Mémoire

Abstract

The North Sea, a semi-enclosed basin with densely populated coastlines, faces substantial anthropogenic pressures due to high urbanization, offshore hydrocarbon extraction, wind farms, navigation, and fisheries. The increasing demand for Critical Raw Materials (CRMs), including Rare Earth Elements (REEs), driven by new technologies and the global energy transition, raises concerns about their environmental impact. Harbour porpoises, with their long lifespan and role as apex predators, serve as effective bioindicators for trace element contamination in the marine food web. However, there is a significant knowledge gap regarding the temporal trends of CRM contamination in marine mammals within the North Sea. This study aims to address this gap by identifying which tissues accumulate CRMs most significantly, examining whether these elements show an adult accumulation trend, and comparing CRM levels in harbour porpoises from two periods: 1999-2001 and 2019-2024. We quantified concentrations of 33 CRMs and 10 other chemical elements in the liver and muscle tissues of 41 harbour porpoises collected from Belgium and Germany using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Results indicate that the liver accumulates higher CRM levels than the muscle, with Bi, Co, Sc, Sm, Ce, La, Nd, and V showing higher concentrations in adults. Temporal analysis revealed significant increases in liver concentrations of Al, Nb, and Sb by 9%, 9%, and 100%, respectively, while no temporal trends were observed for REE, suggesting they do not bioaccumulate in the marine food web. Thus, marine mammals, alongside other species like mollusks, could serve as effective bioindicators for Light Rare Earth Elements (LREE) in the North Sea, but not for Heavy rare Earth Elements (HREE). Although REE concentrations are low, potential toxicity, especially in combination with other contaminants like mercury and organic pollutants, warrants further investigation.