Lighter than water, yet sinking to the deep: the surprising reason microplastics reach the ocean floor

Korean researchers have uncovered why microplastics — lighter than water and expected to float on the ocean surface — are found thousands of meters down in the deep sea. The culprit turns out to be phytoplankton, the tiny organisms drifting through the water.

A research team led by Dr. Baek Seung-ho of the Ecological Risk Research Division at the Korea Institute of Ocean Science and Technology (KIOST) announced Wednesday that field experiments conducted in conditions closely mimicking the real ocean showed that the type of phytoplankton community present significantly affects how much microplastic sinks.

The team set up five cylindrical tanks — each one meter in diameter and 2.5 meters deep — in the waters off Geoje, South Gyeongsang Province. Seawater and phytoplankton were placed in each tank, with nutrient concentrations of nitrogen and phosphorus varied across the tanks to replicate how rainfall changes nutrient levels in estuaries and coastal waters.

Three days after the experiment began, the quantity and species of plankton in each tank had diverged markedly. The team then introduced 30 million microplastic particles into each tank and measured how much settled to the bottom.

The researchers had expected tanks with more phytoplankton to show greater sinking, but the actual sinking amounts did not correlate with total biomass.

In the tank with the highest phytoplankton concentration, only 11.6 percent of the microplastics reached the bottom. By contrast, a tank with roughly half the plankton biomass saw 34.1 percent settle — a nearly threefold difference in sinking rate.

The key factor was the species composition of the plankton, not the quantity.

Tanks dominated by fast-reproducing diatoms saw far more microplastics sink. When phytoplankton die, they secrete a sticky mucus — and diatoms, which have short life spans, produce this mucus in large amounts. The mucus acts like glue, binding surrounding plankton and microplastic particles into clumps denser than water.

In tanks where dinoflagellates — which have comparatively longer life spans — were dominant, mucus secretion was lower and the clumps did not form in sufficient quantities.

The team also found that phytoplankton mucus forms within two days, while the clumps that bind with microplastics take about 10 days to develop. Once formed, those clumps sank at an average rate of about 80 meters per day.

"Through field experiments, we confirmed how phytoplankton communities — which shift depending on nutrient concentrations — alter the movement of microplastics within the ocean," Baek said. "We will continue observing how the dominant plankton species change by season and sea area to determine where microplastics travel and accumulate."

The findings were published in the international journal Water Research.