A century of sediment metal contamination of Mar Menor, Europe's largest saltwater lagoon

Coastal enclosed ecosystems, such as lagoons, are vulnerable to anthropogenic impacts because they favor the accumulation of contaminants from the surrounding watersheds, particularly in their sediments. Europe's largest saltwater lagoon, the Mar Menor (SE, Iberian Peninsula), is a highly impacted ecosystem and the first in the continent to be granted personhood rights. Based on a high-resolution spatial and temporal dataset, we present the historical reconstruction of metal contamination in this ecosystem during the last century. Our results highlight that sediment metal contamination has been mainly driven by the development of the mining industry in the nearby Sierra Minera de Cartagena-La Unión in the late 19th and until the mid-20th century when premeditated mining spills were forbidden. Runoff from former mining areas still transported metals to the lagoon even after mining ceased in the 1990s. The southern sector of the lagoon, closest to mining-affected ephemeral streams, is the most impacted by metal contamination and holds the highest metal stocks. Stocks since 1900 for the entire lagoon reached values of 9200, 1.6, 450, 270, 10,000, and 12 tons of Pb, Hg, As, Cu, Zn, and Ag, respectively. Maxima concentrations were reached in the mid-20th century, with values of 3400, 0.53, 100, 50, 3700, and 5.5 mg·kg-1 for Pb, Hg, As, Cu, Zn, and Ag. Afterward, while some metals' concentrations declined, others were still supplied to the sediments through runoff from former mining areas and sources related to urban expansion. Metal concentrations reported in this study surpass sediment quality guidelines and are generally higher than those found in similar ecosystems globally. Current surface metal concentrations are lower than during most of the 20th century, and sediments seem to retain metal stocks effectively. However, climate change and eutrophication could increase the risk of metal remobilization, further impacting this already vulnerable ecosystem. This study emphasizes the need for attention to managing coastal enclosed ecosystems, where global change impacts can exacerbate the impact of legacy contamination.