Microplastic pollution is an increasingly important component of urban atmospheric contamination, yet its fine-scale spatial patterns remain poorly understood. Spider webs have recently emerged as effective passive samplers of airborne particles and may provide a low-cost tool for biomonitoring atmospheric microplastics. We investigated microplastic occurrence in spider webs along an urbanization gradient in Torino (NW Italy) to assess the influence of landscape and local environmental factors. Spider webs were collected from shrubs across a range of urban intensities. Urbanization was quantified as impervious surface cover at multiple spatial scales, while local variables included shrub area and web type. Microplastic presence was assessed under a stereomicroscope and analysed using generalized linear mixed models (GLMMs). To evaluate whether webs intercept biological and abiotic airborne material through similar mechanisms, we also analysed prey capture patterns.

A total of 547 webs were collected, of which 42% contained microplastics. Microplastic occurrence was higher in spring than in autumn and was more frequent in sheet webs. Analyses restricted to adult female webs collected in spring showed that urbanization intensity at the 1000 m scale and shrub area were significant positive predictors of microplastic occurrence at both web and site levels. Sheet webs also had a higher probability of containing microplastics than other web types. Overall, 1,943 prey items were recorded. Prey capture was strongly influenced by web architecture, with sheet webs capturing significantly more prey, whereas urbanization had no significant effect on prey abundance. At the site level, prey density increased with shrub area and artificial light intensity.

Spider webs therefore provide a robust, fine-scale tool for monitoring atmospheric microplastic pollution while highlighting differences between the drivers of biological and abiotic particle interception.