Singapore's urbanised coral reefs: Changes in heavy metal pollution between 1994 and 2021.

The heavy metal load in coral reefs of Singapore has not been comprehensively assessed since 1994. Here, we repeated the surveys conducted 27 years ago to quantify the levels of Cd, Cu, Cr, Fe, Pb, Ti, Zn in sediment and the sea urchin Diadema setosum from seven reefs. Cu and Cd showed significant reductions, Pb and Cr remained stable, while Fe, Ti and Zn had increased significantly. Overall, based on the Pollution Load Index (PLI), Singapore's reefs would not be considered polluted. Nevertheless, elevated concentrations of Cu, Pb and Zn were detected in sediment, with Cu and Pb exceeding the Threshold Effect Level. The spatial distribution of metal loads between sediment and sea urchin tissues were decoupled, underscoring the complexity of metal uptake and bioavailability. We reveal a mixed temporal trend among the heavy metals examined, each presenting different toxicity potentials and hence risks to local marine assemblages.