Mechanical weathering of polystyrene microplastics intensifies biological stress but enhances ecosystem multifunctionality in a freshwater-sediment microcosm.

Microplastics are ubiquitous pollutants in freshwater ecosystems, yet their impacts on freshwater-sediment ecosystems remain poorly understood. Here, we investigated how pristine and mechanically weathered polystyrene (PS) microplastics influence organisms, sediment properties, and overall ecosystem multifunctionality (EMF) within an integrated freshwater-sediment microcosm containing the rooted macrophyte Vallisneria spiralis, the benthic snail Lymnaea stagnalis, and sediments. Results showed that mechanical weathering transformed the originally smooth spherical PS microplastics into particles with rougher and more irregular surfaces. Weathered PS microplastics induced stronger biological stress in both plants and snails, reflected in higher integrated biomarker response (IBR) scores. Weathered PS microplastics reduced chlorophyll a and carotenoid content and also altered plant nutrient content, indicating impaired photosynthesis and altered carbon metabolism. In snails, weathered PS microplastics triggered oxidative stress and significantly increased caspase-3 activity, suggesting elevated apoptotic responses and ionic regulation demands. Despite organism-level impairment, both pristine and weathered PS microplastics enhanced sediment EMF. This enhancement was primarily driven by increased dissolved organic carbon, elevated β-glucosidase activity, and intensified microbial respiration, reflecting stimulated carbon cycling. These results reveal an ecological trade-off between organism health and ecosystem performance. Short-term functional stability was maintained through accelerated microbial metabolism, potentially masking underlying biological deterioration. Our findings demonstrate that microplastic weathering amplifies biological stress while simultaneously promoting ecosystem-level functional buffering, highlighting that apparent stability may entail long-term ecological risk. Integrating environmentally representative mechanically weathered microplastics with multi-species ecosystem assessments provides a more comprehensive framework for evaluating the ecological risks of microplastic contamination.