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The distribution of subsurface microplastics in the ocean
The distribution and behaviour of microplastics in the ocean is influenced by their size, with some microplastics potentially entering carbon cycles. An analysis published in Nature explores the distribution of microplastics across ocean depths. Larger microplastics between 100 and 5,000 micrometres in size are more effectively trapped by stratification, while microplastics smaller than 100 micrometres have a more uniform distribution and a longer lifespan in the water column. The findings highlight the importance of studying microplastic distribution, which is needed to inform effective strategies to reduce the impact of plastics on the environment.
Although microplastics have been widely recorded across the ocean, many studies on microplastics only sample the top 50 centimetres of the sea surface. Shiye Zhao and colleagues synthesized depth profile data from 1,885 recording stations across the global ocean between 2014 and 2024 to evaluate the concentrations and behaviours of microplastics at depth. These data suggest, for example, that microplastics may constitute a measurable fraction of the carbon cycled in the ocean, increasing from 0.1% of carbon at 30 metres to 5% at 2,000 metres deep. High abundances of microplastics were consistently observed in deep waters, with over 1,100 particles per cubic metre detected at a depth of 100–270 metres in a transect of the Atlantic, 600 particles per cubic metre at a depth of 2,000 metres in the North Pacific Subtropical Gyre, and 13,500 particles per cubic metre at a depth of 6,800 metres in the Mariana Trench.
The authors note that their study highlights that a lack of standardized methodologies leads to substantial uncertainties that make it challenging to come to firm conclusions and to better understand microplastic distribution in the ocean.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr Karin Kvale, Aotearoa Blue Ocean Research, an author of this study
We compiled data from ship observations from all over the world to try to understand where microplastics are building up in the global oceans.
"What we found is that the larger sized microplastics, the ones you can see or nearly see, tend to be found floating near the surface in ocean gyres or are getting trapped along ocean fronts. The smaller microplastics, the ones that would be impossible to see without a really good microscope, are just everywhere, through the whole water column, at high concentrations from many hundreds to many thousands of particles per cubic metre. They are abundant enough to be changing the chemical fingerprint of ocean carbon.
"These findings are worrying for a few reasons. We know that as plastic pollution that is already in the environment continues to break down, the microplastics contamination problem will continue to get much worse. Microplastics accumulate in ocean food webs and this has consequences that are still poorly understood.
"Here in Aotearoa New Zealand we know basically nothing about the current water column microplastic situation. But we do know that already the ocean is a major regional source of microplastics getting caught up in winds at the surface and blowing onto both the North and South Islands. It’s therefore reasonable to expect that our coastal oceans are highly contaminated through the whole water column, but more sampling and study is needed.
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