Simulating microplastic distribution in oceans: The impact of morphology and sources

Abstract

Microplastics are widespread in the ocean with known accumulation hotspots in the subtropical gyres, yet their horizontal and vertical distribution is not fully understood. Microplastics are found under various shapes and sizes, reflecting multiple sources. This morphological variability influences their ocean distribution and hence needs to be considered in global models. Here, we studied the influence of microplastic morphology on its global oceanic distribution with the global numerical model NEMO/PISCES-PLASTIC. We explicitly simulated the ocean distribution of microplastics of different morphologies (fibres versus fragments) reflecting their source categories (laundry and household dust fibres, personal care products, car tyre wear, fragments from larger objects) and source vectors (rivers and continental runoff). Our results demonstrate that accounting for the variability in microplastics vertical behaviour linked with their morphology significantly influences the global spatial distribution and budgets of microplastics. Furthermore, our results indicate that fibres, that constitute a large proportion of the primary MPs, are the dominant microplastics in the mesopelagic layer (100-1000 meters water depth). Finally, global microplastic budget derived from our model simulations show that it is essential to consider in situ fragmentation of macroplastics into microplastics to reconcile the latest assessment of microplastic sources in the ocean with existing estimates of the ocean microplastic budget. These conclusions have strong implications for marine plastic pollution assessments and mitigation measures.