The Menace Of Plastic On Water Surfaces

It is a common misconception that the ocean is covered with plastic bags and islands of plastic floating on the surface. In reality, less than 1% of plastic floats on the surface, while the rest sinks to the seafloor. However, recent studies by Dutch scientists from the University of Utrecht in the Netherlands suggest that a larger fraction of plastic, around 60%, may be floating on the ocean surface. This finding highlights the urgency of addressing plastic pollution in our oceans and the need for more accurate measurements and effective cleanup solutions.

Around 60% of plastic in oceans is floating on the surface

Plastic is the most common form of marine debris. It is found everywhere, from the equator to the poles, from Arctic ice sheets to the seabed. A recent study by Dutch scientists found that around 60% of plastic in the oceans is floating on the surface, which is significantly higher than previously estimated. This challenges the widely held assumption that only 1% of plastic in the oceans floats on the surface. The study also found that the total amount of buoyant marine plastic litter is around 3,000 to 3,400 kilotons, with the majority of plastic mass consisting of large plastic items, ranging from 90% to 98%.

The discrepancy between the amount of plastic entering the ocean and the amount floating on it has puzzled researchers. Some estimates suggest that tens to hundreds of thousands of tonnes of plastic are floating on the surface, which is a large amount but not close to the tens of millions of tonnes that have flowed into the oceans over the years. This has been termed the ''missing plastic problem'. One possible explanation is that ultraviolet light breaks down plastics very quickly, leading to the presence of large amounts of microplastics in deep-sea sediments.

The fate of plastics in the ocean is a complex issue. While some plastic sinks, a significant portion of it floats, with about 80% of floating plastic beaching on coastlines within a month of leaking into the ocean. However, some objects may be washed back out to sea, and plastic can accumulate in huge subtropical oceanic areas called gyres, which trap floating plastic for decades or even centuries. These gyres, such as the infamous Great Pacific Garbage Patch, located between Hawaii and California, are a significant source of plastic pollution.

The impact of plastic pollution on marine life is devastating. Thousands of seabirds, sea turtles, seals, and other marine mammals die each year due to ingesting plastic or becoming entangled in it. Even endangered species like Hawaiian monk seals and Pacific loggerhead sea turtles are affected, with plastic debris found in their habitats and nurseries. The problem is expected to worsen, with predictions that nearly 99% of seabird species will have ingested plastic by 2050.

Addressing the plastic pollution crisis requires urgent action. Strategies to reduce plastic use, improve waste management, and participate in cleanup efforts are crucial. By increasing awareness and committing to changing our habits, we can help mitigate the devastating effects of plastic pollution on our oceans and marine life.

Microplastics are found at all ocean depths

While the ocean's surface is not covered with plastic bags or "islands of plastic", it is estimated that less than 1% of plastic in the ocean floats on the surface. The remaining 99% of plastic in the ocean is located deeper in the water column or has sunk to the seafloor.

Microplastics, in particular, have been found in some of the most remote places in the world, including the deepest point in the world's oceans, the Mariana Trench, and atop Mount Everest, the world's tallest peak. These microplastics are consumed by animals of various sizes, shapes, species, and habitats, causing a range of impairments, from behavioural changes to DNA damage, altered metabolism, decreased growth, and reproductive harm.

Microplastics are plastic particles smaller than 5 millimeters in size, which are either degraded from larger materials or intentionally manufactured at a very small size. They are derived from a variety of sources, including synthetic textiles, city dust, tires, road markings, marine coatings, personal care products, and engineered plastic pellets. Ordinary consumer products are the primary source of microplastics in the ocean, with synthetic textiles being the single greatest contributor, accounting for 35% of the total volume.

Water tank simulations and laboratory experiments have demonstrated that underwater avalanches, known as "turbidity currents", can transport plastic particles to great depths in the ocean. These currents create drift deposits, similar to underwater sand dunes, which can be tens of kilometers long and hundreds of meters high. As a result, microplastics are not only found at all ocean depths but also in remote and unexpected locations.

Only 1% of plastic in oceans floats, the rest sinks

It is a common misconception that the ocean is covered with plastic bags and that there are ""islands of plastic" floating on the surface. In reality, less than 1% of plastic in the ocean floats, while the rest sinks to the seafloor. This is because plastic has a certain density, and if its density is greater than that of seawater, it will sink.

The fate of plastic in the ocean depends on its buoyancy. Nearly half of the plastic sinks directly because of its low buoyancy, while the other half floats. Of the plastic that floats, about 80% will beach on a coastline within a month of leaking into the ocean. Some objects may be washed out to sea again, but coastlines are the final resting place for most floating ocean plastic. This has serious consequences for the coastal environment, the fishing and tourism industries, and cleanup costs for coastal communities. For example, a PET bottle is likely to sink as it fills up with water, but the cap, made of a different type of plastic (HDPE), will stay afloat for much longer. High-density polyethylene (HDPE) products are most likely to travel long distances.

