Assessing the Potential Entrapment of Meso- and Macroplastic Debris by Tidal Marshes in Shallow Estuaries

Plastic pollution is a critical global challenge, with annual production surpassing 400 million tonnes and only a small fraction properly managed. The remainder accumulates in terrestrial and aquatic environments, with rivers acting as major conduits to the ocean. Causing ecological, social and economic impacts that affect the fulfilment of the Sustainable Development Goals (SDGs). Estuaries, located at the land–sea interface, represent key zones for intercepting this flux. In order to address this environmental problem, given the strategic location of estuaries, this study assesses the potential of tidal marshes to retain meso- and macroplastic debris in shallow estuaries.

A multi-scale approach was implemented, combining controlled hydraulic flume experiments, field campaign, and GIS-based spatial modelling in the Santoña Marshes (northern Spain). Laboratory tests quantified plastic entrapment by salt-marsh vegetation (Juncus maritimus, Halimione portulacoides and Spartina maritima) under variable hydrodynamic condition representative of tidal flows varying water levels, flow rates and wind speeds, and the most common plastic materials found in salt marshes. Field surveys tracked floating buoys simulating plastics, under natural conditions, providing qualitative evidence of retention pathways. Geospatial analysis integrated one-year tidal flooding probabilities with vegetation distribution, local bathymetry, plant height, tidal patterns and laboratory entrapment results to estimate the probability of plastic reaching vegetated areas and its potential retention over a year, identifying possible accumulation hotspots.

Results indicate high entrapment efficiency under controlled conditions (≈90% per species), supported by field observations. Spatial modelling revealed significant variability across the entire marsh. Quantitative values for vegetation trapping potential were obtained with a resolution of 10 m. These results were visualised by classifying the area into categories of high, medium and low potential retention. Zones with high potential retention are associated with longer flooding durations throughout the year, which increase the likelihood of plastics reaching vegetated areas. These zones often coincide with the presence of secondary channels and dense vegetation, which slow water flow and enhance plastic trapping. Conversely, areas with low potential retention are typically located near the coastline, where flooding occurs for shorter periods, limiting the time available for plastics to interact with vegetation.

This work delivers species-specific, quantitative evidence of plastic trapping in estuarine environments, offering critical insights for management strategies and informing numerical models aimed at mitigating plastic pollution at the land–sea interface.