Dutch and Chinese researchers have independently unearthed more bad news about nano- and microplastics, focusing on their infiltration of freshwater and seawater sediment.
The first study investigated the long-term impact of these microscopic particles on freshwater bottom dwellers, finding they drastically reduced populations of Nadidae worms with implications for entire ecosystems.
Writing in the journal Science Advances, Paula Redondo-Hasselerharm from Wageningen University and colleagues highlight the dearth of research on the long-term ecological risks of nano- and microplastics.
To address this gap, they mixed trays of natural sediment with five different concentrations of the plastic particles and embedded them in a ditch containing a community of organisms representative of those occurring naturally in canals, ponds and lakes where these particles often accumulate.
The trays colonised with increased numbers of organisms over 15 months as expected, but contaminated soils were less populated than cleaner soils, and the Nadidae worms were most affected.
In the control samples, their numbers increased by factors of 13 and 70 compared to two and 30, respectively, in samples with higher plastic concentrations. Other organisms had smaller but significant declines over time.
Worms have vital roles in ecosystems such as burrowing, breaking down organic matter, helping the flow of nutrients, oxygen and water and providing a food source for fish and other bottom dwellers.
Although the community was not impacted at the lowest concentrations over the observed time frame, the authors note that nano- and microplastic concentrations are predicted to rise due to continued emissions and fragmentation.
The second study, published in the journal Environmental Science & Technology, discovered that marine microplastic pollution is likely to be far greater than previously thought – and again, worms could be involved.
Baoming Xue and colleagues from Guangxi University collected 52 samples of sediment, “generally considered to be a final sink for microplastics”, from Beibu Gulf, a traditional fishing ground in China, and nearby rivers, drilling deeper than their predecessors.
After extracting and counting microplastics under a microscope, they found the particles hidden up to 60 centimetres below the ocean floor.
The concentrations were correlated with heavy fishery activities and, accordingly, most were made from polypropylene or polyethylene. These materials are used abundantly in fishing gear, such as nets, ropes and pots, and can be worn away during use or when gear is lost or thrown into the sea.
Surprisingly, microplastics were found to be five times more concentrated at depth than in surface sediments, and were even found in sediments laid down as long ago as 1897 – before plastics existed.
The researchers suggest that marine organisms such as peanut worms (Sipunculus nudus) and lugworms (Arenicola marina) could have carried them deep into the sediment through ingestion and defecation, preserving “fresh microplastics” in “old sediment”.
In this case we can only guess at the ecological impact on the worms and their communities.