Plastic pollution shapes ecosystem response to a storm event in intertidal sediments

Plastic pollution is an emerging driver of ecological change in coastal sediments, yet real-world ecosystem responses remain poorly understood. During a two-month in situ experiment on an intertidal sandflat in Mahurangi Harbour, New Zealand, a storm event provided a unique opportunity to test whether plastics modulate ecosystem responses to short-term environmental change. We measured sediment oxygen consumption (SOC), nutrient and N₂ fluxes, pore-water nutrient pools, and sediment organic matter in control and plastic-polluted plots across six time points spanning pre- and post-storm conditions. Plastic pollution was realised through vertical insertion of polyester nettings (6 × 2 cm, 10 µm thick, microplastic) into the sediment. Multivariate analyses revealed an interaction effect between treatment and sampling time indicating that the plastic-polluted sediments reacted differently to the storm. In control plots, the storm reduced SOC (− 24%) and pore-water NH₄⁺ (− 53%), and increased NH₄⁺ efflux (+ 186%) and NOₓ − pools (+ 166%). Compared to the storm response in the controls, the storm impact on plastic plots caused a 43% increase in SOC and a 53% increase in NH₄⁺ efflux, and lower pore-water NOₓ − (− 63%). These results demonstrate that plastics can modulate benthic ecosystem responses to storms, potentially altering nitrogen source–sink dynamics. As both plastic pollution and storm frequency intensify, such interaction effects may have growing consequences for coastal ecosystem functioning.