Tide-dominated estuaries as gateways and filters of plastic pollution to the Ocean: insights from the PLASTINEST project

Around three-quarters of oceanic plastic waste come from land-based sources and is mainly transported via rivers and estuaries. However, not all plastics entering estuarine environments reach the open Ocean. Plastic litter and particularly microplastics, can accumulate in estuaries, creating pollution hotspots. This retention may be especially pronounced in macrotidal estuaries, where strong landward residual flows enhance trapping. The ANR PLASTINEST project aims to advance our understanding of the transport, trapping, and dispersion of microplastics within estuarine environments dominated by tides. Using innovative field measurements, controlled physical experiments, and enhanced numerical modeling, PLASTINEST offers new insights into the physical processes that govern particle transport under varying environmental conditions. Ultimately, this research contributes to the ongoing scientific debate on the assessment of plastic river input to the ocean and the existence of a “missing ocean plastic sink”, which to this day ignores the potential trapping role of tide-dominated estuaries. In this work, we present the methodology and key results of PLASTINEST, through three work packages:In the first work package, laboratory experiments provided insights into the erodibility, bottom trapping, and settling dynamics of microplastics in the presence of muddy sediments characteristic of estuarine environments. Both, bottom resuspension and settling behavior were primarily influenced by particles physical properties—shape, density, and size. The presence of cohesive sediment has a secondary influence on microplastics transport in turbid estuaries by increasing the critical shear stress of microplastics deposited on the bed and by promoting flocculation in the water column. These two processes favor the retention and accumulation of particles in turbid estuaries.In work package 2, the spatio-temporal variability of microplastics is evaluated across a macrotidal estuary, in relation to key physico-chemical parameters, through ongoing field campaigns. A novel protocol for sampling microplastics has been implemented in the Gironde estuary, using an innovative in-situ filtration system. Data post-processing will elucidate on the role of hydrodynamics (tides, river discharge, longitudinal tidal and salinity gradient, vertical mixing) on microplastic concentration variability and distribution patterns within the estuary.Numerical modelling tools for the transport of plastic debris were improved to include key microplastic transport processes in work package 3. The implementation of these in the Gironde Estuary provided key insights on the role of environmental factors and transport mechanisms on the trapping and dispersion patterns of microplastics across different temporal scales (from intratidal to seasonal) for floating and sinking particles. Beaching, convergent currents and tidal pumping were identified as key trapping mechanism retaining particles inside the estuary. Interestingly, during wet period, the high river discharge flushes important amount of floating particles into the Ocean , while settling particles remain in the estuary.By synthesizing insights from earlier findings, PLASTINEST will provide a comprehensive view of how plastic particles are transported, accumulate, and are periodically exported to the Ocean within tide-dominated estuarine systems.