal enrichment under confined eutrophic conditions without exhibiting sig-nificant morphological abnormalities, supporting their role as integrative long-term bioindicators in confined lagoon environments. Overall, the results demonstrate that trace element and REE distribution and bioaccumulation are strongly modulated by the degree of hydrodynamic confine-ment and anthropogenic pressure. The principal contribution of this thesis lies in the generation 9 of new datasets on REEs in Mediterranean ecosystems and in their interpretation within a process-based framework linking hydrodynamics, contaminant retention, and biological response. The findings also highlight important regulatory implications, as the absence of environmental quality standards for REEs represents a critical gap in current European legislation, supporting the need to include these elements in monitoring programs under the Water Framework Directive and the Marine Strategy Framework Directive. The persistence and bioaccumulation potential of REEs are the primary reasons for their ecological importance in marine ecosystems. Although the con-centrations found in this study were generally low, their retention in semi-enclosed and confined systems may increase chronic exposure pathways, possibly causing sub-lethal physiological ef-fects and long-term ecosystem changes rather than acute toxicity events