環境科学：海洋における水面下のマイクロプラスチックの分布

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.