Microplastic pollution in the urbanised reefs of Singapore with Diadema setosum as a biomonitor

Microplastic (MP) pollution poses a significant threat to marine ecosystems globally, with coral reefs being of particular concern due to their high biodiversity and capacity to 'trap' MPs. This study provides the first multi-matrix and biomonitor-based assessment of MP pollution in the degraded reef systems of Singapore, integrating sediment, seawater, and the sea urchin Diadema setosum as a biomonitor. Across seven coral reefs in the Singapore Strait, all sample types exhibited lower MP abundance than reported from most other regions. Sediment MP counts were positively correlated with the percentage of silt, corroborating past evidence that finer sediment enhances MP accumulation. Fibre and polyester dominated the MP composition, implicating synthetic clothing as a local source. MP abundance in urchin gonads decreased with increasing gonad weight-likely reflecting dilution from gamete proliferation-and gut MP levels were negatively correlated with gut sediment load, contrary to the assumptions about sediment as an MP sink. No significant co-occurrence was observed between MPs and heavy metals in gonad tissues. These results show that relatively low MP pollution can occur even in highly urbanised waters, while underscoring the need for method harmonisation to improve inter-study comparability.