Trillions of microplastics can be found floating in the surface waters of oceans around the world, a fact that has been well documented through hundreds of studies and raises concerns about the effects of this pollution source on natural ecosystems. Beyond the surface, however, a thorough inventory of the size and types of plastic debris that can be found throughout the entire water column has yet to be undertaken. Now, researchers at Florida Atlantic University have taken a deeper look at the issue – both literally and figuratively – uncovering how plastic particles distribute throughout ocean depths and revealing small microplastic “hotspots” far beneath the surface.
The researchers sampled plastic particles from the surface to the abyssal depths of the South Atlantic Subtropical Gyre using in situ high-volume filtration, Manta net and MultiNet sampling methods. Micro-Fourier transform infrared (FTIR) imaging was used to analyze the particles collected from different areas and depths. The team found small microplastics, less than 100 μm in length, underwent diverse and complex redistribution processes, causing variations in abundances and distribution patterns vertically and geographically. There were also large horizontal and vertical variations in the abundances of these small microplastics, which displayed inverse vertical trends in some cases. Notably, plastic particles were prevalent throughout the entire water column, and the findings showed that weak ocean current systems can contribute to the formation of small microplastic hotspots at depth, where they are more likely to be encountered by subsurface particle feeding organisms such as zooplankton.
The team also looked at differences in abundance and distribution of particles of different sizes and from different types of plastic material. The researchers discovered that the abundance of small microplastics collected from pump samplers was more than two orders of magnitude higher than that of large microplastics, greater than 300 μm in length, that were collected concurrently in MultiNet samplers. Additionally, higher-density polymers such as alkyd resins and polyamide made up more than 65% of the pump sample counts, while previous ocean surveys have shown that buoyant polymers like polyethylene and polypropylene are the most common microplastics found in surface waters. Compared with the larger microplastics, the smaller microplastics were more highly oxidized and appeared to have a longer lifetime in the water column, suggesting that these tinier particles could poste a greater risk to the marine ecosystem through bio-uptake. This study was published in Global Change Biology.
“Our study highlights the urgency for more quantification of the deep-ocean microplastics, especially the smaller size fraction, to better understand ecosystem exposure and to predict the fate and impact of these microplastics,” said Tracy Mincer, senior author on the study.
Mincer noted that extremely small plastic particles could move across the gut epithelium of organisms and become trapped within biomass, potentially being transferred throughout the food web, which ultimately includes humans. The researchers stressed the importance of studying the potential biogeochemical impacts of microplastics pollution and the extent of this pollution from the surface to the bottom of the world’s oceans.
Photo: Post sampling in the water column in the southern Atlantic Ocean. Credit: Members of the Pelagia cruise 64PE448/Florida Atlantic University