The estuarine plastics budget: A conceptual model and meta-analysis of microplastic abundance in estuarine systems

Rivers and estuaries are a key transport pathway for microplastics to enter the world's oceans. Despite this, knowledge of the fate of plastic debris in estuaries is limited. Our study assesses the current state of knowledge on the distribution of microplastic debris in estuaries and their intertidal habitats. The literature is reviewed, and an estuarine plastics conceptual model is presented. The conceptual model is populated with data to show the globally averaged standing stocks of microplastic (<5 mm) debris in estuarine waters and sediments. Estuarine plastic debris studies have previously focused on subtidal sediment and surface waters more than intertidal habitats. Estimated mean microplastic abundance is found to be greater (H = 53.136, P = 0.000175) per volume of sediment than per volume of water, although there are large standard deviations within habitats and some caveats regarding variation in methods and limited data in published studies. We found that the most commonly recovered microplastics align with the most commonly used polymers, and that the rates of microplastic transfer between estuarine compartments remain poorly understood. This study informs global plastic transport models by demonstrating the ability of estuaries to trap large quantities of microplastic debris, but substantial differences in the methodologies between estuarine microplastic studies limit the value of global assessments of estuarine microplastics. The main recommendations from this study focus on improving data availability and standardisation to better enable study comparisons and to improve our ability to examine global trends.