A study, with participation of IFISC (UIB-CSIC) researchers, describes how microplastics sink in the Mediterranean Sea and analyses the consequences for their dispersion and distribution throughout the water column. The work, published in Ocean Science, proposes a model for the vertical movement of microplastic particles that are denser than water, and analyses which parameters influence their sinking from the surface to the seabed.
Of the approximately 8 million tons of plastics that end up in the oceans annually, only 1% remains floating on the surface. The remaining plastic can wash up on beaches, be ingested or sink to the bottom. The latter is the subject of the published paper, which aims to differentiate which types of microplastics are found in the water column on their way to the seafloor, as well as characterizing the properties that affect their dynamics in the ocean, such as their density, size and shape. The researchers estimate that at each instant of time, about 3 tons of microplastics, with an approximate diameter between 0.1 and 0.5 mm, can be found in the water column in the Mediterranean Sea.
Knowing the type of plastic particles to be studied, it is possible to analyze what physical phenomena affect their movement in the ocean. Thanks to this, the study concludes that, for example, the inertia of microplastics, the Coriolis effect (associated with the Earth's rotation), or the vertical variation of seawater salinity itself (which directly affects its density) do not play an important role in describing how microplastics sink.
Based on this, the researchers conducted a simulation of the Mediterranean Sea using realistic current data and found that the distribution of plastics throughout the water column is almost uniform. However, the study does observe differences in the dispersion of microplastics as they sink depending on depth. The most significant change occurs at about 100 m depth, when the particles change from an initial super-diffusion regime, i.e. very large vertical dispersion, to normal diffusion with more moderate dispersion. Such studies are key to current and future strategies for cleaning the oceans of microplastics.
Sinking microplastics in the water column: simulations in the Mediterranean Sea. de la Fuente, Rebeca; Drotos, Gabor; Hernandez-Garcia, Emilio; Lopez, Cristobal; van Sebille, Erik. Ocean Science. DOI: http://dx.doi.org/10.5194/os-17-431-2021