Halocline-associated microplastic accumulation in a weakly mixed estuary: Insights from the Oyabe River, Japan

Microplastic (MP) pollution is a critical global issue, yet its oceanic concentrations are far lower than predicted riverine inputs due to temporary estuarine retention. Currently, the vertical behaviour of MP in weakly mixed systems remains poorly understood. In this study, we investigated the Oyabe River estuary in Toyama Bay, a microtidal salt-wedge estuary where haloclines frequently develop. Fine-interval vertical sampling using a land-based pumping system captured the thin halocline layer, revealing a mean MP abundance of 2.0 ± 2.5 items m^-3, peaking at 3.0 ± 2.6 items m^-3 in the halocline. This vertical pattern (halocline > freshwater > seawater) exhibited the highest polymer diversity, mainly dominated by PE and PP, including PA6 and PU; this indicates that halocline retention acts as a sink buffer. Film-like fragments predominated, likely remaining suspended longer and more readily trapped within stratified layers. MP accumulation intensified when the halocline was thin and sharply defined. Strong stratification developed when river discharge exceeded 40-45 m³ s^-1, values typical of the Oyabe River under normal flow. These findings identify weakly mixed estuaries as dynamic filters of MPs. In systems such as the Oyabe River, halocline-associated retention promotes prolonged accumulation and enhances MP deposition in coastal and benthic environments.