Microplastic accumulation dynamics in two Mediterranean beaches with contrasting inputs

Plastic debris is becoming a growing environmental concern and microplastics (size <5 mm) are one of the most important fractions due to their environmental and human potential deleterious effects. Despite the recent effort done on estimating their concentrations in several ecosystems, the dynamics of microplastic accumulation are not sufficiently studied. In this work, short-and long-term microplastic accumulation dynamics have been studied on two beaches located in the Western Mediterranean with low and high anthropogenic pressure by sampling twice per week the surface of the sediment for one month, and in-depth, respectively. As regards short-term accumulation dynamics, no clear correlation was detected between microplastics levels and oceanographic variables. As regards long-term accumulation dynamics, on both beaches, fibres were the predominant shape and no clear accumulation pattern of microplastics along depth was observed. Only at the beach with high anthropogenic pressure, there was a correlation between the levels of microplastics and the 125 μm fraction of the sediment. The obtained results suggest that, in areas where there are no direct sources of pollution, the accumulation dynamics of microplastics can be driven by oceanographic variables to a greater extent than areas with direct sources of microplastics. The present study suggests that the accumulation dynamics in the long-term are difficult to be monitored on beaches due to their high dynamism. Additionally, special care needs to be taken when studying microplastics levels in the sediment since the shape of the microplastic can greatly affect their accumulation.