Untangling the Toxicity Dilemma of the Orbetello Lagoon Sediments in Paracentrotus lividus Bioassay: Trace Metals vs. Ammonium

This study assesses sediment toxicity in the historically contaminated Orbetello Lagoon (southern Tuscany) using Paracentrotus lividus embryo development bioassays. Elutriates from 15 sites were analysed for trace metals, organic matter, and ammonium. Despite elevated mercury concentrations, toxicity did not consistently correlate with metal levels. Instead, Principal Component Analysis (PCA) identified ammonium as a key driver of developmental toxicity, suggesting that it significantly influences both biological effects and metal bioavailability. These results demonstrate that ammonium, often overlooked, can confound sediment toxicity assessments and should be integrated into risk evaluation frameworks for coastal systems affected by legacy pollution.