Microplastics dynamics under low freshwater discharge conditions in the Guadalquivir estuary

Estuaries behave retention zones for anthropogenic pollutants, including urban  waste and plastics. Under tidal influence, the river flow can be reversed depending on the strength of the tide (neap and spring conditions) and the freshwater discharge in the river basin. In the Guadalquivir River (south Spain), the basin is highly regulated, and most of its drainage area is constrain by a dam at the head of the estuary (ca. 100 km from its mouth). Here, we presents the results of a microplastic monitoring program in the Guadalquivir Estuary. Microplastic samples in sub-superficial waters (triplicates) were collected by net (200 microns mesh) on a bi-monthly basis (neap and spring conditions) in the period 2020-2022 (120 samples), under a wide range of environmental conditions. Most of the samples coincided with very low precipitation and freshwater discharge  periods. Microplastics concentrations varied from 0 to 7 items per cubic meter, mostly fragments and films, and comprised a majority of polyethene and polypropylene particles (75%). Microplastics variability was not correlated with river current speed, salinity and turbidity, indicating the complexity of processes involved in the dynamics of microplastics under low freshwater discharge, when the tide dominates the river flow. These results can be considered as a current baseline, where concentration of microplastics is under 1 item per cubic meter (Quartile 3) for extended periods, while pollution peaks during flood periods in the estuary can exceed 2 orders of magnitude such baseline level.