From coastal regions to the offshore ocean, pollution is rife, and its dramatic consequences for marine life are now known. As the production of plastic has increased exponentially since the 1950s, it has recently been shown that without a strategy to reduce this waste, this contamination will triple by 2040.
To reduce them, let's first try to understand the paths they take. This involves identifying their sources and sinks, where they originate from and where they “end” their course. According to estimates, this marine pollution by plastics is 80% of coastal origin - that is to say resulting from the river supply or the coastal population. The rest would come from maritime activities.
However, we still lack data and observations on this waste. To understand the transport and dispersion of plastic in the oceans, numerical models are an ideal tool to overcome these observation gaps and to test hypotheses on the behavior of particles in water.
This is exactly what we are trying to do in the study that I conducted with other researchers in the physical and spatial oceanography laboratory. Published next April in the Marine Pollution Bulletin, it aims to know the fate of floating plastic debris that is rejected along the coasts, based on a model of ocean circulation on a global scale.
Plastic waste modeled as digital particles
More precisely, the objective of this study is to compare the trajectory of floating plastic particles according to how they arrived in the water from the coasts.
In the first scenario, called rivers, the input of waste therefore comes from rivers and follows a model established by researchers in 2017 according to which approximately 2 million tonnes of plastics enter the ocean every year. The most polluting rivers are mainly located along the coasts of the western Pacific and account for about 70% of the global input in this scenario.
Based on the human population present on the coast, the second scenario used is proportional to the amount of poorly managed plastic waste. The model is based on the estimates collected in a study of 2015, which estimated that between 5 and 13 million tonnes of plastic debris were released into the ocean through this for the year 2010. In this scenario, known as the coastal population, the contributions are distributed more evenly along the coasts than in the rivers scenario.
To study their dispersion and their fate, we modeled coastal plastic waste in the form of digital particles, which follow the evolution of currents on a global scale, daily for 23 years (from 1993 to 2015).
These digital particles do not faithfully reflect reality, this is a theoretical case of pollution by plastics: here we only consider the floating part of the contamination, that is to say the waste transported to the surface of the oceans. - in the facts they constitute 50% of plastic pollution at sea. Therefore, our particles never settle to the bottom of the water.
To mimic the continued influx of plastic pollution into the ocean, 20 particles are released each month in both scenarios - a total of around 000 million particles over the 6 years of numerical simulation. Although in reality the contributions by rivers and those by the coastal population represent different levels of contamination, we have chosen to model the same number of particles in each scenario in order to be able to compare their path.
Particles everywhere ...
At the end of the simulation, we recover the geographical position of the digital particles: we then find both those which are 23 years old, released at the start of the process, and the youngest, released only a few months ago.
If we had a satellite that detected plastic particles on the surface of the oceans, this is the picture we would get - assuming the pollution of the surface particles was only coming from rivers or from the coastal population. , which are the two origins taken into account here.
What is interesting here is to observe that in both cases the particles are present almost everywhere in the oceans. From the coast to the middle of ocean basins, with a much higher concentration in the middle of each ocean gyre: these are called subtropical convergence zones.
There are 5 of them, famous for accumulating plastic waste: in the center of the Indian Ocean, the North and South Pacific, the North and South Atlantic. If the physical dynamics appear similar in the two simulations, we observe significant differences in concentration: in the rivers scenario, the amounts of particles are much lower in 3 of the ocean basins - the South Pacific, the North Atlantic and the South Atlantic. With the data available to us, it seems that the coastal population scenario more closely reproduces accumulation in subtropical convergence zones than that of rivers.
In both cases, the same quantity of particles is present in the heart of the North Pacific and Indian convergence zones, with a rapid accumulation: after only 5 years, they reach the heart of these vortices. This proves that the distances traveled between the source of the pollution (the coast) and the sinks (the heart of the gyres) are relatively short.
In the South Pacific Gyre, on the contrary, particles accumulate very slowly - in the population scenario; this illustrates that the particles travel a long time, and over great distances before reaching this region: the main source is therefore not necessarily located on the coasts of the South Pacific.
From a statistical point of view, less than 20% of the particles rejected by the coast are found in the heart of the subtropical convergence zones. Only 29% end up in the ocean in the river scenario, against 45% in the population scenario. Where did the rest go?
54 to 70% stranded on the coast
In the rivers scenario, 70% of the particles were stranded, compared to 54% in the population scenario. Despite this difference in number and a distinct distribution of sources, their final distribution is similar in both cases: the particles are stranded, in both cases, on almost all coasts. This homogeneity is perhaps explained by the fact that they follow the same currents. This redistribution between sources and sinks reveals that there are potentially specific connections between certain coastal regions.
To study this connectivity and understand the relationship between sources and sinks, we have divided the ocean into several parts: the large basins are cut between north and south, except the Pacific equally divided between east and west. .
We evaluated the amount of particles that wash up from one region (sources) to the coasts of another region (sinks). 85% of the particles that land on the coast do so in their region of origin in both scenarios, and 15% of those previously travel distances of up to 8 km, allowing connectivity on a global scale.
This digital study therefore highlighted several elements. First, that the waste from rivers and the coastal population constitute two key sources of marine pollution by plastics, with up to 20% of the total particles released from the coast that accumulate in the heart of the converging areas .
Then, that there are significant differences between the two scenarios: that of the coastal population better estimates the accumulation of particles in the areas of convergence, and the proportions vary from one scenario to another between the particles that end in sea and those that run aground.
Finally, we have shown that floating debris can travel thousands of kilometers before being stranded, which means that waste that started from one coast somewhere in the world could end up on another coast 8000 km away.
This theoretical study therefore allows us to better assess the impact of the sources of plastics on their future, on the coasts and offshore. Researchers in the Physical and Space Oceanography Laboratory are focusing on these questions, and more findings on the role of ocean dynamics on digital particle stranding are forthcoming.