Recent studies have challenged the idea that only 1% of plastic in the ocean floats. Researchers from the University of Utrecht in the Netherlands developed a new numerical model to estimate the amount of plastic in the ocean. They found that the largest fraction of plastic mass is located at the ocean surface, between 59% and 62%. This is in stark contrast to previous assumptions and suggests that the total amount of buoyant marine plastic litter is much higher than previously estimated, at around 3,000 to 3,400 kilotons. The study also found that the majority of plastic mass is made up of large plastic items, constituting most of the total buoyant plastic mass.

Despite these findings, it is important to note that the presence of plastic in the ocean, whether floating or sunken, remains a significant environmental concern. Floating plastic debris can disrupt marine ecosystems by allowing coastal organisms to spread into the open ocean and outcompete native species. Additionally, plastic in the ocean often contains harmful additives, chemicals, and toxins, which can be ingested by marine life and make their way up the food chain, potentially impacting human health as well.

Larger plastic items make up most of the buoyant mass

While it is commonly believed that the ocean is covered with plastic bags and there are 'islands of plastic' floating on the surface, the reality is quite different. In fact, less than 1% of plastic floats on the ocean's surface, with the remaining 99% either floating deeper in the water column or sinking to the seafloor. This is because plastic has a certain density, and if it is denser than seawater, it will sink.

However, recent studies have challenged this notion, suggesting that a larger fraction of plastic mass may be located at the ocean surface. Dutch scientists from the University of Utrecht in the Netherlands developed a new numerical model to estimate the amount of plastic in the ocean. Their results indicated that between 59% and 62% of plastic mass is found at the surface, contradicting previous assumptions.

The study also revealed that the majority of this plastic mass is comprised of large plastic items, with estimates ranging from 90% to 98%. These larger items contribute significantly to the total buoyant plastic mass. The findings suggest that the total amount of buoyant marine plastic litter is much higher than previously thought, estimated at around 3,000 to 3,400 kilotons.

The discrepancy between the amount of plastic entering the ocean and the amount floating on it has been a puzzle for researchers. The new model indicates a lower plastic input into the marine environment, resulting in a higher residence time for plastics. This means that even if new plastics were to suddenly stop entering the ocean, the removal of plastic mass would be a slow process.

The fate of plastics in the ocean is a complex issue. While larger plastic items make up most of the buoyant mass, there are also microplastics that are practically invisible to the naked eye and can be found at various depths, including the seafloor. The presence of plastics and microplastics in the ocean, regardless of their size, underscores the importance of reducing plastic usage and finding sustainable alternatives to mitigate this global issue.

Ocean mixing processes push debris below sampling devices

The amount of plastic in the ocean is a pressing environmental concern. While it is commonly believed that plastic pollution is spread from the ocean surface to the seabed, this is not the case for buoyant plastic bigger than a sand grain (0.5mm). Conventional methods to study ocean plastic, such as standard plastic sampling devices, underestimate microplastic loads by up to 97% due to ocean mixing processes pushing debris below the sampling depth. These mixing processes occur on a small scale and must be parameterized in climate simulations to understand their impact on the large-scale ocean climate.

Ocean mixing processes are driven by wind, buoyancy, internal waves, and shear forces. For example, strong and variable winds create momentum, CO2, and heat fluxes that interact with the ocean. Storms add a rotational component to the wind, inducing Ekman pumping and shear-driven mixing. Internal waves, driven by buoyancy and shear forces, also play a role in mixing the ocean. These small-scale processes have a significant impact on the large-scale ocean climate, influencing heat, salt, and other tracer fluxes.

The density of plastic and ocean currents also play a role in ocean mixing processes. Plastic with a density greater than seawater will sink, while less dense plastic will float. Oceanic overflows, or dense gravity currents, can originate in semi-enclosed basins or on continental shelves and mix vigorously with surrounding waters, changing their density and transport behaviour. The speed of a downslope gravity current into a stratified environment is approximately constant, and the ambient density of the current will spread out at a height above the level of D below the source.

The Multi-Level Trawl, a new research tool, allows for the measurement of 11 water layers simultaneously, from the surface down to a depth of 5 meters. Results from this tool show that most vertical mixing of microplastics occurs during rough water conditions, and smaller particles are more sensitive to these conditions, sinking deeper when the wind picks up. This has important implications for understanding the distribution and impact of plastic pollution in the ocean.

Frequently asked questions