Environmental drivers of microplastic distribution within a highly urbanized reef system

Microplastic pollution has been an ecological problem since the early 1970s, with contemporary surveys indicating their presence in all major marine environments-including coral reefs. Among coral reefs, microplastic distribution is known to be heterogeneous, however, the environmental drivers for this variation remain understudied. Here, we determine the abundance of microplastics in the water column and sediment among urban reefs in Singapore across three geomorphic zones (i.e., the reef flat, upper slope, and lower slope). For the sediment compartment, the effects of water energy, reef rugosity, macroalgal cover, coral cover, and sediment size were assessed for their influence on microplastic distribution, with rugosity, macroalgal and coral cover representing novel parameters. We found, on average, 80.0 ± 97.0 microplastics kg^-1 and 9.0 × 10^-3 ± 12.5 × 10^-3 microplastics L^-1 (mean ± SD) in sediment and water, and show that abundances varied significantly among sites, but no significant differences were detected among zones. Microplastic abundance in sediment was significantly associated with fine sediments and water energy. There was no detectable association with benthic cover and rugosity-potentially due to the low relief of the macroalgal assemblage and extensive degradation of the study reefs. Considering the co-occurrence and detrimental effects of fine sediments and microplastics on reef-associated organisms, reefs with low water energy are possibly at greater risk than higher-energy systems.