How the dispersion of water masses affects transport trajectories

Marine microplastics (1 μm to 5 mm in diameter) are an increasingly pressing concern, given their longevity in the environment (> 100 years) and the effects they have on the organisms that inhabit them, ‘especially since ocean currents transport particles over great distances, even reaching polar basins.

Often these microplastics wash up on beaches and mix with the sandy shores we are familiar with, but some areas have become particularly hot spots for microplastic pollution. There are “waste zones” associated with the five subtropical ocean gyres (circular currents in the North and South Atlantic Ocean, the North and South Pacific Ocean, and the Indian Ocean) that extend millions of kilometers.² zone of microplastics with floating surface.

Previous studies have suggested that 15.6 trillion particles per year are adding to the global floating marine debris crisis.

New research published in Frontiers of marine science focused on the beaches of the Canary Islands, popular Spanish tourist resorts, investigating the potential origin and transport pathways of microplastics both across ocean basins and through the water column, up to so they end up stranded on these beaches.

It is important to note that the Canary Islands are located in the North Atlantic subtropical gyre, as well as in the direct path of the Canary Current driven by surface winds. Four north-northeast oriented beaches across the archipelago were investigated for large-scale (>100 km) and meso-scale (10-100 km) transport mechanisms, namely: Playa Grande (Tenerife) , Playa de Famara (Lanzarote), Playa Lambra (La Graciosa) and Arenas Blancas (El Hierro).

These beaches are known for their marine debris problems, exceeding 100 g of plastic/m² and 3,000 particles/m².

Dr. Daura Vega-Moreno, assistant professor at the University of Las Palmas de Gran Canaria, Spain, and colleagues focused particularly on current speeds in the latitude range 27°-32°N over a period of several years, and on its impact. transport of marine debris through the water column in the open ocean.

It is important to focus on the open ocean, because the factors affecting the transport of microplastics differ from those occurring along coasts, where the latter are generally related to density, size and shape, being therefore sorted by wind and waves, with the smallest and lightest materials being carried further inland.

Conversely, for open waters, the internal composition of the plastic and its state of degradation play additional important roles, as do the temperature of the water column, convection and the Coriolis effect (air deflected to the right in the northern hemisphere and to the left in the southern hemisphere).

The research team used data from numerous projects in which microplastic particles have been released into the ocean since 2017, monitoring progress using marine technologies as well as directly sampling the water column during of a research cruise.

After processing, a microscope was then used to identify each type of marine debris, totaling 260 microplastic fragments and fibers. All of this data was then used in modeling software to determine trajectories from the source to the beach sink and produce distribution maps.

Dr Vega-Moreno and colleagues identified a water depth of 200m as a key accumulation window for fragments and fibers, while a water depth of 1,100m is a critical point for a strong accumulation of fragments <200 μm et >200 μm. While the former may be due to natural sinking through the water column, the latter is probably partly related to Mediterranean water flow, the high-salinity flow of the Strait of Gibraltar.

This mass of water flows beneath the central waters of the North Atlantic to the seafloor of the Atlantic basin to depths of 1000–1200 m and disperses towards the Canary Islands. The Mediterranean Sea where the water comes from is known to have a high concentration of large pieces of plastic and microplastics, with estimates that up to 10% of global marine plastic pollution is in the Mediterranean basin alone.

Small microplastics (<1 mm) and those that are flattened or degraded are likely to be entrained in this descending water mass and therefore transported vast distances across the deep ocean basin.

It is important to assess the transport routes of marine microplastics and, therefore, where they may wash up on shore, to implement targeted removal techniques, as well as to mitigate the consequences on the marine and coastal biota. This research highlights the role that deeper water bodies must play in dispersing plastic pollution. It is therefore vital to continue work on intermediate (~1000 m) to deep (>2000 m) water formation locations, especially as this includes the polar regions.

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